WATER USE, GROWTH AND YIELD OF SORGHUM AND SOYBEAN
UNDER SELECTED TILLAGE-MULCH PRACTICES IN NKPOLOGU
SANDY LOAM SOIL, SOUTHEASTERN NIGERIA
BY
OBALUM, SUNDAY EWELE
(PG/MSc/2005/39769)
DEPARTMENT OF SOIL SCIENCE
FACULTY OF AGRICULTURE
UNIVERSITY OF NIGERIA
NSUKKA
SEPTEMBER, 2009
i
WATER USE, GROWTH AND YIELD OF SORGHUM AND SOYBEAN
UNDER SELECTED TILLAGE-MULCH PRACTICES IN NKPOLOGU
SANDY LOAM SOIL, SOUTHEASTERN NIGERIA
A dissertation submitted to the Department of Soil Science,
University of Nigeria, Nsukka, in partial fulfillment of
the requirements for the award of the degree of
Master of Science (MSc) in Soil Science
(Soil Physics and Conservation)
BY
OBALUM, SUNDAY EWELE
(PG/MSc/2005/39769)
DEPARTMENT OF SOIL SCIENCE
FACULTY OF AGRICULTURE
UNIVERSITY OF NIGERIA
NSUKKA, NIGERIA
SEPTEMBER, 2009
ii
CERTIFICATION
This is to certify that as a postgraduate student in the Department of Soil Science, University
of
Nigeria,
Nsukka,
OBALUM
Sunday
Ewele,
with
registration
number
PG/MSc/2005/39769, has satisfactorily completed the research work for the degree of Master
of Science in Soil Science (Soil Physics and Conservation option) at the University Teaching
and Research Farm. The work embodied in this dissertation is original and has not been
submitted in part or in full for any other degree or diploma of this or any other University.
…………………...
Professor M.E. Obi
(Project Supervisor)
Date ......................
………………………..
Professor C.L.A. Asadu
(Head of Department)
Date .............................
iii
DEDICATED TO:
My Late Father,
Mr Raphael Obikwelu Obalum,
for instilling in me some virtues about life;
and
My Daughter,
Miss Kasiemobi Nnennia Obalum,
whose birth brought consolation to my heart.
iv
ACKNOWLEDGEMENT
This MSc work in the Soil and Water aspect of Soil Physics was begun and completed
under the advisory and supervision of Professor M.E. Obi. I am very grateful to him for the
adequate attention given to me in the course of this work, especially during the writing of this
dissertation which he supervised painstakingly. I also thank all the respectable senior
academics in the Department for their academic mentoring in general, and for their invaluable
inputs – in forms of pieces of advice and useful suggestions – to this work in particular. They
include Professors C.A. Igwe (Dean of the Faculty of Agriculture), C.L.A. Asadu (Head of
Department), F.O.R. Akamigbo, N.N. Agbim, and C.C. Mba. I equally thank posthumously
late Professor J.S.C. Mbagwu, for being a motivator onto many, including me; may his soul
rest in peace. I appreciate the sharing of ideas with other senior colleagues of mine in the
Department, Mr C.M. Jidere and Miss I.M. Okpara; as well as with my fellow Graduate
Assistant and postgraduate student, Miss C.B. Okebalama.
All forms of technical assistance from the field and laboratory staff of the Department
are gratefully acknowledged. Deserving of a special mention among them are Messrs
Cajethan Iwueze and Bernard Umeh, who assisted with the tedious collection of soil and crop
data respectively in the greater part of the second year trial. I cannot thank them enough for
their level of devotion in the assignment.
My beloved wife, Mrs Maureen C. Obalum (Nee Ugwoke), has been very supportive.
I sincerely appreciate her words of encouragement and high level of understanding.
Finally, I believe in God, the Almighty Creator and King of Ages, who alone knows
the end from the beginning.
Sunday E. Obalum
September, 2009
v
TABLE OF CONTENTS
Content
Page
Designation………………………………………………………………………
i
Certification……………………………………………………………………..
ii
Dedication……………………………………………………………………….
iii
Acknowledgement………………………………………………………………
iv
Table of contents………………………………………………………………..
v
List of tables…………………………………………………………………….
viii
List of figures…………………………………………………………………...
ix
Abstract………………………………………………………………………....
x
CHAPTER ONE: INTRODUCTION………………………………………….
1
CHAPTER TWO: LITERATURE REVIEW…………………………………...
4
2.1 Agronomic values and utilization of sorghum and soybean……………...
4
2.2 Ecology of sorghum and soybean………………………………………...
6
2.3 Soil moisture: meaning, sources, and occurrence………………………...
7
2.4 Moisture retention and water availability in soils…………………...........
8
2.4.1 Moisture retention in soils…………………………………..……......
8
2.4.2 The concept of availability of soil water………………………..…….
9
2.5 Factors controlling the moisture characteristics of soils………..………...
11
2.6 Agronomic significance of soil moisture information……........................
12
2.7 Crops’ consumptive use of available water…………………….………...
13
2.8 Tillage and mulch soil management techniques………………………….
14
2.8.1 Essentials of tillage systems…………………………………………..
14
2.8.2 Use of surface mulches on soils………………………………………
17
2.9 Agronomic effects of tillage-mulch management practices……………...
19
2.9.1 Selected physicochemical properties of soils…………………...........
19
2.9.2 Effects of tillage-mulch practices on soil moisture conservation…….
22
2.9.3 Grain or seed yield and water use efficiency…………………............
26
2.9.4 Specific responsiveness of sorghum and soybean to tillage-mulch practices 30
vi
CHAPTER THREE: MATERIALS AND METHODS………….…………….
34
3.1 Project environment……………………………………………………
34
3.2 Field preparation, experimental layout and design and cultural practices
35
3.3 Sampling and measurements…………………………………………...
37
3.3.1 Monitoring of profile soil moisture contents……………………..
37
3.3.2 Measurement of soil water suction……………………………….
38
3.3.3 Agronomic data collection………………………………………..
39
3.4 Laboratory methods…………………………………………………….
39
3.5 Determination of the crops’ water use………………………………….
43
3.5.1 Soil moisture depletion…………………………………………...
43
3.5.2 Water balance…………………………………………………….
44
3.5.3 Estimation of crop water consumptive use……………………….
45
3.6 Yield-water use relationship………………………………………..
46
3.7 Data analyses……………………………………………………….
48
CHAPTER FOUR: RESULTS AND DISCUSSION……………………………
49
4.1 Rainfall during the field study………………………………………….
49
4.2 Soil physicochemical properties………………………………………..
49
4.3 Management-induced changes in the soil………………………………
54
4.3.1 Activity of earthworms…………………………………………...
54
4.3.2 Physical properties………………………………………………..
54
4.3.3 Chemical properties………………………………………………
58
4.4 Moisture in the monitored soil profile...………………………………..
60
4.4.1 Pattern of depletion ……..………………………………………..
61
4.4.1 Drainage and storage……………………………………………..
70
4.4.2 Soil water suction……..…………………………………………..
78
4.5 Crop water use or evapotranspiration…………………………………..
82
4.5.1 Soil moisture depletion approach………………………………...
82
4.5.2 Water balance approach…...……………………………………...
82
4.5.3 Water use estimated with Blaney-Criddle Equation……………..
87
4.5.4 Variations in the crop water use………………………………….
89
4.5.5 Cumulative water use…………………………………………….
91
vii
4.6 Components of yield…………………………………………………...
91
4.6.1 Plant height and girth: sorghum………………………………….
91
4.6.2 Number of leaves and leaf area: sorghum………………………..
92
4.6.3 Plant height and number of leaves: soybean……………………...
93
4.7 Seed yield and total dry matter…………………………………………
99
4.7.1 Sole- and intercropped sorghum…………………………………
99
4.7.2 Sole- and intercropped soybean…………………………………..
102
4.7.3 Overall trend in yield the allied factors…………………………..
104
4.7.4 Yield under sole- and intercropping……………………………...
106
4.7.5 Instability of yield in the two cropping seasons………………….
107
4.8 Yield-water use relationship: water use efficiency…………………….
109
4.9 Responses to intercropping…………………………………………….
112
CHAPTER FIVE: SUMMARY, CONCLUSION AND RECOMMENDATION
114
References………………………………………………………………………....
116
Appendices…………………………………………………………………………
138
viii
LIST OF TABLES
Table
Title
Page
1:
Some agronomic information from the three experimental plots……........................42
2:
Mean temperature and relative humidity at the study site in 2006 and 2007…...…...47
3:
Rainfall classes and the associated number of rain events in the study site covering the
crops’ growth cycle in the two growing seasons of 2006 and 2007…….……………50
4:
Pentade distribution of rainfall in the months of the two-year study duration............52
5:
Some physicochemical properties of the top (0-10 cm) soil at the start of the study..53
6:
Mean density (no. ha-1) of earthworm casts resulting from the tillage-mulch treatments
in the experimental plots in the second cropping season………….………….……...55
7:
Some physical properties of the top (0-10 cm) soil under the tillage-mulch treatments
at the end of the two-year study………….………………………………………….56
8:
Some chemical properties of the top (0-10 cm) soil under the tillage-mulch treatments
at the end of the two-year study…………….……………………………………….59
9:
Total water use (mm) of the sole- and intercropped sorghum and soybean based on
soil moisture depletion under the tillage-mulch practices….....……………………..83
10:
Total water use (mm) of the sole-cropped sorghum as computed from the other
components of the water balance equation in the two years of the study…………...84
11:
Total water use (mm) of the sole-cropped soybean as computed from the other
components of the water balance equation in the two years of the study…...………85
12:
Total water use (mm) of the intercropped sorghum-soybean as computed from the
other components of the water balance equation in the two years of the study……..86
13:
Some components of yield of the sorghum crop under the tillage-mulch treatments at
specific stages of development in the second season: plant height and girth………..95
14:
Some components of yield of the sorghum crop under the tillage-mulch treatments at
specific stages of development in the second season: number of leaves and leaf area96
15:
Some components of yield of the soybean crop under the tillage-mulch treatments at
specific stages of development in the second growing season…………….…………98
16:
Seed yield, total dry matter, and harvest index of sole- and intercropped sorghum
under the tillage-mulch treatments in the first and second growing seasons…..…...100
17:
Seed yield, total dry matter, and harvest index of sole- and intercropped soybean
under the tillage-mulch treatments in the first and second growing seasons……….103
18:
Water use efficiency of sorghum and soybean under the tillage-mulch treatments in
the first and second growing seasons…..………………………..………………….110
ix
LIST OF FIGURES
Figure
Title
Page
1:
Field layout of the treatments in the three experimental plots……..………………...36
2:
Mean monthly distribution of rainfall during the 2006 and 2007….............. ………..51
3:
Moisture contents of the soil layers (mm) under the treatment combinations in the
sorghum plot in the 2006 (a) and 2007 (b) growing seasons…..……………........61/62
4:
Moisture contents of the soil layers (mm) under the treatment combinations in the
soybean plot in the 2006 (a) and 2007 (b) growing seasons…..………..………...63/64
5:
Moisture contents of the soil layers (mm) under the treatment combinations in the
intercropped plot in the 2006 (a) and 2007 (b) growing seasons…..…………..…65/66
6:
Profile soil moisture storage (mm) in the 50-cm depth zone of the sorghum plot
during the 2006 (a) and 2007 (b) growing seasons…………..……………………....71
7:
Profile soil moisture storage (mm) in the 50-cm depth zone of the soybean plot during
the 2006 (a) and 2007 (b) growing seasons…………….……………………………72
8:
Profile soil moisture storage (mm) in the 50-cm depth zone of the intercropped plot
during the 2006 (a) and 2007 (b) growing seasons…………….…………………….73
9:
Soil water suction in the 0-30 (a) and 30-60 (b) cm depth zones of the sole sorghum
plot in the second year……………………………………………………………….79
10:
Soil water suction in the 0-30 (a) and 30-60 (b) cm depth zones of the sole soybean
plot in the second year…………………...…………………………………………..80
11:
Soil water suction in the 0-30 (a) and 30-60 (b) cm depth zones of the intercropped
plot in the second year……………………..………………………………………...81
12:
Estimated water use of the sole-cropped sorghum (a) and soybean (b) with BlaneyCriddle formula in comparison with values obtained using moisture depletion and
water balance approaches…….....................................................................................88
13:
Cumulative water use of sole-cropped sorghum under the different treatment
combinations in the 2006 (a) and 2007 (b) growing seasons……..………………….92
14:
Cumulative water use of sole-cropped soybean under the different treatment
combinations in the 2006 (a) and 2007 (b) growing seasons……..………………….93
15:
Cumulative water use of the intercropped sorghum-soybean under the different
treatment combinations in the 2006 (a) and 2007 (b) growing seasons……..……….94
x
ABSTRACT
The study, aimed at optimizing conservation and efficient use of soil moisture,
evaluated the effects of no-till (NT) and conventional tillage (CT) as main plots, and bare
fallow (B) and mulch cover (M) as sub-plots of a split-plot in a randomized complete block
design in a sandy loam soil at Nsukka. Four replications of the combinations (NTB, NTM,
CTB, and CTM) were cropped to sorghum, soybean, and their intercrop in separate layouts.
Soil moisture contents were monitored at 10±1 day intervals from crop establishment till
harvest in the 0-10, 10-20, 20-30, and 30-50 cm soil layers. Other key agronomic variables
were equally measured at specific stages during the growing seasons and at harvest.
The density of earthworm casts was significantly (P ≤ 0.001) higher in the NT
compared to the CT in the intercrop. Total porosity and saturated hydraulic conductivity of
the soil were generally lower with the CT compared to the NT. Few cases of significant (P ≤
0.05) differences in the sorghum and the intercrop plots indicated higher moisture storage
under the CT compared to the NT in the first year, and vice versa in the second year. Mulch
consistently enhanced the soil moisture status mainly in the upper layers and at the initial
stage, the effect of which was more pronounced in the second than in the first year. Tillage x
mulch interaction indicated an overall trend of moisture storage being highest in the CTM,
followed by the NTB (for sorghum) and NTM (for soybean). The tillage systems significantly
(P ≤ 0.05) influenced total WU of only the intercrop, being lower with the CT in both years.
There was significant (P ≤ 0.05) reduction in the total WU of soybean (in both years) and the
intercrop (only in the second year) due to mulch. Interaction was significant (P ≤ 0.05) only
in the intercrop; the least total WU occurred in the CTM in both years. Apart from the
significantly (P ≤ 0.05) enhanced leaf area of sole-cropped sorghum under the NT,
components of yield were generally not influenced by the treatments. Each of NT and mulch
significantly (P ≤ 0.05) enhanced the seed yield of sorghum, but not soybean – whose only
yield response was significant (P ≤ 0.05) increase in total dry matter in the second year. The
water use efficiency (WUE) of sorghum was significantly (P ≤ 0.01) enhanced under the NT
in the second year. Mulch significantly (P ≤ 0.05) enhanced the WUE of sorghum (in the first
year) and soybean (in the second year). Seed yield in the CTM, NTB and NTM were
comparable. The CTM out-yielded the NTB/NTM only for the intercropped sorghum. Either
of the NTB and the NTM is suitable for enhancing the growth, yield, and WUE of these and
related crops in this environment. The CTM may, however, be more suitable in intercropping
– which even proved to be a more profitable alternative to sole-cropping of the test crops.
1
CHAPTER ONE
INTRODUCTION
Among the cereals and legumes, sorghum (Sorghum bicolor [L.] Moench) and
soybean (Glycine max [L.] Merrill) seem to be very promising for many of the smallholder
farmers in West Africa. Sorghum is the most important food crop in the semi-arid tropics
(FAO, 1995a), where it constitutes the main grain food for over 750 million people in the
region (Food Security Department, 2003). On the other hand, soybean is a major source of
high quality protein and vegetable oil. The crop offers a variety of wholesome products that
are affordable to many, and so, has great potential to reduce undernourishment. Although the
bulk of sorghum and soybean is grown in the savanna region of Nigeria, there is growing
evidence of impending less annual rainfall in the region which, coupled with temperature
increases, would reduce soil moisture availability (Adejuwon, 2004). In the last few decades,
there has been a steady decline in productivity of sorghum in the region, due mainly to the
occurrence of drought (Chiroma et al., 2006). As for soybean, its cultivation is on the
increase when compared to other crops of vegetable origin, as the crop has been introduced to
some other parts of Nigeria owing to its economic importance (Lasisi and Aluko, 2009).
Both sorghum and soybean have been found to be successfully grown in Nsukka
Agroecological Zone. Sorghum is such a resource-friendly crop that it could thrive even on
marginal soils, whereas soybean has the inherent ability to nodulate freely; the two attributes
of which are desirable under the low-input agriculture common in the zone. Most of the
resource-poor farmers in the zone could take advantage of the comparatively low input
requirement in the cultivation of these crops to maximize returns. However, even though the
southern part of Nigeria is more humid than the northern part, management-responsive water
deficits often occur in the area (Aina, 1993), due mainly to the erratic distribution of rainfall
that contributes to suboptimal growth and reduced yield of key crops (Chukwu, 1999).
Likewise, Babalola and Opara-Nadi (1993) attributed the incidence of drought in the West
Africa savanna to the erratic nature of both the onset and cessation of rains. Rainfall in this
region is also bimodally distributed, a phenomenon that contributes substantially to poor crop
performance in the region (Odurukwe et al., 1995). There are, therefore, indications that the
expected decrease in rainfall in the savanna region of Nigeria would most unlikely elude the
Nsukka Agroecological Zone (Igwe, 2004), which is in the derived savanna zone. In spite of
the prevailing sub-humid climate, the zone is often characterized by high evaporative demand
of the atmosphere. With the increasing competition for water from many sectors and the
global scarcity of water resources, irrigation seems a less realistic and unsustainable resort.
2
In soils, the proportion of water retained in crops’ root zone could even be more
crucial to increased productivity than the total rainfall (Payne et al., 1990). In that regard,
water availability to crop plants in Nsukka area is further limited by the edaphological factor
of weakness in structure of the soils. Poor water retention in these soils under field conditions
stems more from the ensuing adverse pore size distribution, which results in high infiltration
rates and conductivity. This structural constraint applies especially to all the fragile Ultisols
of southeastern Nigeria (Mbagwu, 1987; 1990). The implication of these limitations is that
future agriculture in this zone may be water-constrained, if no means are devised for coping
with the situation. The soil resources of West Africa have great potentials for high
productivity, but for the glaring soil and water management failures (Babalola and OparaNadi, 1993; FAO, 1995a). According to Lal (1997a) one of the key conditions for improving
soil productivity in the sub-Saharan zone is to ensure effective infiltration and storage of
water in the soil. Appropriate soil and water conservation practices that encourage soil
moisture retention are, therefore, needed for maximizing rainwater resource and achieving
the optimum yields of sorghum and soybean in Nsukka zone. Such measures as no-till, mulch
application, and mixed cropping enhance moisture storage by providing groundcover or live
vegetation on the soil (Obi and Nnabude, 1990), and appear more practicable.
Tillage systems modify soil structure, temperature, and water distribution; and, hence,
they influence root distribution (Waddell and Weil, 1996) and ultimately crop yield.
However, they are characteristically inconsistent in their agronomic effects. As a result, the
existing relationships among tillage systems and crop yields are neither strong (Hatfield et
al., 2001) nor fully defined and understood (Agriculture and Food, 2004). Whereas the
conventional tillage (CT) conserves soil moisture through some known mechanisms (Hillel,
1982; Smith 1993; Agele et al., 2000); the increasingly popular no-till (NT) system may, by
virtue of promoting organic matter build-up, likely improve soil aggregation and thus water
infiltration and storage in the soil (Appropriate Technology Transfer for Rural Areas
[ATTRA], 1999). Conversely, the effect of surface mulch is almost always predictable. It
normally has positive effect on infiltration (Adekalu et al., 2007), soil hydrothermal regime
and fertility, and crop yield (Thiagalingam et al., 1996). Combination of CT or NT with
mulch modifies the soil surface and may have much greater impact on moisture status. Such a
modification changes the soil water balance in terms of profile storage, drainage, and
evaporation; and so would ultimately affect how efficiently crops use rainwater input
(Hatfield et al., 2001). Generally, tillage systems and mulch practices employ modification in
soil physical condition and/or formation of a physical barrier in conserving soil moisture.
3
The NT combined with mulch has severally been recommended in the tropics (Lal,
1983; Kamara, 1986; Mbagwu, 1990; Obi and Nnabude, 1990; Franzen et al., 1994; Lal,
1995; Alhassan et al., 1998). However, the above cited studies concentrated on other crops
than sorghum and/or soybean. Hatfield et al. (2001) remarked that the impact of the many
changes due to tillage and mulch varies across locations and crops. Since the study of crop
responses to soil moisture is basic to understanding adaptation and yield stability (Agele et
al., 2002), a quantitative assessment of crop WU throughout the growing season under
selected soil management practices is worthwhile. Attempts, using water balance approach,
have been initiated for sorghum in the semi arid tropical environments, including Northern
Sudan, BurkinaFaso (Zougmore et al., 2004), Central Rift Valley of Ethiopia (Mesfine et al.,
2005), Northeast Nigeria (Chiroma et al., 2006). Similar information for sorghum and
soybean is scanty in the Southeastern Nigeria. Thus, there is the need for research on tillagemulch-moisture relations with WU and yield of these crops in this location, for informed
adoption of tillage-mulch combinations or validation of existing findings. Intercropping of
these crops may result in more efficient use of resources (Willey, 1990) and, so, could be an
additional means of saving water.
At present, no other research has evaluated the performance of sorghum and soybean
with regard to water storage and utilization under tillage and mulch practices in this zone. A
two-year field study was, therefore, conducted in Nsukka with a view to assessing the relative
efficacy of NT and CT systems with and without straw mulch for improving rainwater
harvesting, conservation, and utilization by sole- and intercropped sorghum and soybean
under rainfed condition. The objective of the study was to show the soil-water dynamics
under the influence of the selected management practices, and the associated effect on the
WU and the overall performance of these crops in the zone. Specific objectives included:
1. to determine any treatment-induced changes in some physicochemical properties of the soil;
2. to determine the crops’ WU, from both progressive moisture depletion and water balance
components, and compare the values with that estimated from climatological data;
3. to determine the effects of the tillage-mulch practices on growth and yield of the crops; and
4. to compare the soil conditions, resource use, and performance of these crops under sole
and intercropping, and evaluate profitability of the latter vis-à-vis yield.
4
CHAPTER TWO
LITERATURE REVIEW
2.1 Agronomic values and utilization of sorghum and soybean
Sorghum is the fifth most important cereal crop after wheat, rice, corn, and barley
(Food Security Department, 2003; Saballos, 2008). In such countries as Kenya and South
Africa, there is even a small but growing market for pearled sorghum as a substitute to rice
(Food Security Department, 2003). It is the most important food crop in the semi-arid regions
(FAO, 1995a), constituting the main grain food for over 750 million people who live in the
semi-arid tropics of Africa, Asia, and Latin America (Food Security Department, 2003).
According to Oyidi (1976), sorghum is used to prepare porridge that forms a smooth
paste and has good flavour and a purple colour; the colour development of which is an
interesting peculiarity of this cereal. He pointed out that the grains of the white variety could
also be roasted with salt and spice and eaten. Food Security Department (2003) noted that the
phenolics of the red-grained variety give a desired flavour and colour in some traditional
foods and beverages. In Nigeria, cake and biscuit have been successfully made from the flour
of sorghum (Oyidi, 1976). Though those made from wheat flour were larger, Oyidi (1976)
reported that the flour from sorghum did not hold moisture; it dried and crumbled easily.
Additionally, it had a certain flavour which the wheat flour did not have, an attribute which
could be used to give variety to cakes and cookies. Sorghum could also be used for making
bread, as has been testified in Ethiopia (Mesfine et al., 2005).
It has been recognized as a potentially valuable industrial crop by the brewing
industries and confectioners (Oyidi, 1976; Food Security Department, 2003; ICS-Nigeria,
2003a). In recent years of renewed interest in biofuel production, the useful traits of this
energy crop has projected it to the point of being seriously considered for exploitation
(Saballos, 2008). According to Food Security Department (2003), livestock feed
manufacturers prefer to use grains from white sorghums or low tannin pigmented sorghums
due to the effect of tannins on protein digestibility.
Soybean is grown primarily for its protein and oil content, which are highly suitable
for human consumption and from which meal are obtained for animal feed. It contains 38 to
44% protein (twice as much as most other pulses) and 13 to 20% oil (Chukwu, 1999; Venter,
1999). Its attributes of being a rich and, yet, cheap source of protein have earned the crop the
appellation as a “miracle bean” or a “golden bean” (Sanginga et al., 1999). Research has
shown common soy foods (such as soy milk, tofu, soy flour, soy yoghurt, soycheese,
5
soyvegetable soup, and miso soy foods) to be easily preparable, highly nutritious, and very
economical (Jain, 1988). According to Jain (1988), one kilogram of soybean makes 10 litres
of soy milk. Other food materials that can be prepared from soybean include fermented bean
flavouring (a substitute for locust beans in daily cooking), fried bean cakes (a snack made
from soybean), and steamed soybean cakes (Sanginga et al., 1999). The crop is such an
economical one that almost every part of it is beneficially utilized. Venter (1999) explained
how, after dehulling, soybean hulls are processed to create fibre additives for breads, cereals,
snacks and livestock feed. Soybean can also provide residues for use as animal feed
(Sanginga et al., 1999). Whole soybeans are a good source of calcium, iron, zinc,
phosphorus, magnesium, thiamin, riboflavin, niacin and folacin (Venter, 1999).
Other products of soybean and their utilization are as follows: full-fat flakes, which
may be used in animal feed; full-fat flour, which is used for various commercial food
purposes; crude oil, which is extracted from the bean and refined to produce cooking oil,
margarine and shortening; defatted soy flakes, which are used to produce animal feed and
form the basis of a variety of products for human consumption, including soy flour, soy
isolates, and soy concentrates (Venter, 1999). These products are used extensively in
manufactured foods to help retain moisture and to improve their shelf life, and they act as
emulsifiers and as substitutes for meat in food products. The protein content of the flour is
approximately 50%. Soy flour adds protein and improves the crust colour and shelf life of
baked goods. Soy isolates have protein content of about 90%, and they contain no fibre or
carbohydrates. They are used in many dairy-like products, including cheese, milk, nondairy
frozen desserts, coffee whiteners and meat products. Soy concentrates contain about 70%
protein and retain most of the bean’s dietary fibre. The concentrates are used in protein
drinks, as soup bases and in gravies. Soy flour and soy concentrates are used in meat
products, primarily because of their fat and water absorption properties. These products are
used in a texturised form as extenders in ground meat products, in convenience foods, in
pizza toppings, meat and fish spreads, and in poultry products.
Furthermore, it may be interesting to know that the judicious substitution of soy for
animal protein reduces saturated fat and cholesterol intakes, indirectly resulting in a more
favourable blood cholesterol level and potentially reducing the risk of coronary heart disease
(Venter, 1999). Venter (1999) reported that soybean has also been shown to have
anticarcinogenic and bone-strengthening effects, and is of use in the management of Diabetes
mellitus. It is in the light of all the above high nutritive value of soybean grains that
production of the crop is being encouraged in many sub-Saharan African (SSA) countries.
6
2.2 Ecology of sorghum and soybean
Sorghum originated from the semi-arid regions of Africa, but has been adapted to a
wide variety of climates, including temperate and humid environments (Saballos, 2008). In
both environments, it is adapted for growing in diverse ecological conditions. It is the most
important and, hence, the most widely grown of all the cereal crops produced by farmers in
the savanna zones of Nigeria (Andrews, 1975; ICS-Nigeria, 2003a; Chiroma et al., 2006). As
a result, even though sorghum equally does well in the derived savanna zone (ICS-Nigeria,
2003a), the bulk of the crop is produced in the savanna zone of Nigeria (Chiroma et al.,
2006). The crop is so unique that it can grow in some of the harshest environments, where
many other crops cannot be produced. Food Security Department (2003) noted that the
transpiration ratio (kg of water required to produce a kg of plant material) of sorghum (141)
is low in comparison with maize (170) and wheat (241). It is highly resistant to drought and
can withstand waterlogging much better than most of the other cereals (Saballos, 2008). It is
usually grown in areas that are too hot, too wet, or too low in fertility for maize production
(Zaongo et al., 1994; Food Security Department, 2003). However, its successful production
is, according to ICS-Nigeria (2003a), most possible when a well-drained, fertile land that has
not been cropped with the crop the previous year is selected. Sorghum is very sensitive to
acidity and, so, the preferable soil pH for optimum yield is 6.5 (Hagan, 2008).
Although yields are adversely affected when mean temperature exceeds 28ºC during
the heading period (Zaongo et al., 1994), sorghum can withstand temperatures above 38ºC
(Food Security Department, 2003). Its production in West Africa requires around 400 to 500
mm of rain (FAO, 1995a). However, with the long growth cycle variety (the late-maturing
cultivar), the water requirement is doubled, ranging from 950 to 1100 mm (Food Security
Department, 2003). The local varieties are typically very tall plants (Chiezey and Egharevba,
1987), and can grow to a height of 3 to 5 m (Andrews, 1975; Olufajo, 1995). Among them
are the long growth cycle varieties, which take about 170 days to mature (Olufajo, 1995). The
recommended sowing date for this late-maturing sorghum cultivar in the forest-savanna
transition zone of Nigeria is 10 to 12 June (Bello, 1999) or early to mid July in the southern
Guinea Savanna (ICS-Nigeria, 2003a).
In Nigeria, the mean seed yield (SDY) of sorghum could be as low as 0.88 Mg ha-1 at
Nsukka in the southeast, as reported by Amana (2008); or 1.18 Mg ha-1 at Owo in the
southwest, as reported by Agbede et al. (2008). However, the commonly obtained mean SDY
in the core sorghum-growing areas of Nigeria included 1.12 Mg ha-1 in the Northeastern Zone
7
(Chiroma et al., 2006) and 1.15 Mg ha-1 at Mokwa in the Southern Guinea Savanna Zone
(Andrews, 1975). Others reported from other semi-arid tropical areas outside Nigeria
included ranges of 0.6-2.5 Mg ha-1 from a sandy loam in Burkina Faso (Zougmore et al.,
2004) and 2.2-4.7 Mg ha-1 in Ethiopia (Mesfine et al., 2005), and a mean value of 1.57 Mg
ha-1 in India (Patil and Sheelavantar, 2006).
Soybean, though a native crop of Eastern Asia (Lasisi and Aluko, 2009), is known to
produce best on soils with good residual fertility (Kratochvil et al., 2006). It is relatively
resistant to low and very high temperatures (but growth rates decrease above 35°C and below
18°C), sensitive to waterlogging, and moderately tolerant to soil salinity (FAO AGL, 2006;
Wani et al., 2007). Soybean does better if the soil pH is in the range, 5-6 (FAO AGL, 2006;
Kratochvil et al., 2006). Water requirement of soybean for maximum production varies from
450 to 700 mm (Dupriez and Leener, 1992; Al-Kaisi, 2000; Kranz et al., 2005; FAO AGL,
2006), depending on climate and length of growing period (FAO AGL, 2006). Soybean is
adapted for cultivation in Nigeria, and the highest of this takes place in the middle belt –
especially in Benue State. The local varieties grow to a height of about 0.5 m and the short
season varieties among them mature in 120 days (Olufajo, 1995). The recommended sowing
date for this early-maturing soybean cultivar is early to mid July in the southern Guinea
Savanna (ICS-Nigeria, 2003b).
Though SDY of 2.5 to 4.0 Mg ha-1 are achievable (Wani et al., 2007), soybean SDY
normally varies from 1.5 to 2.5 Mg ha-1 (Willatt and Olsson, 1982; FAO AGL, 2006). The
mean is 1.4 Mg ha-1 in the world (Jain, 1988) and 1 Mg ha-1 in the semi-arid tropics (Wani et
al., 2007). A range of 1-2 Mg ha-1 was obtained from a trial involving 26 varieties in the
southern guinea savanna environment of Nigeria (Akande et al., 2007). In a sandy loam soil
in the southwestern derived savanna zone of Nigeria, Lasisi and Aluko (2009) recently
reported a range of 0.7-0.9 Mg ha-1, following a two-year field trial.
2.3 Soil moisture: meaning, sources, and occurrence
Soil moisture is that constituent of the soil that makes it characteristically wet. It
represents the liquid phase component of the three-phase (solid, liquid, and gas) soil system.
Sources of moisture to the soil include precipitation (especially rainfall, snowfall, and
throughfall), irrigation, and groundwater recharge. With rainfall, overland flow sometimes
occurs, depending on some characteristics of the rainfall, as well as on the topographic,
structural, and cover management features of the soil. This overland flow, termed runoff,
8
does not enter the soil from where it emanates. If, however, the rainwater is impounded in
depressions or a reasonable proportion is trapped by some techniques geared primarily
towards conserving soil water, the ‘harvested’ runoff automatically becomes part of the soil
moisture, provided the soil physical conditions favour infiltration. Strictly speaking, water
lost to runoff never contributes to soil moisture, since soil moisture refers to the water which
had already infiltrated into, and is part of the soil. Runoff is, therefore, considered an
important process of loss of rainwater from the soil surface. On the other hand, deep
percolation is another important process of loss of soil moisture, and is influenced mainly by
soil texture and climate, being more pronounced in sandy soils and in the wetter regions than
in clayey soils and in the drier regions. Based on the foregoing, effective rainfall rather than
total rainfall aptly describes the rainfall contributing to soil moisture – total rainfall less
runoff and deep percolation.
In a hypothetical medium-textured soil, moisture constitutes about 25% by volume;
the value is generally lower and higher in coarse- and fine-textured soils respectively, all
other factors remaining constant. Under natural field conditions, the soil is never devoid of
moisture, which exists as films of water at the peripheries of the solid phase. Soil moisture is
actually water in the soil; the only difference is that it is neither in its pure state nor in ponded
condition. As such, the terms ‘moisture’ and ‘water’ are regularly used interchangeably with
respect to the soil. Since water is a universal solvent, its dissolution of solutes in a mineralrich system like the soil is naturally inevitable. Therefore, the use of ‘soil water’ should be
with caution, to denote that which ordinarily has some solutes (mainly minerals) dissolved in
it, and which is not ponded in any occlusions in the soil. In the whole of this dissertation,
‘moisture’ is preferably used; ‘water’ is used only when connoting the soil moisture properties
(such as movement and energy status) and its availability or utilization by crop plants.
2.4 Moisture retention and water availability in soils
2.4.1 Moisture retention in soils
Jury and Horton (2004) indicated that of all characteristics of the soil liquid phase, the
two most important are the amount of moisture in a given amount of soil (which influences
such processes as gas exchange with the atmosphere, diffusion of nutrients to plant roots, soil
temperature, and the speed with which dissolved chemicals move through the zone during
irrigation or rainfall) and the forces holding water in the soil matrix (which influences such
processes as efficiency of water absorption by plant roots, the amount of drainage occurring
9
due to gravity, and the extent of upward movement of water and solutes against gravity). The
amount of moisture in any soil is traditionally expressed as the moisture content of the soil;
usually on gravimetric, volumetric, or depth basis. The retaining force is expressed as the soil
matric potential or moisture tension/water suction, which is actually the energy of soil water
– which when expressed per unit volume is equivalent to the negative pressure of the soil
water. Hence, this force has the same units as pressure, of which the most convenient in
agrometeorological parlance is the “bar”. The two – amount and force – tend to exhibit an
inverse relationship; as one increases, the other decreases.
Moisture is retained in the soil by the solid framework of the porous medium for
plants roots to extract what they need to survive, and the mechanisms involved need to be
understood. The soil, being a porous medium, has an amazing property of retaining water for
substantial periods of time, despite the incessant pull of gravity; and so, acts like a water
storage reservoir (Jury and Horton, 2004). The relative amount of pores increases with
decreasing soil grain size. In deed, the soil pore system is quite a complex one, especially as
regards the size, shape, and generally the geometry of the pores, upon which the movement of
liquid water through soils depends. This complexity of the pore system makes for
inefficiency in the achievement of absolute theoretical saturation and dryness under field
conditions, as ought a water tank. In other words, ‘completely’ saturated or dry soil hardly
exists under field conditions. The achievable limits of moisture (saturation – or more
appropriately, satiation – and dryness – or more appropriately, droughtiness) in a given soil
mark the extremes of moisture amounts in that soil.
2.4.2 The concept of availability of soil water
Since the amount of moisture in the soil is subject to fluctuations between the two
stated extremes, it is more realistic to think of available moisture relative to the bulk soil. A
‘saturated’ soil has a tension near zero, but at any given moisture content below satiation,
water is held in the soil by some force and, as the moisture reserve of the soil depletes further,
the force increases correspondingly. The implication is that plant roots have to work too hard
or at least have slightly above equivalent energy of the soil water, known as the soil matric
potential or soil moisture tension, to be able to extract water from the soil (Scherer et al.,
1996). In the course of depletion of the soil moisture reserve, three different remarkable
stages are reached, assuming no water input. The first is field capacity (FC), normally
reached 2-3 days after rainfall or irrigation, when all the gravitational or superfluous moisture
10
held in the macropores must have been freely drained. In other words, downward movement
of water practically ceases, with the soil retaining all the moisture it can hold against gravity.
At this stage, the extractive force of the plants’ roots exceeds that with which moisture is held
to the soil matrix. Below the FC, soil water is partly liquid and partly vapour and, so, its
subsequent movement is limited.
The second is incipient wilting point (IWP) presumably reached at the onset of exertion
of additional force by the plant’s roots, to be able to extract capillary or available water from
the micropores of the progressively drying soil; the plant leaves show signs of loss of
turgidity, which could still be arrested with timely interventional irrigation. The third is
permanent wilting point (PWP) reached when the two forces are at equilibrium; the
hygroscopic or unavailable moisture left in the soil is held tightly to the matrix with a force
equal to the much the plant roots could exert. Water extraction temporarily ceases. The roots
exert stress to remove water from micropores in the soil. Immediately beyond this point, the
roots suffer loss of water to the surrounding soil due to osmotic pressure gradient, which
instantly results in an irreversible drop of osmotic pressure of the plant leaves due to water
stress. Morphologically, the plant loses turgidity, which culminates in wilting – a situation
that could not be revived.
Scherer et al. (1996) indicated that the value of soil moisture tension at FC varies with
soils of different textures with a range generally between 0.05 and 0.33 bars. They held that
for most agronomic crops and soils, PWP occurs at about 15-bar tension, and that this tension
with which the soil holds the water equals the maximum energy exerted by the plant at that
stage. The soil moisture contents at FC and at PWP are regarded as the upper and the lower
limits of available water respectively, and the difference between the two gives the available
water capacity (AWC) of the soil (Salter and Williams, 1965). All the water held between FC
and PWP are available to plants, and all tension values from FC to PWP correspond to given
moisture contents for any soil. These moisture contents on volumetric basis when graphed
against their corresponding tension values give a soil-specific curve known as the soil
moisture characteristics (SMC) curve. Such a curve represents the moisture release pattern of
the soil. Salter and Williams (1965) observed that though the shapes of the curves
representing this relationship gave an indication of availability of water to plants, it was not
equally available over this tension range.
Based on the above, another very important consideration in water availability is the
proportion of the AWC or total available water (TAW) that is readily available. Mbagwu et
11
al. (1983) recorded 113 mm m-1 as the maximum value of the AWC of Ultisols at Onne in
southeastern Nigeria. Scherer et al. (1996) gave the AWC of loamy sand- and sandy loamtextured soils to generally be in the range, 60-120 mm m-1 and 110-150 mm m-1 respectively.
The readily available water (RAW) for the soil under investigation had been observed to be
about 0.7 of TAW; the moisture released between 0.1- and 0.5- or 1-bar tensions (Mbagwu,
1985; 1987). It was based on this RAW consideration that the concept of minimum allowable
balance (MAB) or maximum allowable depletion (MAD) was developed to express the soil
moisture status, using the soil water balance method or soil moisture depletion method
respectively. Each of MAB and MAD (given in mm) is simply a fraction of the AWC (given
in mm m-1) in a specified root zone depth (m); both fractions always add up to one.
2.5 Factors controlling the moisture characteristics of soils
Generally, when the curve runs more or less parallel to the soil moisture tension axis,
it implies that little water is lost between FC and PWP, as observed for some fine-textured
soils (Obi and Akamigbo, 1981); but when the curve slopes down quickly and then
subsequently becomes gentle, it implies that much of the AWC of the soil sample is lost at
very low tension, as observed for some coarse-textured soils (Mbagwu et al., 1983; Obi and
Nnabude, 1988). In their own study, Obi and Nnabude (1988) reported that on a sandy loam
soil at Nsukka, up to 33% of the saturation water content was lost between saturation and
0.06-bar tension, and another 24% lost between tensions of 0.06 and 0.33 bar. It appears,
therefore, that texture has the most influence on SMC, even though some other factors
influence SMC alongside texture. Organic matter content of soils, for instance, has been
commonly associated with soil moisture retention. Nnabude and Mbagwu (1999) reported
that amending soils with organic matter decreased bulk density and increased total porosity,
thereby increasing moisture retention, the effect of which was most pronounced at PWP.
Generally, organic matter content of soils is inversely related to bulk density. In the work of
Rawls et al. (1991), increases in soil organic matter (SOM) augmented water retention at all
matric potentials with the greatest effect of such at matric potential above 0.8 bar.
Petersen et al. (1968) had earlier observed that the magnitude of this effect of SOM
and bulk density on soil moisture content decreased from C through B to A horizon, as
different horizons of a soil profile normally had variable textures. They also reported that
additions of organic matter to textures finer than silt loams would not significantly increase
AWC. According to these authors, this was because the influence of organic matter appeared
12
to be related to texture, as it favoured structure development, aggregation, and formation of
large pores which retained little moisture. Hence, air capacity increased at the expense of
moisture storage capacity. Mbagwu (1989a) indicated that the drop in AWC with texture
fineness was most prominent at PWP. He worked with three different soils, each of which
was treated with five rates of manure; and he observed that the higher the clay content, the
smaller the relative improvement in moisture retained at PWP. He also observed that at all
tensions ranging from saturation to 3-bar tension; pore size distribution took control with the
influence of total porosity becoming less prominent after 0.3-bar tension.
Though the role of texture seems to be of paramount importance, its effect on soil
moisture content may not always produce a linear relationship, just as it may be masked
under some conditions. For instance, Mbagwu (1987) reported that, considering only particle
size fractions, available storage increased with silt content and decreased with clay or sand
content. Texture (especially above 10% clay), together with clay mineralogy, influences
water retention prominently only at low water contents (Rawls et al., 1991). These workers
indicated that while the 2:1 layer silicates, especially montmorillonite, had the greatest effect,
the 1:1 non-expandable clays (kaolinite) had the least impact. Similarly, Obalum (2004)
found texture especially sand and clay content to be very important only at water retention at
PWP, with coarse sand having a negative influence, while fine sand and clay had positive
influence. Sometimes, the influence of texture is interwoven with that of structure in the
determination of soil moisture retention, especially at high soil water potentials, as observed
by Obi and Akamigbo (1981).
Chemical properties of the soil such as CEC and salt contents were noted to be also
important in soil water retention (Rawls et al., 1991), especially if they affected the integrity
of the soil aggregates or clay. Scherer et al. (1996) also stated that the presence of soluble
salts in the soil reduces the amount of water available to plants, even when the soil moisture
tension remains the same. Some workers (McCown 1972; Babalola and Opara-Nadi, 1993;
Oluwasemire et al., 2002) have detected other factors which bring about variations in
favourable soil moisture conditions, to include vagaries in local climatic conditions and soil
characteristics, which affect evaporation rates.
2.6 Agronomic significance of soil moisture information
Despite the form of occurrence of soil moisture, many physical, chemical, and
biological properties of the soil are influenced by the relative proportion of its liquid phase.
13
Soil moisture is a medium for many biochemical reactions in the soil, and so needs to be
maintained in adequate amount for optimal functioning of the soil.
Soil moisture tension is the most important soil water property to a growing plant
(Scherer et al., 1996). This is so because the SMC, by setting the limits of FC and PWP,
determines the AWC and, so, gives an indication of availability of water for plants’ use.
Having known the factors that control the SMC, moisture retention in soils, when deemed
unfavourable, could be manipulated through appropriate management. Important agricultural
uses of soil moisture retention data include the assessment of crop water requirement,
planning irrigation schedules, prediction of probable crop responses to water supply on soils
of different textures (Salter et al., 1966), generation of large-scale probability estimates of
potential runoff or the potential fate of agricultural chemicals (Rawls et al., 1991). Water
budgeting for irrigation planning also requires local soil moisture retention information. Poor
budgeting due to paucity of such information could lead to water stress, which manifests in
reduced crop yields. Besides the constraints to crop production, unfavourable soil moisture
status hampers the capacity of soils to carry out their environmental regulatory functions.
2.7 Crops’ consumptive use of available water
Scherer et al. (1996) defined crop water use (WU) as an estimate of the amount of
water transpired by the plants and the amount of evaporation from the soil surface around the
plants. The two processes occur simultaneously and become known as evapotranspiration
(ET). However, as the growing season progresses and canopy cover increases, evaporation
from the wet soil surface gradually decreases, such that when the crop reaches full cover,
approximately 95% of ET is due to transpiration and evaporation from the crop canopy where
most of the solar radiation is intercepted (Al-Kaisi, 2000). In other words, at full cover, a crop
is at its potential ET (PET) (the maximum ET rate), if soil water is not limiting, namely, if the
soil in the root zone is at FC. Water depleted through ET is taken to have been used by the
crop and so, crop WU is used interchangeably with ET. All crops have a similar WU pattern,
which changes predictably from germination to maturity (Scherer et al., 1996). Crop WU
should not be limited by soil moisture, especially at a certain stage of growth; or else yield
may be adversely affected.
A unit amount of water defined as ‘consumptively’ used on a given area comprises
that lost during evaporation from adjacent soil and that utilized in transpiration for the
building of plant tissues (Erie et al., 1981). Soil evaporation is influenced mainly by albedo
14
of ground surface, and the impact is felt more in drier regions. Transpiration is controlled
mainly by the plant’s C-4 characteristics. Crop WU can, therefore, change from growing
season to growing season due to changes in climatic variables (such as rainfall, humidity,
temperature, solar radiation, wind movement, etc.) and soil differences between fields
(Collinson, 1996; Scherer et al., 1996). Other factors that influence crop WU include variety
selection or growing season length, management, topography, crop species, growth stage,
prevailing weather conditions, and available water in the soil (Erie et al., 1981; Hattendorf et
al., 1988; Al-Kaisi, 2000).
Evaporation from the surface and transpiration by plants are, in deed, the major
avenues of depletion of available soil water. Obi (2000) noted that ET often accounts for over
90% of water absorbed by the soil in the tropics. Measurement of crop WU in the field can be
tedious and time-consuming. There are two traditional methods – the soil moisture depletion
approach and the water balance approach –, either of which can be used for such. The use of
a water balance model to estimate changes in soil water at a specified interval enables
synchronization of crop growth with WU (McCown, 1972).
2.8 Tillage and mulch soil management techniques
2.8.1 Essentials of tillage systems
Tillage systems are integral parts of soil management and conservation techniques.
They fall under two broad categories; conventional tillage and conservation tillage.
Conventional tillage (CT), as the name suggests, is the socio-culturally acceptable
system of land preparation for cultivation in virtually all societies of the world. One of the
beneficial effects of CT on soils is that it minimizes soil hardening (Aina, 1993). The
operations involved normally result in inversion of the topsoil and, sometimes, in burying of
crop residues; and could be achieved either mechanically or manually. Mechanized
operations entail mechanical manipulation of the soil in an entire field by ploughing,
followed by one or more harrowing; with the degree of soil disturbance depending on the
type of implement used, the number of passes, soil, and intended crop type (Opara-Nadi,
1993). Manual operations entail the use of native or traditional tillage tools to loosen,
pulverize, and manipulate the soil. The two most important tools in the humid and sub-humid
regions are cutlasses and hoes.
Though the CT system had been in use since time immemorial, it has some
limitations. One of such limitations is that for it to be effective over a longer term, the soil
15
aggregates must be stable and resistant to breakdown under raindrop impact (Aina, 1993).
This author further noted that it must be carried out under appropriate conditions of soil
moisture and at reduced frequency too, to avoid structural breakdown and smearing.
Discontinuity of macroporosity frequently occurs under CT, especially when practised
annually; subsoil compaction occurs at the plough depth within the early years, which worsen
as years roll by (Birkas et al., 2004). As a result, CT may not be as effective in the humid
and sub-humid regions as it is in the arid and semi-arid regions. Other limitations include
steady diminution of the effectiveness with increase in slope; laboriousness, especially when
hand-operated tools such as hoes and other local implements are used; and the inevitable
disruption in the life cycle and/or interruption in the activity of some beneficial living
components of the soil – which constitutes a disturbance in the entire ecological balance,
especially when practised on a continuous basis.
A necessitated alternative to the seemingly monotonous CT is the conservation tillage.
Conservation tillage, as defined by the Conservation Technology Information Centre (CTIC)
in West Lafayette, Indiana, USA, is any tillage or planting system in which at least 30% of
the soil surface is covered by plant residue after planting, to reduce erosion by water; or
where soil erosion by wind is the primary concern, with at least 1.12 Mg ha-1 flat small grain
residue on the surface during the critical wind erosion period (Opara-Nadi, 1993).
Sometimes, the term, conservation tillage, is properly used for all types of tillage that are
designed to minimize erosion; in which case some earthworks (such as contour ploughing)
are regarded as examples (FAO, 2001). Five types of conservation tillage system identified
by the CTIC include no-till (NT) or slot planting, reduced or minimum tillage, strip or zonal
tillage, ridge till (including NT on ridges), and mulch tillage (such as stubble mulch tillage).
Explanations of the unique features of each and every one of these types of conservation
tillage could be found in Opara-Nadi (1993) and/or FAO (1995b). It is, however, a common
practice among authors to regard each of them as synonymous with conservation tillage.
According to the Natural Resources Conservation Services (NRCS) and the CTIC,
any tillage and planting system that leaves all or some of the previous crops residue on the
soil surface is described as crop residue management (Liang and Wang, 2002). One of the
potentials of crop residue management is that it aims to maximize cover and the principles
are equally effective in any conditions; it is effective in the control of wind erosion in largescale mechanized cereal production, precluding the need for terraces or other permanent
structures (FAO, 1987). Its other potentials include savings in machinery investment and in
the time required for seedbed preparation, reduction in storm runoff and erosion, suppression
16
of weed growth, reduction in WU, improvement of nutrient and water use efficiency (WUE),
and sustenance of economic productivity (Aina, 1993; Opara-Nadi, 1993); as well as
prevention of soil salinity and promotion of crop nutrition (Liang and Wang, 2002). In the
semi-arid and sub-humid regions, this tillage system may likely improve moisture storage and
movement within the soil, organic matter concentration and aggregation of the soil; and
consequently decrease susceptibility of the soil to erosion.
Conservation tillage equally has some limitations. It is less applicable to low input
level crop production or subsistence agriculture; the application is mainly in mechanized high
production farming with good rainfall (FAO, 1987). Thus, there are many reports which
indicate that the beneficial effects of the NT system (especially when crop residue is retained
on the surface) in soil and moisture conservation are more pronounced in the humid and subhumid regions (Aina, 1993; Babalola and Opara-Nadi, 1993; Opara-Nadi, 1993) than in the
arid and semi arid regions where soils – such as Ustropepts, Ustalfs, Alfic Entrustox, and
other Aridisols, characterized by weak structure, low porosity, and low infiltration rates – on
which NT is less effective, are commonly found (Aina, 1993). Other factors identified by
FAO (1987) which hinder the application of conservation tillage in semi-arid conditions
include the value of crop residues as feed for livestock, difficulty of planting through surface
mulches by ox-drawn planters, and the incompatibility of dense plant covers with the welltested strategy of using low plant populations to suit low moisture availability. The lack of or
even complete absence of crop residues mulch and other biomass on the soil surface is
perhaps the most important constraint limiting the adoption of NT farming in developing
countries (Lal, 2007). The ox-drawn planters need to be replaced with appropriate seeding
equipment which, according to Lal (2007) are conspicuously not available.
On the other extreme, NT systems have proven less effective on some soils, especially
hydromorphic soils with poor internal drainage and those with compacted surface and
subsoils (Aina, 1993). Because ideally, weed is chemically controlled under this system, Aina
(1993) and Lal (2007) also reason that increasing costs of herbicides might be a deterrent to
the continuous practice of NT even in those areas it is found suitable. Moreover, the heavy
dependence on agrochemicals could lead to serious water pollution (Opara-Nadi, 1993).
The NT system of soil conservation is generally defined as planting crops in
unprepared soil with at least 30% mulch cover (Triplett and Dick, 2008). The system has
lately been enjoying so much enthusiasm among researchers, having shown promise with
respect to yield in several geographical regions under a range of conditions (Jones et al.,
1969; Blevins et al., 1971). According to these authors, the NT system provides not only
17
protection against short duration droughts by contributing to a more efficient WU, but also
offers effective erosion control against severe storms. Research on NT cropping was,
therefore, begun in the semi-arid tropics of north-west Australia by the Animal Industry and
Agriculture Branch of the Northern Territory Administration in the early 1970s but, because
of lack of suitable planting machinery, little was achieved (Thiagalingam et al., 1996). At
about the same time (1970s), the beneficial impacts of the NT system were also documented
in South America and West Africa (Lal, 2007). Yet, as at 2006, NT was practised on less than
100 million hectares worldwide, or on merely 6% of the global cropland area (Lal, 2007).
The observation that some reduced tillage systems suitable for some soils and crops in
the tropics may be unsuitable for others (Lal, 1982) elicited curiosity among many
researchers. This has led to the intensification of search for the most appropriate tillage
management systems for different ecological zones. Many of those research efforts, as
sufficiently evident from the literature, show that the NT system can produce differential
effects in various soil-crop-location combinations. This may have been part of its slow
popularization in the region. Lal (2007) remarked that the extent of adoption of NT farming
by resource-poor small landholders of sub-Saharan Africa, where the potential benefits of this
promising innovation of the 20th century is probably the highest, is practically negligible.
Some of the benefits of NT system include superior soil and moisture conservation,
long-term build-up of organic matter, and increased water infiltration thereby reducing runoff
(ATTRA, 1999); lower production cost, greater production efficiency, arresting or reversing
soil degradation processes, and reducing nutrient and pesticide losses by reducing runoff
volume and soil loss (Liang and Wang, 2002). Others include soil fertility enhancement and
carbon sequestration (Lal, 2007). Triplett and Dick (2008) remarked that, unlike CT, the
benefits of NT are most likely to be realized with continuous practice.
2.8.2 Use of surface mulches on soils
Mulch is usually defined in a wider sense to mean any form of protective coverage on
the surface of soils, aimed at minimizing evaporative moisture losses and soil crusting. Many
different watertight or water-retardant materials could be used as mulch; including straw or
stubble, wood bark, cotton burs, sawdust, gravels, loose soil materials, latex oil, paper
asphalt, plastic films, and metal foils. Surface crop residue mulch has the advantage of
providing protective cover at a time when crop cover is not practical (Opara-Nadi, 1993),
thereby reducing evaporation, decreasing flow velocity, and controlling erosion (Aina, 1993).
This form of mulch also helps to maintain the quality and quantity of water running off the
18
agricultural land, and regulates soil temperature and moisture regimes (FAO, 1995b). Stubble
or crop residues mulch helps the soil to avoid raindrop strike, keeps soil natural structure,
reduces soil evaporation, conserves soil moisture, improves infiltration, and greatly reduces
runoff (Liang and Wang, 2002). Such mulch materials could decompose to release nutrients,
and provide other special effects such as avoiding formation of hardpan, promoting soil
microbial activity, increasing soil organic matter content, promoting soil aggregate formation,
improving soil physical conditions, and greatly reducing soils’ scours and wind erosion
(FAO, 1995b; Liang and Wang, 2002). Interestingly, straw mulch can easily be implemented
by local farmers and can be extended on regional scale because the low cost straw is easily
accessible and does not contaminate the soil (Ali and Talukder, 2008).
When gravels are used as mulch material, they reduce wind and water erosion,
conserve soil moisture, and, if light coloured, cool the soil and regulate its temperature. On
the other hand, paper and plastic mulch helps to increase soil temperature; increase in soil
temperature is beneficial in hastening early germination of seeds, seedling growth, and
development in areas where temperature is cold during the planting season. Plastic and oil
mulch could be used to collect runoff water in depressions in arid areas, which is a form of
runoff farming. Liang and Wang (2002) noted that plastic films used as mulch are highly
translucent and easily penetrable by light, yet they have less thermal conductivity and no air
permeability. Hence, they have proved to be a good mulch material, especially in China
where their use is on the increase. These authors also noted that the use of soil as a mulch
material (soil mulch) is still favoured because of its low input and easy operation.
Additional benefits of mulch application include stabilization of loose soils to
improve their physical characteristics, promotion of microbial activity, suppression of weed
growth in the field, reduction of build-up of soil salinity, and promotion of crop development
(Liang and Wang, 2002). Babalola and Opara-Nadi (1993) observed that application of mulch
decreased the effect of tillage on changes in the clay and silt content. In particular, the
benefits of mulch practices in the semi-arid tropics, according to a review by Tilander and
Bonzi (1997), include not only soil moisture conservation, soil temperature modification,
runoff and erosion reduction, nutrient recycling following decomposition, soil microfauna’s
activity enhancement; but also trapping of nutrient-rich, wind-borne dust and improving root
growth. Furthermore, all forms of mulch are helpful in controlling weeds (Erenstein, 2002).
Suppression of weeds by mulch prevents transpiration losses from those weeds. All forms of
mulch also improve the quality of some fruits (e.g. tomato, eggplants, and pepper) by
preventing contact with soil.
19
Some possible limitations of mulch practices include mismatch between requirement
and availability of crop residue; liability of organic mulches to rapid oxidation by high
temperatures; problems of pests, diseases, or N lock-up; and lack of implements that can drill
through the mulch (FAO, 1987). In many farming systems, the use of straw or organic mulch
is not practicable because it is in great demand as fuel, livestock feed, and building materials
during the long dry season (Payne, 1999), especially in dryland Africa (Smith, 1993). Where
available, the transportation would entail substantial labour and cost, and so may not be
economical (Mbagwu, 1991; FAO, 1995b; Sarkar and Singh, 2007). Additionally, the
potential benefits of mulch are less in the humid and sub-humid regions than in the arid and
semi-arid regions (Payne, 1999).
However, according to Smith (1993), the prevalent high temperatures in these drier
regions frequently lead to fast breakdown of organic mulch incorporated into the soil and the
associated decreases in available N levels. Occasional application of N fertilizers to
supplement for the deficiency of N is another avenue of increasing cost with mulch practice.
Payne (1999) lamented that the use of contoured plastic mulch is unlikely to be practised
widely either, because such materials are too expensive or generally unavailable in most of
West Africa. Besides being contestable, plastic mulch has generally been implicated to
increase transpiration from vegetation, caused by the transfer of both sensible and radiative
heat from surface of the plastic cover to the adjacent vegetation (Allen et al., 1998).
Whatever be the mulch materials, application at the end of the growing season is ineffectual
for moisture conservation because conserved water rapidly drains through the soil profile
during the dry season (Payne, 1999). Finally, if convenience and effectiveness are anything to
go by, mulch practice is sensitive to land configuration and cropping system.
2.9 Agronomic effects of tillage-mulch management practices
2.9.1 Selected physicochemical properties of soils
Babalola and Opara-Nadi, (1993) observed that most research work on effects of
tillage had been done without measuring the effects on soil properties, but had quantified
tillage effects in terms of crop performance. They shared the same view with Mbagwu
(1990), who had noted that continuous application of CT in the cultivation of the structurally
unstable and fragile Ultisols, predominant in the humid and sub-humid regions, could lead to
adverse effects on beneficial soil properties and crop yield, especially on a long term basis.
20
On the other hand, conservation farming practices and better management of cropping
systems can improve and maintain soil physical and chemical properties (Connolly, 1998).
Structure, perhaps the most dynamic physical property of soils, is the most amenable
to changes by tillage operations, because tillage influences soil cementing agents,
arrangements of soil particles, aggregate sizes, porosity, pore size distribution, and aeration
(Babalola and Opara-Nadi, 1993; Connolly, 1998). These structural changes could lead to
reduced moisture retention capacity or to reduced hydraulic conductivity of surface soils,
depending on whether the micropores or the relatively more stable macropores are destroyed
more by compaction due to tillage. Babalola and Opara-Nadi (1993) noted that since soil air
composition and aeration are connected with soil structure, they are easily modified by CT.
Ploughing the soil helps to enhance porosity of the soil for at least a short time,
especially in compact soils, or to break a pan which restricts infiltration and permeability
(FAO, 1987; Aina, 1993). The soil porosity and, hence, infiltration could be enhanced further
by the expected increase in organic matter due to any ploughed-in weeds (Smith, 1993).
Increased porosity with the CT has severally been attested to in different locations (Hamblin
and Tennant, 1981; Laddha and Towatat, 1997; Omer and Elamin, 1997; Lipiec et al., 2006),
including that of the present study (Anikwe et al., 2003). In the pore system of soils,
information is needed not just on the total porosity but also on the pore size distribution.
Whereas some long-term studies indicated that CT caused significantly larger macroporosity
(Glab and Kulig, 2008; Martinez et al., 2008); other long-term studies indicated that NT
caused significantly larger macroporosity (Buczko et al., 2006), microporosity (Azooz and
Arshad, 1996), and mean weight diameter (Mahboubi and Lal, 1998; Martinez et al., 2008).
It appears that porosity would normally not be affected when no change in bulk
density occurs. Obi and Nnabude (1988) reported that bulk density of a sandy loam in the
Nsukka agroecology was not significantly affected by CT and NT treatments after two years
of cultivation. Similar observation of CT and NT treatments having no significant effect on
bulk density has been reported elsewhere (Xu and Mermoud, 2001; Johnson-Maynard et al.,
2007; Martinez et al., 2008). However, bulk density is more commonly lower with CT. This
has been attested to by findings from several short term studies (Ojeniyi and Adekayode,
1999; Baumhardt and Jones, 2002b; Fabrizzi et al., 2005; Osunbitan et al., 2005; Agbede,
2006; Anikwe and Ubochi, 2007; Glab and Kulig, 2008; Zhang et al., 2008), as well as from
long term studies (Barber et al., 1996; Unger and Jones, 1998; Dam et al., 2005; Czyz and
Dexter, 2009). There is, however, a risk that over time, CT would increase bulk density (Hill,
21
1990; Czyz and Dexter, 2009). For instance, intensive soil cultivation had been reported to
increase bulk density (with the associated reduction of porosity and change in pore size
distribution) at Nsukka – the location of the present study (Obi, 1989).
The infiltration rates on CT and NT cocoyam seedbeds were found not to be
significantly different (Obi and Nnabude, 1988). Some other workers (e.g. Xu and Mermoud,
2001; Johnson-Maynard et al., 2007) reported similarly that infiltration rates were not
significantly changed by tillage systems. Harrowing destroys clods and so decreases
infiltration (Smith, 1993). The greater infiltration under NT has also been associated with a
greater thermal contact area and, consequently, a greater saturated hydraulic conductivity
(Ksat) (Azooz and Arshad, 1995). Consequently, Ksat has been reported higher under NT by
some authors (e.g. Cresswell et al., 1993; Christensen et al., 1994; Azooz and Arshad, 1996;
Moreno et al., 1997; Osunbitan et al., 2005; Zhang et al., 2008). Contrary to this, many
previous studies have shown that Ksat could still be higher under CT (Creswell et al., 1993;
Laddha and Towatat, 1997; Guzha, 2004; Lipiec et al., 2006; Anikwe and Ubochi, 2007;
Martinez et al., 2008). These trends normally result from the extent of compaction following
loosening (under the CT system) in comparison with the effect on porosity of the
uninterrupted biological activities going on within the soil (under the NT system). Besides,
Xu and Mermoud (2001) remarked that significant temporal variations characterize tillageinduced changes in topsoil properties, particularly bulk density and conductivity.
Connolly (1998) contended that though CT breaks the soil surface into a desirable
condition for crop establishment, it leads to accelerated breakdown of organic matter by soil
microorganisms because of changes in soil water relationships, aeration and temperatures. In
corroboration of the above, investigations by some researchers (Unger, 1991; Laryea and
Unger, 1995; Barber et al., 1996; Buschiazzo et al., 1998; Ojeniyi and Adekayode, 1999;
Anikwe, 2000; Ghuman and Sur, 2001; Li et al., 2007; Zhang et al., 2008) revealed lower
SOM content with CT compared with NT. Although the degree to which CT results in
breakdown of organic matter is related generally to the intensity of the tillage operation, the
increase in soil temperature due to CT may not last in some cropped lands before reversal of
the trend. Dalmago et al. (2004) found that 30 days after emergence of maize seedlings,
higher soil temperatures prevailed under the NT system – which had been recording lower
values from the inception of the tillage treatments and sowing. They noted, however, that
differences between the two tillage systems decreased as the crop cover of the soil surface
increased in extent. In such soils, tillage-induced soil temperatures and may be undermined.
Anikwe and Ubochi (2007) reported higher nitrogen content under NT compared to CT.
22
Application of mulch was found to result in greater proportion of water stable
aggregates (WSA) and total porosity, and lower bulk densities of an Ultisol in southeastern
Nigeria (Mbagwu, 1991). Similar beneficial effect of mulch has also been reported from
Alfisols in western Nigeria, where total porosity and Ksat were enhanced (Lal et al., 1980) and
bulk density was reduced (Franzen et al., 1994). Improvements in these physical properties,
in turn, result in greater ability of the soil to withstand disruption by raindrops impact.
According to FAO (1995b), other benefits of mulch application include increase in both
sorptivity and transmissivity of the soil, and this increases as the mulch rate increases (FAO,
1995b). Mulch has also been reported to improve infiltration and reduce soil temperature
(FAO, 1987; Tilander and Bonzi, 1997). The moderating effect of mulch on soil temperature
has been attested to by some workers in different environments (Mbagwu, 1991; Gill et al.,
1996; Dahiya et al., 2007), and this helps to minimize losses due to desiccation. Connolly
(1998) recommended the use of straw or mulches to reduce temperature that leads to rapid
oxidation of organic matter by soil microorganisms. Unlike tillage, the beneficial effect of
mulch is not likely to change for the worse with time. For instance, continuous application of
mulch for 11 years was found to increase total porosity (Mulumba and Lal, 2008).
2.9.2 Effect of tillage-mulch practices on soil moisture conservation
There is the need to think of how to effectively conserve water supplied by
precipitation. The major objective of soil and water management systems is to encourage
water infiltration rather than runoff (FAO, 1995a; Kovac et al., 2005). This may be achieved
by enhancing soil surface storage and improving the soil’s physical and hydro-physical
properties (Brady and Weil, 1999). Tillage systems and mulch practices, which employ
modification in soil physical condition and/or formation of a physical barrier in conserving
soil moisture, are useful management options in this regard. They are typical examples of
simple physical or mechanical soil moisture conservation measures or techniques. When
appropriately selected, these management options could improve the conditions of those
variables which affect the moisture characteristics and AWC of soils, and which are
responsive to management. In other words, through tillage and mulch management measures,
the AWC of the soil could be indirectly enhanced, thus implying an elevation in its moisture
status. Buschiazzo et al. (1998) observed that, among texturally contrasting soils, moisture
contents of the sandier ones were least affected by tillage systems. This underscores the role
of texture on variations in soil moisture due to tillage. Ojeniyi and Dexter (1979) noted that,
23
to conserve soil moisture with tillage, the aim would be to minimize proportions of voids
larger than about 8 mm. If that be the case, it implies that structure of the soil is another very
important factor in how tillage influences soil moisture.
Research results on tillage trials obtained by Hamblin and Tennant (1981) from the
West Australian wheat belt showed that when three intensities of tillage disturbance were
imposed on deep loamy sand for three consecutive years, air permeability, soil water
diffusivity, and the distributions of moisture down the first 70 cm of profile were all found to
increase with intensity of tillage. They found too that the ploughed layer had its maximum
moisture content deeper in the profile early in the rainy season. Willcocks (cited by FAO,
1987) observed that, by increasing the rooting depth, the amount of moisture available to crop
roots was also increased with CT on a dense sandy Luvisols in Botswana. Payne (1999)
reported an increase in soil water storage in the upper 2.4 m of bare plots by up to 47 mm,
when tilled with hilaire, a traditional shallow-cultivating hoe. For cropped plots, the increase
with tillage in soil water storage in the upper 1.4 m was up to 32 mm. Kovac et al. (2005)
found that within the first 80 cm of a loamy Haplic Chernozems in Slovakia, the soil under
CT had significantly higher moisture content than the one under NT treatment.
Some mechanisms would help to explain the moisture retention benefits of CT. In the
first place, presence of large clods in conventionally tilled fields helps to improve the amount
of infiltration since excess water is temporarily held in the spaces between the clods as it
infiltrates into the soil, which is at the same time made to present larger surface area (Aina,
1993; Smith, 1993 Hatfield et al., 2001; Ali and Talukder, 2008). This enhances the soil
moisture status under CT. The choice of an appropriate type of tillage – and the implements
to use – that would leave large clods on the soil surface for optimum infiltration of water
seems to depend on texture and antecedent moisture content of the soil. Sometimes,
fragmentation of the aggregates following CT leads to sorting of soil particles through voids
produced during ploughing (Ojeniyi and Dexter, 1979; Agele et al., 2000). The finer particles
tend to sink to the bottom while the coarser ones remain at the top, implying greater potential
for moisture retention beneath the surface.
Another very effective mechanism through which CT conserves soil moisture is by
decreasing conductivity of the profile, particularly of the surface zone. Hillel (1982) noted
that shallow cultivation loosens the moist soil, which becomes pulverized at the surface –
where the induced high but transient evaporation rate makes desiccation faster and more
complete. The pulverized “dry” soil at the surface eventually breaks the continuity of
24
capillary pores to bring about the decrease in conductivity, thereby acting as surface mulch
for conserving more moisture beneath the surface (Hillel, 1982; Agele et al., 2000). Hillel
(1982) argues, therefore, that the CT produces a hysteresis effect, the accompanying decrease
in conductivity of which helps to arrest or retard subsequent outflow. This, which is one of
the best ways of modifying evaporative flux at the surface, helps in conserving soil moisture
over a long period of time (Hillel, 1982). According to (Payne et al., 1990), this phenomenon
obtains in especially sandy soils, which conduct water very rapidly at high moisture content
and slowly at low moisture content.
In what looks like a contrast, Hatfield et al. (2001) contends that CT leads to
increased soil moisture losses by evaporation; since tillage moves moist soil to the surface
(which predisposes the soil) and breaks apart any soil crust (which provides greater surface
area). Sometimes, these evaporative moisture losses may be of the magnitude to offset the
increased moisture storage by increased infiltration (due to the large clods associated with
CT) into the soil (Hatfield et al., 2001). By and large, tillage-induced complexity of changes
in soil properties affecting moisture need to be understood in order to appreciate how tillage
changes soil moisture (Agriculture and Food, 2004).
Significant differences in soil moisture reserves under different tillage practices have
been reported in favour of NT practice (Allen et al., 1980; Maurya and Lal, 1980). Johnson et
al. (1984) reported that more soil moisture was available in the upper 1 m under NT
compared with other tillage practices in Wisconsin. Mean values of available water of 21.2
and 19.0% respectively were obtained for the NT and CT plots after a study that lasted two
years on a sandy loam Ultisol at Nsukka, Eastern Nigeria (Obi and Nnabude, 1988). These
authors equally reported that at high matric potential, moisture retention was higher in NT
than in CT, especially when crop residue was retained on the soil surface. Azooz and Arshad
(1995) found higher soil moisture contents under NT compared with moldboard plough in
British Columbia. They observed too that the increased soil moisture storage under long-term
NT produced a greater thermal contact area. Generally, moisture conservation benefits of
conservation tillage usually result from improvement in soil structure in such a way that
favours preponderance of micropores.
Mulch application is another viable and effective soil moisture conservation
technique. Whereas the two tillage systems are characteristically inconsistent as regards their
effect generally on soil properties, and specifically on the pattern of moisture availability, and
the associated WU and WUE; the effect of mulch practices is almost always predicted with a
high degree of accuracy. The effectiveness of mulch application in conserving soil moisture
25
has been recognized in different regions by so many researchers (Adetunji, 1990; Mbagwu;
1991; Gill et al., 1996; Moitra et al., 1996; Thiagalingam et al., 1996; Sow et al., 1997;
Rathore et al., 1998; Agele et al., 2000; Ramalan and Nwokeocha, 2000; Sharma and
Acharya, 2000; Liang and Wang, 2001; Gicheru et al., 2004; Li et al., 2005; Chiroma et al.,
2006; Cook et al., 2006; Adekalu et al., 2007; Dahiya et al., 2007; Sarkar and Singh, 2007;
Sarkar et al., 2007; Chakraborty et al., 2008; Mulumba and Lal, 2008).
Lal (1974) reported that mulch application was effective in equilibrating the soil
temperature and increasing soil moisture content. Mbagwu (1991) found that, beneath the soil
surface, mulch contributed positively to moisture retention in the root zone and led to
enhanced water transmissivity. These observations were attributed to improvements in
percent WSA > 0.5 mm, total porosity, and bulk density; and to reduced evaporative moisture
losses. Grass mulch was found to ameliorate the soil hydrothermal and moisture regimes in
southwestern Nigeria (Agele et al., 2000).
Mulch application plays a complementary role in many ways to any chosen tillage
system. The need to minimize unproductive water losses at the soil surface by encouraging
infiltration and reducing runoff and evaporation (FAO, 1995a) could, therefore, be met by
choosing the tillage-mulch management combination most suitable for the purpose. This,
which is needed for the conservation of soil moisture in situ, could ultimately enhance
production. According to Babalola and Opara-Nadi (1993), research results obtained in
humid and sub-humid regions have pointed out that CT brings about reduction in infiltration
rates when compared with the NT plus mulch treatments. They indicated that NT plus straw
mulch seedbeds modified soil temperature better than CT plus straw mulch seedbeds. The
structure and, hence, conductivity of a tropical Alfisol were maintained at a favourable level
by the NT plus mulch (Franzen et al., 1994). The suitability of the NT plus mulch in
promoting the structural stability and hydraulic conductivity of structurally fragile soils has
been attested to elsewhere (Zhang et al., 2008).
Use of mulch in soil moisture conservation is based on some important principles.
One of such is the ‘barrier’ principle, especially in areas of extraordinarily high evaporation
rate due to high insolation and/or wind intensity and speed. Mulch application on the soil
surface, no doubt, has been established to minimize soil moisture losses to evaporation (FAO,
1995b), the extent of which is dependent upon the depth of organic mulch and the fraction of
the soil surface covered (Allen et al., 1998). Another very important principle is that when
porous materials are used as mulch, they encourage infiltration – by virtue of better trapping
26
and the transient detention storage – of rainwater. Such materials also enhance infiltration –
by creating rough surface that constitutes physical impedance to the movement – of runoff
(including impounded runoff). Sometimes, mulch also results in reduction, through sorption,
of downward percolation of infiltrated and absorbed water to the deeper horizons (from
where they might be irretrievably lost to the groundwater), especially when partially
incorporated into, and good contact established with, the soil. Sorptivity has been found to
increase with increasing mulch application (FAO, 1995b).
Additionally, energy supply to the site of evaporation could be controlled by
modifying soil albedo through colour (Hillel, 1982). Soil colour affects reflectance which, in
turn, affects the rate of evaporative moisture losses. However, it is only when the mulch
materials are organic that soil colour could be modified, through addition to the soil of
organic matter, following the decomposition of the mulch materials. Above all, the potential
of mulch to control weed growth (Erenstein, 2002) implies conservation of water that the
weeds ordinarily would transpire. Technologies used to increase infiltration and reduce
runoff, drainage, and evaporation would be useless if weeds use the additional stored water.
2.9.3 Grain or seed yield and water use efficiency
Many authors (including Mbagwu, 1990; Babalola and Opara-Nadi, 1993) have
observed that data on tillage systems, especially reduced tillage systems comparing the
effects of NT and CT on soil properties and crop response, are often contradictory, and the
results are confounded by many other factors. This makes such findings to lack universal
application. It should be noted, however, that absolute yield of crops is not the only factor
considered in assessing the efficacy or superiority of a given tillage system over the other.
Blevins et al. (1983) indicated that when one tillage management system does not maintain or
enhance the soil productivity better, but only has equal or higher yields than the other, it
should not be judged superior to the one that gives the lower yield.
It is one thing for moisture to be conserved in the soil, it is another to optimize and
make more efficient its subsequent utilization by crop plants. Since ET is the major avenue of
depletion of soil water, the management aim should be for the productive transpiration to
predominate, while enhancing soil moisture storage in the profile through impedance of the
‘little’ unproductive drainage below the root zone. FAO (1995a) opined that, in order to
favour productive transpiration and attain increased water productivity (WP), unproductive
water losses in the root zone could be minimized by increasing the ratio of root water uptake
27
to deep percolation. Such approaches to efficiently utilize the limited available soil moisture
should aim at realizing maximum yield from the scarce resource (Agele et al., 2002), more so
when the crops can be grown only once a year (Zaongo et al., 1994).
In crop production system, water productivity (WP) is used to define the relationship
between crop produced (in terms of total dry matter or seed (grain) yield – gravimetrically
quantified) and the amount of water involved in crop production, expressed as crop
production per unit volume of water (Ali and Talukder, 2008; Tolk and Howell, 2008). It
follows that the higher the value of WP, the better the production system, especially in recent
times of increasing competitive demand for water from civic, domestic, industrial, and
recreational uses. The amount of water involved could be the total supplied to the soil
through rainfall or irrigation, or the crop WU (ET). When it is the ET, the WP is better
specified as evapotranspiration water productivity (EWP), which turns out to be the WUE – a
term for expressing the ratio of seed yield (in kg ha-1) to total ET (in mm).
Maurya and Lal (1980) reported that maize and soybean had greater root density in
the surface (0-10 cm) layer in NT plots; while in the subsoil horizons, they had greater root
densities only in the ploughed plots. They stressed that vigour and grain yields in NT soil
may not always be better than with CT techniques. Maurya and Lal (1980) argued that plants
grown under NT are occasionally stunted and show symptoms of nutritional deficiencies
because greater bulk density, low porosity and inadequate nutrient distribution in the soil
profile slow down root growth and development.
In spite of the above, NT resulted in taller mustard and chickpea plants elsewhere
(Rathore et al., 1998). Lawrence et al. (1994) found that NT consistently provided an average
grain yield advantage of 0.5 Mg ha-1 for years compared with CT. Lawrence et al. (1994)
associated the yield advantage of the NT over the CT to greater WU and increased WUE.
Research results obtained in highland regions of Loess Plateau for 10 years showed that the
grain yield of winter wheat was 21-25% higher under NT condition than under CT treatment
(Liang and Wang, 2002). These authors also found that the consumption of soil moisture
under NT was approximately 29-39% lower than under CT treatments.
On the other hand, Obi and Nnabude (1990) observed that CT led to a better yield of
groundnut, though nominally. The CT enhances root development and extension (Aina,
1993), and this is, indeed, one of the reasons behind its yield advantage. The active deep roots
ensure adequate soil moisture abstraction and help to reduce drainage losses (Ali and
Talukder, 2008). Ali and Talukder (2008) further noted that the roots normally do access sub-
28
soil at the mid- or late-season stage, when a minor gain in photosynthetic activity contributes
to an accelerated rate of grain yield. Payne (1999) reported a grain yield increase of CT over
NT by 68 and 70%, respectively in the first and second seasons of a two-year trial. Payne
(1999) observed that CT reduced WU and increased WUE in the second year.
According to Agriculture and Food (2004), tillage influences crop growth and yields
by changing soil structure and moisture removal patterns over the growing season. So many
previous studies tend to suggest that these effects of tillage may be dependent on soil and
environmental conditions. Whereas some workers found the CT to increase yield over the NT
(Klaij and Ntare, 1995; Payne, 1999; Thompson, 2001; Pedersen and Lauer, 2002; 2003;
Wilhelm and Wortmann, 2004; Fabrizzi et al., 2005; Agbede, 2006; Anikwe and Ubochi,
2007; Anikwe et al., 2007; Tulema et al., 2008; Lasisi and Aluko, 2009; Shemdoe et al., 2009);
the reverse was the case for others (Christensen et al., 1994; Lawrence et al., 1994; Norwood,
1994; Ojeniyi and Ogbonaya, 1994; Cassel et al., 1995; Radford et al., 1995; Moreno et al.,
1997; Rathore et al., 1998; Ghuman and Sur, 2001; Liang and Wang, 2002; Stone and Schlegel,
2006; Temperly and Borges, 2006; Jin et al., 2007; Sarkar et al., 2007; Agbede et al., 2008).
Yet, others reported non-significant differences in crop yield between the CT and the
NT (Kramer and Alberts, 1988; Obi, 1989; Gibson et al., 1992; Unger, 1994; Moitra et al.,
1996; Ojeniyi and Adekayode, 1999; Anikwe, 2000; Thompson, 2001; Wilhelm and
Wortmann, 2004; Fabrizzi et al., 2005; Mesfine et al., 2005; Aboudrare et al., 2006; Sarkar
and Singh, 2007; Alvarez and Steinbach, 2009; Koga and Tsuji, 2009; Rodrigues et al.,
2009). Even within a location, tillage trials seem to be characterized by vagaries, yielding
inconsistent data from year to year (Al-Darby and Lowery, 1984; Johnson et al., 1984;
Webber et al., 1987; Tessier et al., 1990; Thiagalingam et al., 1996; Azooz and Arshad,
1998; Norwood, 1999; Baumhardt and Jones, 2002a; Dam et al., 2005; De Vita et al., 2007).
Many of these results are, as remarked by Agriculture and Food (2004), indications that
tillage trials are usually non-repeatable even at the same site.
These findings on tillage system-crop yield dichotomy point to the need to intensify
research on the subject matter. This is to enable adoption of tillage systems suitable for
diverse soil types, crops and ecoregions, with a view to curtailing soil degradation and
decline in crop yield. It is imperative that conservation and utilization of water resource under
the different systems be considered in such studies, so as to know if the vagaries in yield
could be explained from that perspective. In line with this, the experiment of Tessier et al.
(1990) showed that changes in soil moisture content due to tillage are not of the magnitude to
influence crop production. Similarly, the commonly observed differential pattern of water
29
availability under the tillage systems in a given location in favour of NT does not always
confer higher yield advantage to the system (e.g. Anikwe and Ubochi, 2007).
Mulch, on the other hand, usually results in enhanced yields of crops. Application of
mulch has been reported to be effective in increasing the yield of maize (Jones et al., 1969;
Lal, 1974). Higher yields due to mulch have been reported from a sandy loam soil at Nsukka
for maize (Okoro, 1986) and maize and cowpea (Mbagwu, 1991). Rathore et al. (1998)
reported that applied mulch regulated proper plant water status, soil temperature, and lowered
soil mechanical resistance; leading to better root growth and higher grain yield of both
chickpea and mustard in straw mulch than in no mulch plots. Similarly, Agele et al. (2000)
reported that in southwestern Nigeria, grass mulch improved the vegetative and flowering
performance, and significantly increased tomato fruit yield over bare soil. Generally, yield
advantage of mulch has been attributed to decreased soil temperature (Adetunji, 1990), better
utilization of soil moisture due to deeper and denser rooting (Sow et al., 1997) or subsequent
release of conserved soil moisture (Rathore et al., 1998).
The NT with mulch on the surface and the CT but left bare were the best and worst
treatments respectively in a long term study (Lal, 1983). This author observed that the NT
with mulch was a promising alternative to the CT left bare in structurally weak Alfisols, and
that the former gave equal or greater yields as a consequence of better soil fertility and
improved moisture conservation. Similar results have been reported by some other
researchers in the tropics (Kamara, 1986; Mbagwu, 1990; Obi and Nnabude, 1990; Franzen et
al., 1994; Lal, 1995; Alhassan et al., 1998). Babalola and Opara-Nadi indicated that the NT
with mulch seedbeds also gave higher yields, especially of cereals, than CT with mulch
seedbeds. Lal (1995) concluded that satisfactory maize yields with continuous and intensive
cropping are possible with the NT plus mulch, especially with the use of chemical fertilizers
and alternation with a legume. It has also been demonstrated elsewhere that the NT system
leaving crop residues on the surface, can reduce surface crusting, increase infiltration, and
reduce runoff and soil loss while increasing crop yield (Cassel et al., 1995). This, however,
does not preclude further trials for best combination of these soil management practices, for
the ‘superior’ NT plus mulch and the ‘inferior’ CT left bare may not automatically always be
the best and the worst combinations respectively across locations. For instance, in the work of
Jones et al. (1969), the CT with mulch gave the highest yield of corn.
30
2.9.4 Specific responsiveness of sorghum and soybean to tillage-mulch practices
Zaongo et al. (1994) defined effective rooting depth as the topsoil depth in which
80% of the roots are located. In their work to determine the rooting system and water
extraction of sorghum under different water levels, they found that root penetration was
controlled by seasonal wetting front and soil pH. The sorghum crops exhibited a low water
extraction rate over a long period, and they tended to have dense rooting system in the
topsoil. Sorghum normally has its highest WU during the critical phenological stages of
booting, heading, flowering (peak), and grain-filling. Peak soil water extraction of sorghum
was observed to occur between 60 and 75 DAS (Laryea and Unger, 1995; Laddha and
Totawat, 1997), which is normally the period from booting through heading to flowering.
Conversely for soybean, peak WU occurs during the pod-filling stage, usually between 20
and 40 DAS (Rasiah and Kohl, 1991; Kranz et al., 2005; FAO AGL, 2006). The above peaks
are the respective stages of highest water requirement by the crops.
Sorghum could be grown with about 490-500 mm of water in the United States
(Hattendorf et al., 1988; Norwood, 1999). Mean values of 605 and 500 mm have been
reported under semi-arid conditions in Ethiopia (Mesfine et al., 2005) and northern Nigeria
(Chiroma et al., 2006), respectively. Christensen et al. (1994) reported higher sorghum yields
with NT under a temperate climate. Similarly, Agbede et al. (2008) found that sorghum seed
yield (SDY) was higher with NT compared to CT in southwestern Nigeria. In contrast,
Shemdoe et al. (2009) reported higher SDY with CT compared to NT in semi-arid Tanzania.
In the work of Mesfine et al. (2005), differences in sorghum SDY between NT and CT were
not significant. These results suggest that many other factors may mask the effect of tillage
systems on yields of sorghum. Stone and Schlegel (2006) found that up to 63% of the
variation in sorghum SDY was explained through variations in profile moisture storage and
precipitation during the growing season. Application of mulch has been reported to increase
yields of sorghum (Bhaska, 1985; Eagleton et al., 1991; Chiroma et al., 2006). The
commonly reported ranges of WUE values for sorghum under dryland conditions in Nigeria
savanna are 1.95-2.48 kg ha-1 mm-1 (Chiroma et al., 2006), though range of 3.5-4.2 kg ha-1
mm-1 has been reported outside Nigeria (Gibson et al., 1992; Varvel, 1995).
Soybean grown on two different soils had SDY in each soil type significantly
correlated with WU (Rasiah and Kohl, 1991). These authors indicated that peak water
requirement occurred during the podfill stage, and that depletion beyond 0.75 of plant’s TAW
in the top 0.75 m spelled water stress, especially when water supply below this depth was
31
inadequate for root uptake. According to their findings, soybean yield is relatively insensitive
to shortage of water during the vegetative growth, but not in podfill stage when WU must not
be less than the potential demand. Rasiah and Kohl (1991) concluded that SDY, in general,
responded to WU at podfill growth stage. The WU of soybean is characterized by the
tendency to vary, even within a region. For instance, approximate seasonal values obtained
from texturally-contrasting soils in the United States were 620 mm (Tolk et al., 1997), 450
mm (Norwood, 1999), and 580 mm (Tolk and Howell, 2008).
The SDY of soybean was reported higher with CT compared to NT in a sandy loam in
southwestern Nigeria (Lasisi and Aluko, 2009). In contrast, some authors have reported
higher yield of soybean with NT in Wisconsin (Pederson and Lauer, 2003; Temperly and
Borges, 2006). In two separate studies that lasted for five years, NT out-yielded CT in only
two years (Thiagalingam et al., 1996) or one year (Norwood, 1999); differences were not
significant in the remaining years. Many other authors have found non-significant differences
in soybean yield due to tillage systems (Kramer and Alberts, 1988; Wilhelm and Wortmann,
2004; Alvarez and Steinbach, 2009; Koga and Tsuji, 2009; Rodrigues et al., 2009).
Specifically, Kramer and Alberts (1988) concluded, following a six-year study, that tillage
systems had no significant effect on SDY of soybean. Alvarez and Steinbach (2009) upheld
same view in a review of tillage systems for many years in Argentine Pampas. The range of
values of WUE for soybean outside Nigeria is 2.0-2.4 kg ha-1 mm-1 (Varvel, 1995; Norwood,
1999), even though values as high as 4-7 kg ha-1 mm-1 are possible (FAO AGL, 2006).
The ever-increasing pressure on the land resources of the tropics had led to taking
seriously the seemingly non-scientific practice of intercropping, and it has now been
recognized as a potential system to boost productivity over space and time. It is a crop
management system involving two or more economic species grown together for at least a
portion of their respective production cycles, and planted sufficiently close to each other so
that interspecific competition occurs (Hiebsch and McCollum, 1987). Intercropping with
short-cycle food crops is widespread among subsistence farmers in tropical latitudes (Hiebsch
and McCollum, 1987). Sorghum is commonly intercropped with soybean in Nigeria (Olufajo,
1995) and other parts of the world (FAO AGL, 2006). According to Olufajo (1995), the most
ideal intercrop combination of these crops for the sub-humid savanna is the long season (latematuring) sorghum variety and the short season (early-maturing) soybean variety. The
soybean component can contribute to the enhanced sustainability of intensified cropping
systems by enhancing soil fertility through atmospheric nitrogen fixation and by permitting a
32
longer duration of ground cover in the cropping sequence (Sanginga et al., 1999), making its
combination with a cereal superb.
Although intercropping has been practised for a long time at subsistence level, there
has been in recent years a desire to evolve an intercropping system for both subsistence and
commercial purposes (Akunda, 2001). According to Akunda (2001), reasons for the increased
yield in intercropping system include, among others, efficient use of solar radiation and
potential compensatory growth from vagaries of the environment. The advantage may be
increased by choosing suitable spatial arrangement. It often results in decreased soil erosion
and increased conservation of soil moisture, because ground cover is greater than with solecropping (Smith, 1993). Reddy et al. (1980) noted that especially in the semi-arid tropics, the
main purpose of intercropping is to produce additional crop without much effect on the base
crop yield, or at most some marginal sacrifice on the base crop.
Recent research suggests that substantial yield advantage due to intercropping
compared with pure cropping is possible only when the component crops make better use of
resources and complement each other (Waghmare et al., 1982). A test of the effect of
intercropping on yield of sorghum in India (Reddy et al., 1980) revealed that out of the 8
crops used as intercrops, yield decreased from cowpea to groundnut to soybean, though with
little variations, in both the base crop and the intercrop. In another study in India, Waghmare
et al. (1982) investigated the compatibility of sorghum with some crops in various spatial
arrangements. They found that the planting of sorghum in single row spaced 60 cm apart,
with an intercrop in between, gave the highest yield of sorghum and the intercrop in the first
year. This was followed by 30 x 90 cm spacing, with two rows of intercrop between the 90
cm space; and then the 30 x 60 cm spacing with one row of intercrop in between the 60 cm
space. In the second year the best two options in the first year interchanged positions. Out of
5 crops (including groundnut) used as the intercrops, yield decreased from fodder cowpea to
grain cowpea to soybean in the first year; but in the second year, it decreased from grain
cowpea to soybean to fodder cowpea. In both years, yields of these intercrops were best in the
30 x 90 cm spacing, followed by 60 cm spacing, and then 30 x 60 cm spacing. Waghmare et
al. (1982) stated that the concept of paired row planting was developed to group crop rows of
diverse heights so as to make available more solar radiation for the dwarf ones.
Yield of maize was significantly reduced only when it was planted with soybean in
the same row, but the grain yield of soybean was very greatly reduced when it was planted
with maize in alternate pairs of rows. This was the findings of Dalal (1977). He reported that
33
reduction in soybean yield when planted with maize was not even alleviated by the
application of N although soybean grown in pure stand responded significantly to N. In a
study with maize (Zea mays) and cowpea (Vigna unguiculata), Hulugalle and Lal (1986)
concluded that one of the benefits of intercropping those crops is higher WUE in relation to
sole-cropping, provided soil moisture is not limiting. Morris and Garrity (1993) indicated that
water capture by intercrops is higher by about 7% compared with sole crops, and that water
utilization efficiency of intercrops was higher by about 18% compared with sole crop.
According to Willey (1990), intercropping makes use of not only water, but other
environmental resources such as radiation and nutrients more efficiently than sole-crops.
Moreover, spatio-temporal relationship in yield under the system relative to solecropping, using the land equivalency ratio, is another means of assessing the profitability or
otherwise of intercropping. Many of such assessments have revealed yield advantage of
intercropping over sole-cropping of sorghum and soybean (Olufajo, 1995; Akunda, 2001), as
well as other cereals and legumes (Olowe et al., 2006; Agegnehu et al., 2006). Perhaps, the
soil management practices of choice of tillage system and application/non application of
mulch may, much as environment, have influence on the magnitude of the yield advantage
(or disadvantage as the case may be) of intercropping over sole-cropping.
34
CHAPTER THREE
MATERIALS AND METHODS
3.1 Project environment
The experiment was carried out at the University of Nigeria Teaching and Research
Farm, Nsukka (06° 52´ N; 07° 24´ E) in Southeastern Nigeria. This location is on an altitude
of approximately 400 m above mean sea level. Generally, the climate is characterized by
mean annual total rainfall of about 1600 mm and mean annual evapotranspiration (ET) of
about 1560 mm. The ET, however, exceeds total rainfall in most months of the year (Igwe,
2004). Rainfall distribution is characteristically bimodal, with peaks during July and October.
The entire wet season lasts from April to October, whereas the dry season lasts from
November to March. During the wet season, there is a soil moisture recharge of 104 mm and
a moisture surplus of about 260 mm, which depletes to an average deficit of about 650 mm in
the dry season (Mbagwu, 1987). Temperature is uniformly high throughout the year, with
mean minimum and maximum annual values of 21°C and 31°C respectively. Rarely does it
exceed 35°C during the hottest months (Asadu, 1990; Obi and Salako, 1995).
The soil at the experimental site belongs to Nkpologu series and had been classified,
according to the Soil Survey Staff (2003) Keys to Soil Taxonomy and the FAO/UNESCO
(1988) revised legend, as Typic Paleustult and Haplic Nitisol respectively. The area has an
ustic soil moisture regime and an isohyperthermic soil thermal regime, and the soils around
are characterized as being well-drained, with very low total exchangeable acidity, total
exchangeable bases, cation exchange capacity, and base saturation (Asadu, 1990). Runoff
rarely occurs on the experimental site not only because of its gentle slope (only about 1-2%),
but also due to high infiltration rate encouraged by the texture. Steady state infiltration rate
ranges from 240 to 1320 cm hr-1 (Obi and Nnabude, 1988). The soil is deep, coarse-textured,
and low in organic matter content, with perennial leaching problem (Igwe, 2004). The 15-bar
soil moisture exceeds 0.10 m3 m-3 in the Ap and Bt horizons of this soil (Obalum, 2004).
Grassland vegetation is predominant in the study location which, according to
Mbagwu (1991), is within the forest-savanna transition vegetation zone. Dominant plants at
the site included such grass species as Andropogon gayanus, Celosia trigyna, Cynodon
nlemfuensis, Emilia sonchifolia, Panicum maximum, Pennisetum polystachion, Oldenlandia
corymbosa, and Spermacoce verticillata; leguminous weeds were represented by
Calapagonium mucunoides and Mucuna urens, broad leaf weeds by Asystasia gangetica. As
at 2006, the field had been under mixed-species fallow for about 10 years.
35
3.2 Field preparation, experimental layout and design and cultural practices
Land clearing was manually achieved in the site with minimal soil disturbance in both
years (2006 and 2007) of the study. Prior to the pre-planting tillage operations during the
rainy growing seasons of the two years, thoroughly mixed organic manure (poultry
droppings) was uniformly applied in the entire field at 5 Mg ha-1; no inorganic fertilizers
were used. This was based on the recommended application rates of 5 Mg ha-1 (minimum) for
sorghum (ICS-Nigeria, 2003a) and 6 Mg ha-1 (maximum) for soybean (Kratochvil et al.,
2006), where available, as substitutes for inorganic fertilizers. Thereafter the field was
partitioned into three equal plots. Two factors – tillage systems and mulch practices – were
under investigation in each plot. Treatments consisted of factorial combinations of two of the
tillage systems (no-till [NT] and conventional tillage [CT]) and two of the mulch practices
(bare surface and mulch-applied surface). Manually prepared seedbeds, tilled to depths of
about 20 cm, represented CT; clean-weeded flat beds represented NT. The only soil
disturbance in the NT treatments occurred during seeding and occasional weeding. The
factors were laid out in 2 x 2 split-plot design (wherein the two tillage systems were the main
plots and the two mulch practices were the sub-plots) in a randomized complete block design
(RCBD), replicated four times. This arrangement yielded four treatment combinations in each
replication: NT, bare (NTB); NT, mulch (NTM); CT, bare (CTB); and CT, mulch (CTM).
The three plots were separately cropped to sorghum (Sorghum bicolor [L.] Moench),
soybean (Glycine max [L.] Merrill), and sorghum-soybean in an intercrop; and were
designated experimental plots 1, 2, and 3 respectively. There were clear demarcations inbetween treatments in a block. Earth bunds were built round the entire field and around each
experimental plot; they were also used to demarcate the blocks/replications. Each
experimental plot measured 18 m × 8.4 m, with 0.9 m margin at the boundaries. The four
treatments each occupied an area of 4.2 m × 2.1 m in each of the four replications. The field
layout of the treatments is shown in Fig. 1.
Late-maturing sorghum (cultivar SAMSORG-16 [FFBL]) and early-maturing soybean
(cultivar SAMSOY-2) were manually sown, after being treated with Apron Star, on all the
three experimental plots on 3 July, 2006 (first year) and 7 June, 2007 (second year). The
sowing date in the first year was within the range (early to mid July) recommended for those
sorghum and soybean cultivars in the study area (ICS-Nigeria, 2003a, b), but in the second
year, the sowing date was only close to the interval (10 to 12 June) recommended for
sorghum in the forest-savanna transition zone of Nigeria (Bello, 1999).
36
NTB
NTM
CTB
EXPERIMENTAL PLOT 3
(INTERCROP)
EXPERIMENTAL PLOT 2
(SOLE SOYBEAN)
EXPERIMENTAL PLOT 1
(SOLE SORGHUM)
CTM
NTB
NTM
CTB
CTM
NTB
NTM
CTB
CTM
CTB
NTM
NTB
CTM
CTB
NTM
NTB
CTM
CTB
NTB
NTM
CTB
CTM
0.4m
NTM
NTB
CTM
0.4m
18m
0.9m
NTB
NTM
CTB
CTM
0.9m
NTB
NTM
CTB
CTM
0.4m
NTM
NTB
CTM
8.4m
CTB
NTM
NTB
CTM
CTB
NTM
NTB
CTM
27m
Fig. 1: Field layout of the treatments in the three experimental plots
NTB – No-till & Bare; NTM – No-till + Mulch; CTB – Conventional tillage & Bare; CTM – Conventional tillage + Mulch
CTB
4.2m
37
Seeding was at 3 per hill in shallow openings (for sorghum) and at 2-4 cm depth (for
soybean). In sole-cropping, crop stands were spaced 60 cm between and 30 cm within rows
for both sorghum and soybean in their respective plots. Spacing of the crops in the intercrop
was based on the recommended spatial arrangement of these crops in an intercrop on a sandy
loam soil (Waghmare et al., 1982); sorghum was spaced 90 cm between and 30 cm within
rows, with two 30 cm × 30 cm rows of soybean in-between the 90 cm gap in all two sorghum
rows. To achieve close to the recommended sole crop plant density in sorghum of 53 333
plants per hectare (Olufajo, 1995), the seedlings were thinned down to one per stand 14 days
after sowing (DAS), giving plant population of 55 555 plants per hectare. The same
population was achieved in the sole soybean plot. In the intercrop plot, there were 39 700
sorghum and 63 500 soybean plants per hectare.
Application of mulch followed immediately after thinning. The mulch material was
composed mainly of dry leaves of Paspalum notatum, and was applied at the rate of 5 Mg ha-1.
All plots were kept free from weeds using a hand hoe or by hand picking throughout the
growing seasons; no herbicides were used. Stem borer attacked the sorghum plants in the first
year, and the pest was controlled by applying Furadan to the whorls.
3.3 Sampling and measurements
3.3.1 Monitoring of profile soil moisture contents
The initial moisture content of the soil down to 50 cm depth was determined
immediately after sowing, on the assumption that the profile had been wetted homogenously.
Subsequent sampling for the monitoring of changes in the profile soil moisture (PSM)
contents was started two weeks after mulch application. Experimental plots 1 (sole sorghum)
and 3 (the intercrop) were sampled 11 times in each year, while plot 2 (sole soybean) was
sampled 9 times in the first year and 8 times in the second year, before harvest.
During the sampling period, two of the four replicates of each treatment were selected
for monitoring the PSM storage. Designated portions, centrally located within the plots, away
from the border rows, were permanently marked for the repeated moisture content
measurements. Sampling was limited to 50 cm depth zone, the zone of greatest root density
of sorghum (Zaongo et al., 1994; Moroke et al., 2005) and soybean (Willatt and Olsson,
1982; Kranz et al., 2005; FAO AGL, 2006). On each sampling occasion, moisture content
was determined using the approach by Hulugalle and Lal (1986), Moitra et al. (1996) and
Zougmore et al. (2004) in their studies on soils of similar texture (sandy loam) as the present
38
soil. Samples were taken with a tube auger from depth ranges of 0-10, 10-20, 20-30, and 3050 cm; and the moisture contents determined gravimetrically. Thereafter, gravimetric
moisture contents were converted to volumetric basis (using pre-determined bulk densities of
soil cores taken from corresponding soil layer) and expressed on depth basis (using the
thickness of each depth range). On a given sampling date, the depth of moisture (mm) found
within the entire monitored depth zone (0-50 cm) was taken to be the PSM storage. This was
obtained by integration of the values from the four soil layers thus:
n
S=
∑ (θ t )(10
i i
−1
);
i =1
where S = the moisture available within the sampled profile (in mm),
θ = the volumetric moisture content (cm3 cm-3),
t = the thickness of each sampled depth zone (in cm),
n = the number of sampled depth zones, and
10-1, the conversion factor from cm to mm.
Since plant water need over periods of about 10 days would usually be met by soil
moisture storage (Stern et al., 1982), the interval for the monitoring of changes in PSM
storage was 10±1 day. The principle of avoiding monitoring immediately after rains was
adopted in choosing the dates, in order to maximize the chances of doing so on days when
differences did appear. Notably, the aim of adopting this principle in soil moisture monitoring
was not to achieve an exact picture of the pattern of WU by the test crops throughout the
growing period, but to identify differences among treatments (Tilander and Bonzi, 1997).
3.3.2 Measurement of soil water suction
Soil water suction was monitored at 30 and 60 cm depths in the field with vacuum
gauge tensiometers (Cassell and Klute, 1986) during the second growing season. A single
tensiometer was installed at each depth in all the treatments. Observations on soil moisture
were made only in the last block of the three experimental plots because of the limited
number of the tensiometers. Gauge readings were taken at the same interval, and on the same
date too, as soil moisture contents determination in the second year trial.
39
3.3.3 Agronomic data collection
Counts of fresh earthworm casts found in the different treatment plots were taken
early in the morning at three-day intervals. The crops’ performance data collected before
maturity included such components of yield as height and leaf count, and additionally for
sorghum crop alone, girth and leaf area. For each of these parameters, 12 crop plants in the
sole-cropped plots; 8 sorghum and 16 soybean stands on the intercropped plot and from
centrally located portions within each treatment plot, were randomly selected for the
assessment. Leaf area of the sorghum plants was determined using the method of Pal and
Murari (1985). At maturity, sorghum and soybean stands were harvested at the ground
surface level, from a designated portion on each treatment plot, consisting of central four
rows of four plants per row. The stovers were separated from the seeds and sun-dried to a
constant weight for about four weeks. Seed yield (SDY) and stover yield (STY) were
assessed on dry matter basis. Total dry matter (TDM) was obtained by adding SDY to the
measured STY. Harvest index (HI) of the component crops was calculated as the ratio of
SDY to TDM. The crops – soybean and sorghum – were respectively harvested on 27
October and 10 December, 2006 (the first year) and on 24 October and 7 December, 2007
(the second year). Table 1 presents some of the relevant agronomic information.
At the end of the experiment, some physicochemical properties were determined for
the topsoil. Auger samples were collected from two of the treatment replications each in a
composite manner; three per treatment plot. They were air-dried and passed through a 2-mm
sieve. Together with the undisturbed soil samples collected with cores, these samples were
analysed for some basic physiochemical properties.
3.4 Laboratory methods
The particle size distribution was determined by the hydrometer method as described
by Gee and Bauder (1986). Soil pH was measured in soil-water/KCl (1:2.5) suspensions, as
described by McLean (1982). The soil organic matter (SOM) was derived from organic
carbon determined using the modified Walkley-Black wet digestion and combustion method
as described by Nelson and Sommers (1996); total nitrogen (N) by the Kjedahl digestion and
distillation method as described by Bremner and Mulvaney (1982); available phosphorus (P)
by Bray 1 Method (Olson and Sommers, 1982); cation exchange capacity (CEC) by
ammonium acetate displacement method as described by Rhoades (1982); and exchangeable
cations as described by Thomas (1982). The bulk density and total porosity were obtained by
40
(a)
(b)
Plate 1: The field experimental plots at about 81 DAS in the second year, showing the
boundary and parts of the: (a) sorghum and soybean plots (b) soybean and
intercropped plots; and the earth bunds for clear demarcations and runoff reduction
41
(a)
(b)
Plate 2: The field experimental plots at about 81 DAS in the second year, showing (a)
the cross-view of the three experimental plots during data collection (b) the relative
heights of sorghum and soybean in the intercrop
42
Table 1: Some agronomic information from the three experimental plots
Seeding rate (1000 plants ha-1)
Sole crop
Intercrop
Year
Crop
Planted
Harvested
Growth
period
2006
Sorghum
July 3
Dec. 10
160 days
56
40
Soybean
July 3
Oct. 27
116 days
56
64
Sorghum
June 7
Dec. 07
182 days
56
40
Soybean
June 7
Oct. 24
138 days
56
64
2007
43
the core method (Blake and Hartge, 1986). Pore size distribution (PSD) was estimated using
water retention data (Flint and Flint, 2002). Moisture retained in the micropores, taken to be
the moisture content of the soil at 60-cm (water) tension, represented field capacity (FC).
Aggregate stability was measured by the mean weight diameter (MWD) of water-stable
aggregates (Kemper and Rosenau, 1986) calculated as:
n
MWD =
∑X W …;
i
i
i=1
where MWD = mean weight diameter
Xi = the mean diameter of a given size fraction (mm),
Wi = the proportion by weight of total aggregates in a given size fraction (g g-1), and
n = the number of sieves used.
The saturated hydraulic conductivity (Ksat) was determined using the method of Klute and
Dirksen (1986), and was calculated as:
Ksat = (Q/At) (L/∆H);
where Ksat = saturated hydraulic conductivity (cm h-1),
Q = steady state volume of outflow from the entire soil column (cm3),
A = the cross-sectional area (cm2),
t = the time interval (h),
L = length of the sample (cm), and
∆H = change in the hydraulic head (cm).
3.5 Determination of the crops’ water use
3.5.1 Profile soil moisture depletion
The amount of moisture depleted from the soil profile during the growing season was
computed using the soil moisture depletion procedure as follows:
N −1
n
U = ∑ { ∑ ([ m 1 − m 2 ] i [ ρ ] i ) t i } I ;
I =1
i =1
where U = the amount of soil moisture depleted during the growing season (mm),
m1 = the gravimetrically determined moisture content on a given sampling date (g g-1),
m2 = the gravimetric moisture content on the following sampling date (g g-1),
44
ρ = the bulk density of a given sampled depth zone (g cm-3),
t = the thickness of a given sampled soil depth zone (mm),
n = the number of sampled depth zones, and
N = the number of times soil moisture was monitored from sowing to harvest.
However, monitoring a progressively drying soil was not possible because of new rain events
between the intervals. On a given sampling date, soil moisture recharge by the rainwater
input beginning from a preceding sampling date was taken care of by using an adjusted value
of m1, corresponding to the altered depth of moisture in the soil due to the rains. This altered
depth of moisture was obtained from the notional water budget equation (Stern et al., 1982):
Wn = Wn-1 + Rn – ETn;
where Wn = moisture stored (mm) in the soil profile on a given sampling date, n,
Wn-1 = moisture stored (mm) in the soil profile on a preceding sampling date, n-1,
Rn = rainwater input (mm) from n-1 to n, and
ETn = evapotranspiration during the interval (mm).
To obtain ETn, the daily ET was estimated as the product of reference crop ET (ETo) and the
crop coefficient (Kc). It involved first converting the mean evaporation from Class A pan in
the study location (5.6 mm day-1) (Mbagwu and Osuigwe, 1985; FAO, AGL, 2006) to ETo,
using the average annual conversion factor of 0.7 (Bruce and Clarke, 1966). The United
States Department of Agriculture (USDA) Kc values for sorghum (0.87) and soybean (0.68)
(Erie et al., 1981) were used. The estimated daily ET for the two crops was comparable to 3
mm day-1 reported for some cereals and legumes in the humid tropics (Verplancke, 1985).
3.5.2 Water balance
A simple water balance equation was used to compute ET or crop WU as follows:
ET = P + I + C – R – D – (S2 – S1);
where ET = the evapotranspirational WU,
P = precipitation (mainly rainfall),
I = applied irrigation water,
C = capillary rise (from the water table by upward water flux) to the PSM,
R = the runoff (positive value) or runon (negative value),
D = the drainage below the maximum rooting depth,
45
S1 and S2 were the initial and final PSM respectively – of which their difference above could
be represented as a change in the PSM storage (∆S).
The daily values of P for the entire period were obtained from the University Meteorological
Station, located about 50 m away from the experimental site. Irrigation was zero in this study,
and so, I dropped out of the equation. When the water table is more than about 1 m below the
bottom of the root zone, C would normally be ignored (Allen et al., 1998); and this condition
was prevailing in the study area. Runoff and runon were reduced by bunds built around each
individual plot. Besides, the lowest steady state infiltration rate that could be recorded on
these soils would normally be greater than the highest likely intensity of average tropical
rainstorms (Mbagwu, 1991). The drainage process was based on the assumption that deep
percolation only took place when FC was exceeded (Oluwasemire et al., 2002). The value of
D was estimated using the equation:
D = P + S − FC; (as developed by Oluwasemire et al., 2002)
where D = the drainage,
P = precipitation (mm),
S = actual moisture in the soil before the rain (mm), and
FC = the field capacity (mm).
Notably, the drainage term might be underestimated by the above equation only if the FC
simulated to be the moisture content at 60-cm-water tension (the theoretical FC) was above
the actual FC of the soil. The ∆S (S2 − S1) was obtained from the PSM on successive
sampling dates. On each new date of monitoring, the new moisture content was taken as S2
while the S2 on the preceding date became S1. Thus, the ET was estimated from the equation:
ET = P – D – ∆S
In any of the monitoring dates when P – D < ∆S, the ET was set to zero. The seasonal ET was
the sum of all the values computed from the first to the last sampling date before harvest.
3.5.3 Estimation of crop water consumptive use
The following relationship developed by Blaney and Criddle (1950) for the USDA
was used to estimate the crops’ water consumptive use:
U = 0.46 Kp (t + 18);
where U = crop WU or ET (mm),
K = crop coefficient (known as K factor),
46
p = mean monthly percentage of daytime hours of Nsukka (06° 52´ N),
t = the mean daily temperature (°C); t = ½ (tmax + tmin), where tmax is the quotient of total t
maximum values in a month and the number of days in a month, whereas tmin is the quotient
of total t minimum values in a month and the number of days in a month.
Values of K for sorghum (0.87) and soybean (0.68) used in the estimation were as given by
the USDA (Erie et al., 1981). The consumptive use factors, p and t, were respectively
obtained from Erie et al. (1981) and from Table 2, showing the prevailing tmax and tmin in the
study location for the two years.
The estimated total WU values of both sorghum and soybean were compared with the
measured ones. For each of the two years, mean value of the measured seasonal WU under
the four treatments, rather than that from any single treatment, was used in the comparison.
3.6 Yield-water use relationship
Water use efficiency (WUE), being a factor which relates crop yield to WU, was first
computed by dividing the total SDY by the seasonal WU. Responses to intercropping were
expressed by comparison of WUEs; the one under the system computed following Morris and
Garrity’s (1993) guideline and those of the sole-cropped sorghum and soybean. Land
equivalency ratio (LER) (IRRI, 1974) and area × time equivalency ratio (ATER) (Hiebsch
and McCollum, 1987) were also used to evaluate intercropping. The steps for obtaining the
values of LER and ATER are respectively presented as follows:
LER = LA + LB + …… + LN = IA/SA + IB/SB + …… + IN/SN = ΣIN/SN;
where LER is the land equivalency ratio
LA, LB … LN are the LER for the individual crops,
IA, IB… IN are the individual crop yields in intercropping, and
SA, SB… SN are their yields as sole crops; and
ATER = [(IA/ YA) (TA) + (IB/ YB) (TB) + … + (IN/ YN) (TN)]/D;
where ATER is the area × time equivalency ratio
I/Y is the relative yield; the ratio of yields in intercrop to sole crop
T is the production cycle length in sole crop for intercrop components
D is the duration of the intercrop system
A, B … N are the intercrop components
47
Table 2: Mean temperature and relative humidity at the study site in 2006 and 2007
Min. temperature
(°C)
Max. temperature
(°C)
Relative humidity
(%)
2006
2007
2006
2007
2006
2007
January
23.1
20.9
33.1
33.3
75.3
54.6
February
23.3
22.6
33.6
35.1
76.0
71.9
March
22.8
23.2
33.1
35.1
73.7
69.5
April
23.3
23.0
35.5
32.7
74.8
74.5
May
21.4
21.9
30.6
31.1
77.9
76.3
June
21.2
21.8
29.9
29.4
77.7
77.5
July
21.6
21.2
28.7
28.5
79.7
78.7
August
20.8
21.9
27.8
27.7
80.0
79.1
September
21.3
21.4
28.2
28.3
79.1
78.1
October
21.3
20.7
29.9
29.5
76.9
76.6
November
19.0
21.3
31.8
30.4
65.5
76.3
December
17.9
20.0
32.7
31.6
57.2
69.4
Mean
21.4
21.7
31.2
31.1
74.5
73.5
48
3.7 Data analyses
Differences among the treatments were tested for significance using analysis of
variance (ANOVA) for a split plot in RCBD. Where significant, separation of treatment
means (for statistical differences) was achieved by the procedure of Fisher’s least significant
difference (F-LSD) as described by Obi (2002). Probability level (P) of 0.05 was used as the
critical limit for distinguishing the degree of significance between means.
The three experimental plots were considered independent of one another. In each
plot, moisture contents of the soil were analysed layer after layer, regarding each sampled
layer as an experimental unit.
49
CHAPTER FOUR
RESULTS AND DISCUSSION
4.1 Rainfall during the field study
The different rainfall classes and the associated number of rain events during the
study period are shown in Table 3. Low-amount classes of rainfall dominated in the first year.
In spite of the fact that less number of rainy months was involved in the first than in the
second year, drizzles (individual rain events < 10 mm in depth) constituted 14.7% and 12.2%
in the first (237.5 mm) and second year (191.2 mm) respectively. On the other hand, fewer
rainstorms were recorded in the first than in the second year. A rain event is regarded as a
heavy rain or a rainstorm when the rainfall amount is ≥ 40 mm (Allen et al., 1998). Such an
event occurred only twice in 2006 (one each in July and October), totalling 92.0 mm; and six
times in 2007 (two each in the months of June, August, and October), totalling 267 mm.
Rainfall also reached its peak in the stated months in both years (Fig. 2). In this
agroecological zone, rainfall normally reaches its peak in the longer and shorter wet season
during the months of July and October respectively; the least rainfall is normally recorded in
the month of August, during which a natural dry spell is experienced. This period of short
break is referred to as August Break. So, the occurrence in 2007 of rainstorms and rainfall
peak in August was somewhat anomalous for this zone. It implied that rainfall was
comparatively erratic in 2007.
Additionally, rainfall seemed to be generally better distributed in the first than in the
second year. The pattern of pentade distribution of rainfall is shown in Table 4. By the
definition of Griffiths (1959), more number of dry pentades (11) was recorded in the second
year in comparison with the first year (5).
4.2 Soil physicochemical properties
Table 5 shows the properties of the top (0-10 cm) soil of the study site at the start of
the experiment. From the particle size distribution, the soil is of sandy loam texture. The bulk
density, total porosity, Ksat, MWD, pH, and available P indicated moderate values, whereas
percent SOM, percent total N, and CEC were generally low. The PSD indicated that
micropores predominated over macropores; the soil was poorly aerated.
50
Table 3: Rainfall classes and the associated number of rain events in the study site
covering the crops’ growth cycle in the two growing seasons of 2006 and 2007
Depth in millimeters
Year
Month
Total
< 10
≥ 10
≥ 20
≥ 30
≥ 40
2006
July
51.1 (15)
25.7 (2)
24.4 (1)
67.1 (2)
45.7 (1)
213.9 (21)
Aug.
63.2 (13)
67.1 (4)
30.5 (1)
34.8 (1)
-
195.6 (19)
Sept.
102.4 (20)
41.4 (3)
46.7 (2)
-
-
190.5 (28)
Oct.
19.3 (6)
92.7 (6)
91.4 (4)
64.3 (2)
46.2 (1)
313.9 (19)
Nov.
1.5 (1)
-
-
-
-
1.5 (1)
Dec.
-
-
-
-
-
0.00 (0)
Total
237.5 (55)
226.8 (15)
193.0 (8)
166.1 (5)
92.0 (2)
915.4 (88)
**1613.4 (129)
2007
June
27.4 (7)
40.6 (2)
30.2 (1)
135.9 (4)
93.5 (2)
327.7 (16)
July
19.7 (11)
42.9 (3)
-
-
-
62.7 (14)
Aug.
26.4 (6)
46.0 (3)
100.8 (4)
150.4 (4)
86.4 (2)
323.6 (17)
Sept.
61.2 (12)
66.0 (5)
42.4 (2)
-
-
169.7 (19)
Oct.
45.7 (9)
95.3 (6)
-
39.1 (1)
87.1 (2)
267.2 (18)
Nov.
10.7 (2)
17.5 (1)
26.9 (1)
-
-
55.1 (4)
Dec.
-
-
-
-
Total
191.2 (47)
308.4 (20)
200.4 (8)
325.9 (9)
0.00 (0)
267.0 (6)
1206 (88)
**1570.2 (112)
The values in parenthesis stand for the number of rain events that yielded the corresponding
rainfall amount
**Annual rainfall
350
350
300
300
250
250
200
200
150
150
100
100
50
50
0
0
Jan
Feb
Mar
Apr
May June July
Aug Sept
Oct
Nov
Dec
Month of the year
2006
2007
PET
Fig. 2: Mean monthly distribution of rainfall during the 2006 and 2007
P otential evapotranspiration (P E T) (m m )
Rainfall am oun t (m m )
51
52
Table 4: Pentade distribution of rainfall in the months of the two-year study duration
Year
2006
2007
1-5
6-10
Rainfall distribution (mm)
Pentade
11-15
16-20
21-25
July
8.9
8.6*
25.9*
9.4
64.5
82.6
8.9
Aug.
60.5
14.0
15.2*
11.4*
19.3
58.9
3.1
Sept.
44.4
32.5
41.9
24.4
10.2
40.9
-
Oct.
58.2
37.6
85.6
69.1
21.3
36.3
0.00
Nov.
20.3*
1.5
0.0
0.0
0.0
0.0
-
Dec.
0.0
0.0
-
-
-
-
-
82.3
5.8
113.8
12.7
39.6*
-
Month
June
26-30
31
July
5.1*
0.0*
16.5*
19.6*
1.8*
20.1*
0.0
Aug.
22.4*
24.9
89.2
32.5
39.6
115.1
0.0
Sept.
42.2
29.7
6.1*
29.7*
23.9
38.1
-
Oct.
36.1
61.7
24.9
15.5
69.6
49.8
9.7
Nov.
17.5
10.7*
26.9
0.0
0.0
0.0
0.0
Dec.
0.0
-
-
-
-
-
-
*Dry pentades
53
Table 5: Some physicochemical properties of the top (0-10 cm) soil at the start of the study
Physical and chemical properties and their corresponding values
Physical properties
Chemical properties
Bulk density (Mg m-3)
1.46
pH (H2O)
6.6
Total porosity
0.57
SOM (%)
0.86
Macroporosity
0.12
Total N (%)
0.11
Microporosity
0.45
Available P (ppm)
28
Ksat (cm h-1)
8.3
CEC (cmol kg-1)
7.0
MWD (mm)
2.3
Exchangeable bases
*AWC (mm 50-cm-1)
90
Ca (cmol kg-1)
1.7
Sand (%)
75.2
Mg (cmol kg-1)
1.2
Silt (%)
16.0
Na (cmol kg-1)
0.4
Clay (%)
8.8
K (cmol kg-1)
0.1
Taxonomy
Ultisol
Textural class
Sandy
loam
Ksat – saturated hydraulic conductivity; MWD – mean weight diameter;
AWC – available water capacity *(moisture held between FC and 15-bar tension);
SOM – soil organic matter; CEC – cation exchange capacity
ppm – parts per million
54
4.3 Management-induced changes in the soil
4.3.1 Activity of earthworms
Table 6 shows the mean density of earthworm casts as found on the different
treatment plots during the second growing season. The density was significantly (P ≤ 0.001)
higher in the NT treatments than in their CT counterparts in the intercropped plot. Many
authors (Radford et al., 1995; Buschiazzo et al., 1998; Chinaka, 1998; ATTRA, 1999; Birkas
et al., 2004; Brevault et al., 2007; Johnson-Maynard et al., 2007; Li et al., 2007) have shown
that earthworm thrives under NT. On the other hand, CT is associated with reduction in
earthworm population. This reduction results from some of the deleterious effects of tillage
implements; drying the soil, burying the plant residue they feed on, destroying their vertical
burrows, and cutting up and killing the worms themselves (ATTRA, 1999). The effect of
tillage x mulch was not significant on any of the three experimental plots.
4.3.2 Physical properties
Table 7 shows the physical properties of the soil at the end of the study. In the
sorghum plot, total porosity was significantly (P ≤ 0.05) higher with the CT than with the
NT. Some authors (Hamblin and Tennant, 1981; Laddha and Totawat, 1997; Moreno et al.,
1997; Omer and Elamin, 1997; Lipiec et al., 2006) have reported similar observation on soils
of various textures. In the tillage x mulch interaction, total porosity was highest in the CTM
and was different from the rest; NTB, CTB, and NTM. The Ksat under the CT treatments was
also significantly (P ≤ 0.05) higher than that under the NT treatments. However, the tillage x
mulch interaction was not significant.
In the soybean plot, neither the tillage nor the mulch factor significantly influenced
bulk density, total porosity, macroporosity, microporosity, and MWD. The non-alteration to
porosity and PSD is consistent with other findings on short-term basis under other legumes in
the study location (Anikwe et al., 2003; Ahamefule and Mbagwu, 2007) and specifically
under soybean elsewhere (So et al., 2009). Interaction effect of these treatments was not
significant on any of the above properties either. The Ksat under the bare treatments was
significantly higher (P ≤ 0.01) than that under the mulch-applied treatments. Effect of the
tillage x mulch on Ksat was significant (P ≤ 0.05); the values were such that the CTB > NTB =
CTM > NTM, each being higher than the one after it by over 100%.
In the intercropped plot, most of the soil’s physical properties were, as observed in the
sole soybean plot, uninfluenced by the management practices. The trend in the sole sorghum
plot, whereby CT significantly (P ≤ 0.05) improved Ksat over the NT system, was maintained.
55
Table 6: Mean density (no. ha-1) of earthworm casts resulting from the tillagemulch treatments in the experimental plots in the second cropping season
Sorghum plot
Mulch practice
Treatments
Tillage
system
B
M
Mean
NT
19048
14966
17007
CT
10431
13832
12131
Mean
14739
14399
†LSD ns, ns, ns
Soybean plot
Mulch practice
Treatments
Tillage
system
B
M
Mean
NT
37188
48753
42970
CT
28118
31179
29648
Mean
32653
39966
†LSD ns, ns, ns
Intercropped plot
Mulch practice
Treatments
Tillage
system
B
M
Mean
NT
43084
32313
37698
CT
13039
16440
14739
Mean
28061
24376
†LSD 6100***, ns, ns
NT = No-till, CT = Conventional tillage;
B = Bare, M = Mulch
ns stands for not significant at P ≤ 0.05;
*** denotes significance at P ≤ 0.001 level of probability.
†Given for tillage system, mulch practice, and tillage x mulch in that order
56
Table 7: Some physical properties of the top (0-10 cm) soil under the tillage-mulch treatments at the end of the two-year study
Bulk density (Mg m-3)
Total porosity
Macroporosity
Ksat (cm h-1)
Microporosity
MWD (mm)
Sole sorghum plot
Treatments
Tillage
system
Mulch practice
B
M
Mean
1.43
1.42
1.42
1.42
CT
Mean 1.42
1.44
1.43
NT
1.43
†LSD ns, ns, ns
B
M
Mean
0.46
0.43
0.44
0.44
CT
Mean 0.45
0.52
0.48
NT
0.47
†LSD 0.03*, ns, 0.06*
B
M
Mean
0.07
0.06
0.07
0.07
CT
Mean 0.07
0.08
0.08
NT
0.07
†LSD ns, ns, ns
B
M
Mean
0.39
0.37
0.38
0.37
CT
Mean 0.38
0.44
0.40
NT
0.41
†LSD ns, ns, ns
NT
B
M
Mean
12.0
8.1
10.1
22.1
18.6
15.0
CT
Mean 13.5
15.1
†LSD 3.4*, ns, ns
B
M
Mean
2.5
2.2
2.3
2.0
CT
Mean 2.3
2.4
2.2
NT
2.3
†LSD ns, ns, ns
Sole soybean plot
B
M
Mean
1.51
1.45
1.48
1.34
CT
Mean 1.42
1.48
1.41
NT
1.46
†LSD ns, ns, ns
B
M
Mean
0.50
0.52
0.51
0.58
CT
Mean 0.54
0.49
0.53
NT
0.50
†LSD ns, ns, ns
B
M
Mean
0.08
0.08
0.08
0.09
CT
Mean 0.09
0.07
0.08
NT
0.08
†LSD ns, ns, ns
B
M
Mean
0.42
0.44
0.43
0.49
CT
Mean 0.47
0.42
0.45
NT
0.43
†LSD ns, ns, ns
NT
B
M
Mean
25.0
8.9
16.9
26.2
41.6
57.0
CT
Mean 41.0
17.6
†LSD ns, 6.9**, 11.1*
B
M
Mean
1.5
1.5
1.5
1.6
CT
Mean 1.5
1.1
1.3
NT
1.3
†LSD ns, ns, ns
Intercropped plot
B
M
Mean
1.44
1.51
1.47
1.36
CT
Mean 1.40
1.38
1.37
NT
1.44
†LSD ns, ns, ns
B
M
Mean
0.47
0.48
0.47
0.48
CT
Mean 0.48
0.48
0.48
NT
0.48
†LSD ns, ns, ns
B
M
Mean
0.07
0.08
0.07
0.07
CT
Mean 0.07
0.07
0.07
NT
0.07
†LSD ns, ns, ns
B
M
Mean
0.40
0.40
0.40
0.41
CT
Mean 0.41
0.41
0.41
NT
0.41
†LSD ns, ns, ns
NT
B
M
Mean
12.7
8.7
10.7
27.5
26.1
24.7
CT
Mean 18.7
18.1
†LSD 5.7*, ns, 2.5*
NT = No-till, CT = Conventional tillage, B = Bare, M = Mulch
Notations of the physical properties are as explained in Table 4
†Given for tillage system, mulch practice, and tillage x mulch in that order
ns stands for not significant at P ≤ 0.05; * and ** denote significance at P ≤ 0.05 and 0.01 levels of probability respectively.
B
M
Mean
2.2
2.9
2.6
2.8
CT
Mean 2.5
2.7
2.8
NT
2.8
†LSD ns, ns, ns
57
Loosening of the soil, associated with CT, could help to explain this observation. Lal (1989)
attributed a similar scenario to a significant change in the soil pore geometry. Higher Ksat
with CT has been severally reported (Creswell et al., 1993; Laddha and Towatat, 1997;
Guzha, 2004; Lipiec et al., 2006; Anikwe and Ubochi, 2007; Martinez et al., 2008). The Ksat
values due to interaction indicated that the tillage x mulch combinations differed significantly
(P ≤ 0.05) from one another; such that CTM > CTB > NTB > NTM.
Each of the tillage and mulch treatments was found not to have any significant effect
on bulk density of the soil in this experiment. Similar observation as regards tillage and bulk
density had been reported from the same location (Obi and Nnabude, 1988; Obi, 1989;
Anikwe et al., 2003) and elsewhere (Xu and Mermoud, 2001; Johnson-Maynard et al., 2007;
Martinez et al., 2008), including a long term study (Lal, 1997b). On the other hand, findings
from both a short term study (Baumhardt and Jones, 2002b) and a long term study (Mulumba
and Lal, 2008) indicated that bulk density was similarly not influenced by mulch treatments.
The mulch treatments had no influence on total porosity of the soil, probably due to
shortness of the study duration. Mulumba and Lal (2008) found continuous application of
mulch for 11 years to increase total porosity. In none of the three experimental plots was PSD
influenced by the tillage and mulch treatments. This corroborates previous findings in soil of
the study site (Obi and Nnabude, 1988; Obi, 1989) and elsewhere (Xu and Mermoud, 2001;
Micucci and Taboada, 2006). It was evident that micropores conspicuously predominated
over macropores. This could be due to the weakness in the structure of the soil, which
frequently predisposes the macropores to destruction. Additionally, the MWD was not
affected in any of the three plots, and this was in accord with the finding of Johnson-Maynard
et al. (2007) in a similar short term study on silt loam in the United States. The view that a
period of 2-3 years might usually not be enough for tillage to affect most soil properties
(Buschiazzo et al., 1998), could be the case with the PSD and MWD, more so with the
inherently weak structure of the soil. Some long-term studies indicated that NT and CT
influenced PSD (Azooz and Arshad, 1996; Buczko et al., 2006; Glab and Kulig, 2008;
Martinez et al., 2008); other long-term studies indicated significantly larger MWD under NT
(Mahboubi and Lal, 1998; Martinez et al., 2008).
In each of the three plots, the Ksat was lowest under the NTM (Table 7). In contrast,
Ksat was enhanced under the NTM in similar studies on an Alfisol in western Nigeria
(Franzen et al., 1994) and an Entisol in China (Zhang et al., 2008). In this study, the Ksat was
always lower under mulch, though significant only in the sole soybean plot. This, coupled
with the lower Ksat with NT, could explain the low Ksat values recorded under the NTM.
58
4.3.3 Chemical properties
Table 8 shows the corresponding chemical properties of the soil. In all the three
experimental plots, soil pH and total N were not significantly affected by the tillage
treatments. Similar non-significant effect of CT and NT systems on pH and total N of this
soil had earlier been reported (Anikwe, 2000). Buschiazzo et al. (1998) attributed similar
effect on pH and N to non-application of any chemical fertilizer and the sandy nature of the
soils respectively. Likewise, chemical fertilizers were not used in this study, and the soil was
sandy. The effect of mulch practice was also not significant, except in the intercropped plot,
where pH and total N of the soil were significantly (P ≤ 0.05) higher under the bare
treatments. Higher N under the bare treatments could be due to N lock-up frequently
associated with mulch application (FAO, 1987; Smith, 1993). The soil pH due to interaction
was significantly higher (P ≤ 0.05) in the NTB than in the rest of the combinations.
None of the management practices significantly affected the SOM content in any of
the three plots. The SOM contents were generally low under both tillage systems. Similar
non-significant effects of CT and NT, and lowness in values of SOM have been reported
from the study location (Obi, 1989). However, there was an overall improvement of the SOM
content under the management practices. This is attributed to the poultry manure applied
before sowing in each of the two years. In each of the plots, the SOM content was lower
under the CT compared to the NT. The differences were, however, not significant probably
due to the short duration of the study. In the interaction, the lowest and highest values were,
remarkably, in the CTB and NTB. Compared with the value (0.86%) at the commencement
of the study in 2006 (Table 6), the values under the CTB and NTB represented 23 and 37%,
30 and 53%, and 30 and 72% increment in SOM in the sole sorghum, sole soybean, and
intercropped plots, respectively. These results are also indicative of greater oxidation of the
volatile SOM with CT compared to the NT (Connolly, 1998; Wright and Hons, 2005).
The mulch material was expected to decompose and increase the SOM content of the
mulch treatments, but this was not so. Some field study results outside the tropics showed a
positive relationship between SOM content and mulch (Sharma and Acharya, 2000; Zhang et
al., 2008), and some only after the third year (Liang and Wang, 2001). Therefore, the nonsignificant effect of mulch on SOM after two years of this study might be linked to its rapid
decomposition (caused by high temperatures) in especially the sandy soils of the tropics
(Ajantha de Silva and Cook, 2003). The generally lower SOM under the CT compared to the
NT (which is the more common phenomenon), stated earlier, substantiates the deduction that
rapid decomposition of SOM was the reason for non-significant effect of mulch.
59
Table 8: Some chemical properties of the top (0-10 cm) soil under the tillage-mulch treatments at the end of the two-year study
pH in H2O
SOM (%)
Total N (%)
CEC (cmol kg-1)
Available P (ppm)
Sole sorghum plot
Treatments
Tillage
system
Mulch practice
B
M
Mean
B
M
NT
5.8
5.4
5.6
CT
5.3
5.1
5.2
Mean
5.5
†LSD
Mean
NT
1.18
1.18
1.18
CT
1.06
1.12
1.09
5.2
Mean
1.12
1.15
ns, ns, ns
†LSD
ns, ns, ns
B
M
Mean
NT
0.08
0.07
0.07
CT
0.07
0.06
0.07
Mean
0.07
0.07
†LSD
ns, ns, ns
B
M
Mean
B
M
NT
34.3
29.3
31.8
CT
31.4
24.4
27.9
Mean
32.8
†LSD
Mean
NT
6.4
4.7
5.6
CT
8.6
5.3
6.9
26.8
Mean
7.5
5.0
ns, 5.4*, ns
†LSD
1.1*, 0.9**, ns
Sole soybean plot
B
M
Mean
B
M
NT
5.3
5.3
5.3
CT
5.1
5.5
5.3
Mean
5.2
†LSD
Mean
NT
1.32
1.26
1.29
CT
1.12
1.30
1.21
5.4
Mean
1.22
1.28
ns, ns, ns
†LSD
ns, ns, ns
B
M
Mean
NT
0.09
0.07
0.08
CT
0.08
0.09
0.09
Mean
0.09
0.08
†LSD
ns, ns, ns
B
M
Mean
NT
22.4
13.4
19.9
CT
14.9
30.3
22.6
Mean
18.6
21.9
†LSD
2.4*, 3.0*, 2.8**
B
M
Mean
NT
6.7
5.3
6.0
CT
6.4
8.3
7.4
Mean
6.6
6.8
†LSD
0.27**, ns, ns
Intercropped plot
B
M
Mean
NT
5.9
5.0
5.5
CT
5.1
5.2
5.1
Mean
5.5
5.1
†LSD
ns, 0.3*, 0.8*
B
M
Mean
NT
1.48
1.34
1.41
CT
1.12
1.30
1.21
Mean
1.30
1.32
†LSD
ns, ns, ns
B
M
Mean
B
M
NT
0.11
0.06
0.08
CT
0.08
0.06
0.07
Mean
0.09
†LSD
Mean
NT
36.3
33.2
34.7
CT
17.4
14.9
16.2
0.06
Mean
26.8
24.1
ns, 0.02*, ns
†LSD
ns, ns, ns
NT = No-till, CT = Conventional tillage, B = Bare, M = Mulch
Notations of the physical properties are as explained in Table 4
†Given
for tillage system, mulch practice, and tillage x mulch in that order
ns stands for not significant at P ≤ 0.05; * and ** denote significance at P ≤ 0.05 and 0.01 levels of probability respectively.
B
M
Mean
NT
9.4
5.9
7.6
CT
7.8
7.2
7.5
Mean
8.6
6.5
†LSD
ns, ns, ns
60
Availability of P was significantly (P ≤ 0.05) enhanced under the bare treatments
compared to the mulch-applied treatments in the sole sorghum plot. Both the tillage and the
mulch treatments significantly (P ≤ 0.05) influenced P availability in the sole soybean plot.
This could be due to the fact that the soybean crop is highly dependent on P (Buschiazzo et
al., 1998). Phosphorus was more available, on one hand, under the CT treatments, and on the
other hand, under the mulch-applied treatments. Tillage x mulch interaction was significant
(P ≤ 0.01) only in the sole soybean plot, with CTM > NTB > CTB = NTM. This result is
indicative of the suitability of the CTM and/or the NTB in coping with P deficiency common
among sandy soils (Klaij and Ntare, 1995). The CEC of the soil was significantly (P ≤ 0.05)
higher with the CT than with the NT in the sole-cropped plots. It was significantly (P ≤ 0.01)
higher under the bare compared to the mulch-applied treatments in the sole sorghum plot.
4.4 Moisture in the monitored soil profile
4.4.1 Pattern of depletion
Figs. 3, 4, and 5 show the moisture contents of the four sampled soil layers on all the
monitoring dates during the 2006 and 2007 cropping seasons in the sole sorghum, sole
soybean, and intercropped plots respectively. The monitored 50-cm depth range represented
the zone of greatest root density and, hence, of highest moisture extraction by sorghum
(Zaongo et al., 1994; Moroke et al., 2005) and soybean (Willatt and Olsson, 1982; Kranz et
al., 2005; FAO AGL, 2006). Hence, the figures depict the general pattern of moisture
depletion in the soil layers across the sampling dates.
The moisture depletion and accretion processes fluctuated with and exhibited
appreciable response to the rainfall pattern in the first year. Since rainfall was the only source
of soil moisture recharge, this would be expected. This was, however, not the case in the
second year, probably due to the erratic nature of rainfall in that season. Overall, the moisture
depletion pattern was of moderate speed and, hence, range. Obi and Nnabude (1988)
reported, for the soil under investigation, similar influence of rainfall pattern on moisture
depletion; which contrastingly, was characterized by rapidity and high range in the 0-30 cm
depth zone. These authors attributed the phenomenon to the sandy nature of the soil and the
associated high proportion of macropores. The disparity would be explained partly by the fact
that deeper profile was monitored in the present study and partly by the preponderance of the
micropores over the macropores, more so as the PSD was not affected by the treatments in
the topsoil (Table 7). Hamblin and Tennant (1981) noted that alterations to porosity in sandy
soils might have a disproportionately large influence on many water-related properties.
61
30
160
140
25
120
20
100
80
15
0-10 cm
10
60
40
5
20
0
160
140
25
120
15
80
10-20 cm
60
10
40
5
20
0
0
MOISTURE
30
160
140
25
120
20
100
15
SOIL
(mm)
100
INPUT
CONTENT
20
80
20-30 cm
60
10
40
5
20
0
0
70
160
60
140
120
50
100
40
80
30-50 cm
30
60
C
20
40
10
20
0
0
36
46
56
65
73
81
91
101
110
120
130
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
Fig. 3a: Moisture contents of the soil layers (mm) under the treatment
combinations in the sorghum plot during the 2006 growing season
RAIN
(mm)
0
30
62
40
160
35
140
30
120
25
100
20
80
15
60
0-10 cm
10
40
5
20
0
0
160
140
25
120
80
15
10-20 cm
10
60
40
5
20
0
0
MOISTURE
25
160
140
20
120
100
15
80
20-30 cm
10
SOIL
(mm)
100
INPUT
CONTENT
20
60
40
5
20
0
0
50
160
140
40
120
100
30
80
30-50 cm
20
60
40
10
20
0
0
72
81
90
101
110
118
127
136
146
156
166
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
Fig. 3b: Moisture contents of the soil layers (mm) under the treatment
combinations in the sorghum plot during the 2007 growing season
RAIN
(mm)
30
63
30
160
140
25
120
20
100
80
15
0-10 cm
10
60
40
5
20
(mm)
0
0
30
160
140
25
120
80
15
10
60
10-20 cm
20
MOISTURE
0
25
160
INPUT
40
5
0
140
20
120
100
15
80
SOIL
(mm)
100
10
60
20-30 cm
40
5
20
0
0
60
160
140
50
120
40
100
80
30
60
30-50 cm
20
c
40
10
20
0
0
36
46
56
65
73
81
91
101
110
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
Fig. 4a: Moisture contents of the soil layers (mm) under the treatment
combinations in the soybean plot during the 2006 growing season
RAIN
CONTENT
20
64
30
160
140
25
120
20
100
15
80
0-10 cm
60
10
40
5
20
0
0
160
140
25
120
20
80
10-20 cm
60
10
40
5
20
MOISTURE
0
30
0
160
140
25
120
20
100
15
80
20-30 cm
60
10
SOIL
INPUT
CONTENT
15
(mm)
100
40
5
20
0
0
50
160
140
40
120
100
30
80
30-50 cm
20
60
40
10
20
0
0
72
81
90
101
110
118
127
136
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
Fig. 4b: Moisture contents of the soil layers (mm) under the treatment
combinations in the soybean plot during the 2007 growing season
RAIN
(mm)
30
65
40
160
35
140
30
120
25
100
80
0-10 cm
15
60
10
40
5
20
0
0
35
160
30
140
120
25
100
10-20 cm
40
5
20
0
0
30
160
(mm)
60
10
140
25
120
20
100
20-30 cm
15
SOIL
80
15
INPUT
MOISTURE
CONTENT
20
80
60
10
40
5
20
0
0
60
160
140
50
120
40
100
30-50 cm
30
80
60
20
40
10
20
0
0
36
46
56
65
73
81
91
101
110
120
130
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
Fig. 5a: Moisture contents of the soil layers (mm) under the treatment
combinations in the intercropped plot during the 2006 growing season
RAIN
(mm)
20
66
30
160
140
25
120
20
100
15
80
0-10 cm
60
10
40
5
20
0
0
160
140
25
120
15
80
60
10-20 cm
10
40
5
20
MOISTURE
0
0
35
160
30
140
120
25
100
20
80
15
60
20-30 cm
SOIL
(mm)
100
INPUT
CONTENT
20
10
40
5
20
0
0
70
160
60
140
50
120
100
40
80
30
60
30-50 cm
20
40
10
20
0
0
72
81
90
101
110
118
127
136
146
156
166
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
Fig. 5b: Moisture contents of the soil layers (mm) under the treatment
combinations in the intercropped plot during the 2007 growing season
RAIN
(mm)
30
67
However, moisture depletion was generally fairly rapid in the sole sorghum and in the
intercropped plots towards the end of the season in both years (Figs. 3 and 5). The
coincidence of the late season sampling with the cessation of the rains in the study area would
explain the observation. It was not so in the sole soybean plot, since the crop was harvested
much earlier. Remarkably, the moisture contents increased with depth in the sole-cropped
plots but not in the intercropped plot. This could be due to stratified utilization of soil
moisture by sorghum and soybean in the intercropped plot and, hence, fairly equal water
extraction from the four sampled layers.
The tested tillage systems influenced moisture contents in the monitored layers of the
three plots. In both years, pattern of moisture contents of the four monitored soil layers
showed signs of inconsistency in effects of the two tillage systems under sorghum. The
effects were most pronounced within the plough depth zone (the uppermost two – 0-10 and
10-20 cm – soil layers) in the second year, when differences were significant (P ≤ 0.05) on a
number of monitoring dates. Moisture contents of the soil, based on those dates, tended to be
generally higher with the CT in the 0-10 cm layer, but with the NT in the 10-20 cm layer.
This result is, with regard to the soil moisture content in the 0-10 cm layer, in conformity
with the findings from a sandy loam in western India (Laddha and Totawat, 1997) and for a
silt loam in Poland following a long-term study (Lipiec et al., 2006). In the sole soybean plot,
the CT consistently maintained higher moisture contents within the plough depth zone in both
years, the differences being significant (P ≤ 0.05) on many of the sampling dates. The effect
in the 20-30 cm layer was inconsistent across the sampling dates. Contrary to the trend in
plough depth zone, the NT maintained significantly (P ≤ 0.05) higher moisture contents
compared to the CT in the 30-50 cm layer on a number of the sampling dates in both years.
The trend in the intercropped plot was similar to what obtained in the sole soybean
plot, except that the inconsistency in effects of the two tillage systems in the 20-30 cm layer
extended to the 30-50 cm layer. As observed also by Azooz and Arshad (1995), the
differences between the NT and the CT generally decreased towards the end of the season.
This could be due to a combination of progressive drying of the soil and gradual narrowing of
the differences in the soil conditions under both tillage systems with the passage of time.
Thus, it was more evident in the sole sorghum and the intercropped plots (with longer
growing seasons) than in the sole soybean plot (with a shorter growing season).
The cases of higher moisture content within the plough depth zone under CT could be
due to the increase in total porosity – with preponderance of micropores over macropores – in
the topsoil under the CT (Table 7). Micro-aggregates have greater potential for moisture
68
retention than macro-aggregates. Sorting of soil particles during ploughing in the CT
treatments may have preferentially accumulated the micro-aggregates beneath the soil surface
(Ojeniyi and Dexter, 1979; Agele et al., 2000), where their intra-particulate moisture was
better protected from loss to the atmosphere. Moisture advantage of the CT over the NT
could further be linked to two complementary processes. The first is that the loosened moist
soil moved to the surface becomes pulverized; inducing high but transient evaporation rate
that facilitates more complete desiccation (Hillel, 1982). In the accompanying second
process, the pulverized “dry” soil at the surface eventually breaks the continuity of capillary
pores, thereby acting as surface mulch (Agele et al., 2000). The resulting hysteresis and the
associated decrease in conductivity help to arrest or retard subsequent outflow, thereby
conserving more moisture beneath the surface (Hillel, 1982). Since this phenomenon obtains
in especially sandy soils (Payne et al., 1990), an evaporation control layer to the depth of
tillage might have been created in the soil under the CT.
Significant difference in moisture contents due to tillage within the 30-50 cm layer in
the sole soybean plot suggested that the effect of the tillage systems on soil moisture was felt
beyond the plough depth zone in the plot. Moisture content was higher in this lower depth
under the NT. This is consistent with the findings of Norwood (1999) in Kansas and Barzegar
et al. (2003) in Iran. In corroboration of the above observation, some other workers (Hangen
et al., 2002; Birkas et al., 2004) reported that macroporosity was continuous under NT but
restricted to the plough zone under CT. Hangen et al. (2002) concluded that the NT had
higher potential for increased water infiltration and displacement to greater depths than the
CT. Therefore, with the minimal soil structure-disrupting attribute of the NT, deeper
penetration of rainwater might have taken place under the treatment (Barzegar et al., 2003),
hence the higher moisture content in the lower depth compared to the CT.
The effect of mulch was significant (P ≤ 0.05) on many monitoring dates; its
application enhanced the moisture retention capacity of the soil mostly in the upper soil
layers. In the sole sorghum plot, the strongest influence of mulch on moisture retention
occurred in the 0-10 cm layer in both years. The positive effect in the 10-20 cm layer was
evident on selected sampling dates in especially the second year. In the 20-30 cm layer,
values were generally higher under the bare treatments compared to the mulch-applied
treatments most of the time in both years. This trend persisted in the 30-50 cm layer in the
first year, but not in the second year when the effects were inconsistent across the sampling
dates. In the sole soybean plot, the mulch-applied treatments distinctly maintained higher
moisture contents compared to the bare treatments in the 0-10, 10-20, and 20-30 cm layers on
69
almost all the sampling dates in both years. The strength of this advantageous effect of mulch
diminished in the 30-50 cm layer, as evident on some sampling dates. Remarkably, moisture
contents were higher under the bare treatments compared to the mulch-applied treatments in
all the four sampled layers at 101 DAS in the first season. The above deviation may have
been due to a sampling error. In the intercropped plot, mulch-applied treatments enhanced
moisture contents of the top 0-10 cm layer and the 30-50 cm layer only in the second year.
The moisture contents of the 10-20 and 20-30 cm soil layers under mulch were marginally
above those under the bare treatments in both years.
The higher moisture retention under the mulch-applied treatments compared to the
bare treatments in the 0-10 cm layer of the three experimental plots implies that the most
effective mechanism through which mulch conserves soil moisture is by reduction of
evaporation from the surface. By providing surface cover to the soil, mulch frequently
minimizes evaporative losses. Allen et al. (1998) noted that 50% reduction in evaporation
from soil surface would occur, if fully and effectively covered by organic mulch. It also
seems that when mulch finally establishes good contact with the soil at the surface, it exerts a
kind of sorption on topsoil moisture against unproductive drainage. Similar beneficial effect
of mulch in the topsoil region was also reported in similar studies by Mbagwu (1991) in the
same location and Cook et al. (2006) in southern England. In the sole soybean plot, the
persistence of the positive effect of mulch down to the 30 cm depth, unlike in the other plots,
was probably due to the dense and closed canopy of the crop. This ensured additional
trapping of rainwater for the mulch, for subsequent infiltration into the soil.
As could be seen from Figs. 3-5, temporal variations in moisture contents of the soil
layers were due to the tillage x mulch interaction. In the sole sorghum plot, the highest was
found in the CTM in all the sampled soil layers, followed consistently by the NTB. The
general trend in all the four layers was decreasing moisture content in the order
CTM→NTB→CTB or NTM (Fig. 3). In the sole soybean plot, the CTM and NTB had the
highest and the least moisture respectively within the plough depth zone (0-20 cm) and in the
20-30 cm layer. The NTB was most likely to maintain the highest temperature near the
surface (Obi and Nnabude, 1988; Dahiya et al., 2007) hence, the least moisture compared to
the rest in the topsoil region. The NTM and CTM had the highest and the least moisture
respectively in the 30-50 cm layer (Fig. 4). The relative moisture contents of the NTM and
CTM in the 30-50 cm layer is consistent with the findings of Jalota et al. (2001) in India.
With the less moisture content under the CT compared to the NT and the decline in effect of
mulch in the 30-50 cm layer of this sole soybean plot, the least moisture content under the
70
CTM would be understandable. In the intercropped plot, the overall trend indicated a
decrease in the order, CTM→CTB→NTM→NTB in the plough depth zone (0-20 cm), and
CTM→NTM/NTB→CTB in the 20-30 and 30-50 cm soil layers (Fig. 5).
4.4.2 Drainage and storage
For the water balance study, the drainage term needed was the deep percolation
occurring below the monitored profile (depth, 50 cm). The cumulative values at the end of
the growing season in the first and second year were respectively 151.6 and 255.8 mm (for
sorghum) and 128.4 and 217.2 mm (for soybean). In an environment where the annual
rainfall rarely exceeds 750 mm, a range of 148-239 mm was obtained under soils similar in
texture (loamy sand-sandy loam) as the one under investigation, cropped to sorghum and
other cereals (Oluwasemire et al., 2002; Chiroma et al., 2006).
The corresponding storage term needed was the change in storage, ∆S. Storage, the
depth of moisture (mm) found within the monitored depth zone (50 cm) at any sampling
instance or simply the profile soil moisture (PSM) storage, is presented for the different
treatments on all the sampling dates in Figs. 6, 7, and 8 respectively for the sole sorghum,
sole soybean, and intercropped plots. Since rainfall was the only source of soil moisture
recharge, this moisture depth tended to fluctuate with the rainfall pattern. Notably, the PSM
in the three experimental plots fluctuated between 63 and 135 mm per the 50 cm depth in the
two years of the study. These lower and upper limits of stored soil moisture were obtained
during the second year trial under the sole sorghum and the intercrop, Figs. 6 and 8,
respectively. This could be a manifestation of the erratic nature of rainfall in the second year.
Sole-cropped sorghum: In the first year, the effect of tillage system was significant
(P ≤ 0.01) plot at 56 and 91 DAS; moisture storage was higher under the CT than the NT
treatments. There were no significant differences in the PSM storage under the bare and the
mulch-applied surfaces on all the sampling dates, owing to the vagaries in moisture status
observed in the soil layers under these treatments. The tillage x mulch interaction indicated
that the CTM and NTB had the highest storage in their profiles on six and five monitoring
dates respectively, with an overall trend of decreasing moisture storage in the order,
CTM→NTB→CTB→NTM (Fig. 6a). Averaged across the sampling dates, increments over
the NTM by the CTB, NTB, and CTM were 4.3, 10.0, and 10.2% respectively. Jones et al.
(1969) reported that, in a corn field, the CTM plots gave the highest values for soil moisture
content in their profiles. In the second year, moisture storage was significantly (P ≤ 0.05)
higher under the NT treatments at 110 DAS. The mulch-applied treatments had significantly
71
DEPTH OF MOISTURE (mm)
160
140
120
100
80
60
40
20
0
36
46
56
65
73
81
91
101
110
120
130
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
FIELD CAPACITY
(a)
DEPTH OF MOISTURE (mm)
160
140
120
100
80
60
40
20
0
72
81
90
101
110
118
127
136
146
156
166
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
FIELD CAPACITY
(b)
Fig. 6: Profile soil moisture storage (mm) in the 50-cm depth zone of the sorghum
plot during the 2006 (a) and 2007 (b) growing seasons
NTB: No-till and Bare
CTB: Conventional tillage and Bare
NTM: No-till + Mulch
CTM: Conventional tillage + Mulch
DAS: Counted as from 3 July, 2006 (a) and 7 June, 2007 (b)
72
DEPTH OF MOISTURE (mm)
160
140
120
100
80
60
40
20
0
36
46
56
65
73
81
91
101
110
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
FIELD CAPACITY
(a)
DEPTH OF MOISTURE (mm)
160
140
120
100
80
60
40
20
0
72
81
90
101
110
118
127
136
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
FIELD CAPACITY
(b)
Fig. 7: Profile soil moisture storage (mm) in the 50-cm depth zone of the soybean
plot during the 2006 (a) and 2007 (b) growing seasons
NTB: No-till and Bare
CTB: Conventional tillage and Bare
NTM: No-till + Mulch
CTM: Conventional tillage + Mulch
DAS: Counted as from 3 July, 2006 (a) and 7 June, 2007 (b)
73
DEPTH OF MOISTURE (mm)
160
140
120
100
80
60
40
20
0
36
46
56
65
73
81
91
101
110
120
130
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
FIELD CAPACITY
(a)
DEPTH OF MOISTURE (mm)
160
140
120
100
80
60
40
20
0
72
81
90
NTB
101
NTM
110
118
127
DAYS AFTER SOWING (DAS)
CTB
CTM
RAIN INPUT
136
146
156
166
FIELD CAPACITY
(b)
Fig. 8: Profile soil moisture storage (mm) in the 50-cm depth zone of the
intercropped plot during the 2006 (a) and 2007 (b) growing seasons
NTB: No-till and Bare
CTB: Conventional tillage and Bare
NTM: No-till + Mulch
CTM: Conventional tillage + Mulch
DAS: Counted as from 3 July, 2006 (a) and 7 June, 2007 (b)
74
(P ≤ 0.05) higher PSM storage than the bare treatments at 72 and 101 DAS. Generally, the
PSM storage indicated higher values under the mulch-applied surfaces during the first six
sampling dates, and vice versa during the remaining five. This trend suggests that the grass
mulch was effective from application of the treatments till around 118 DAS. Beyond this
point, the mulch material, being organic and hence liable to rapid oxidation by high
temperatures (FAO, 1987), had probably deteriorated to the point that it could no longer offer
full coverage to the soil. This was visually evident in the experimental plot towards the later
stage of the growing season, hence the observed decline in the effect of mulch.
The interaction was significant (P ≤ 0.01) at 72 and 127 DAS; in each case, the
highest moisture storage occurred in the CTM, followed by the NTB (Fig. 6b). Chiroma et al.
(2006) reported similar few cases of significant effects of tillage-mulch treatments cropped to
sorghum on PSM storage. The overall trend was moisture storage in the decreasing order,
NTB→CTM→NTM→CTB. Averaged across the sampling dates, increments over the CTB
by the NTM, CTM, and NTB were 0.9, 7.1, and 7.4% respectively. The relative positions of
the CTM and NTB in the first and second year’s trends is noteworthy, so also are those of the
CTB and NTM. Such a swop of positions is viewed as a pointer to the similarity in effects of
the concerned treatment combinations.
Sole-cropped soybean: In this plot, no significant differences in PSM storage were
found between the two tillage systems on any of all the sampling dates in both years. The
values were, however, numerically higher under the CT compared to the NT about 67 and
75% of the time in the first and in the second year respectively. But for the deviation at 101
DAS in the first year, the mulch-applied treatments consistently maintained higher PSM than
their bare counterparts on all the sampling dates in both years. In the first year, the effect was
significant (P ≤ 0.01) at 36 DAS, with the PSM storage being higher under the mulch-applied
(CTM and NTM) than the bare (CTB and NTB) treatments. On the same date, the interaction
was significant (P ≤0.05): the NTM had the highest storage and was different from the rest;
the NTB had the least and was different from the rest. Altogether, each of the CTM and NTM
had the highest storage in its profile on four out of the nine observations made before harvest
in the first year (Fig. 7a). In the second year, moisture storage at 72 DAS was significantly (P
≤ 0.01) higher under the mulch-applied treatments; with significant (P ≤ 0.05) interaction,
indicating higher storage in the CTM and NTM than in the CTB and NTB – which recorded
the least. The enhanced PSM with mulch was equally significant (P ≤ 0.05) at 101 and 136
DAS; but in these cases, on neither of the dates was interaction significant. Altogether, the
CTM had in six while the NTM had in two out of the eight observations in the second year
75
the highest storage in their profiles (Fig. 7b). The overall trend in both years was decreasing
storage in the order, CTM→NTM→CTB→NTB. Averaged across the sampling dates,
increments over the NTB by the CTB, NTM, and CTM were 2.3, 6.0, and 6.4% respectively
in the first year; and 5.4, 7.5, and 8.3% respectively in the second year.
Intercropped sorghum-soybean: The CT maintained higher PSM storage over the
NT on almost all the sampling dates in the first year, effect of which was significantly (P ≤
0.05) different only at 91 DAS. Neither the mulch-applied nor the bare treatments remarkably
influenced the PSM storage. The interaction was significant (P ≤ 0.05) at 36 and 120 DAS; in
each case, moisture storage in the CTM and NTB was higher than that in the CTB and NTM.
Moisture
storage
indicated
an
overall
trend
of
decreasing
in
the
order
CTM→NTB→.CTB→NTM (Fig. 8a). Averaged across the sampling dates, increments over
the NTM by the CTB, NTB, and CTM were 2.4, 5.2, and 11.9% respectively. In the second
year, effect of the tillage systems was similar to what obtained in the first year, whereas the
PSM storage was consistently higher under the mulch-applied treatments compared to the
bare treatments. However, neither of these effects was significant on any of the sampling
dates. The CTM was, with respect to moisture storage, always above the other treatment
combinations. The PSM storage indicated an overall trend of decreasing in the order,
CTM→NTM→NTB→CTB (Fig. 8b). Averaged across the sampling dates, increments over
the CTB by the NTB, NTM, and CTM were 3.7, 4.5, and 16.6% respectively.
Notably, the influence of the tillage systems on PSM storage was neither consistent in
the two years of the study nor well pronounced all through the sampling dates in either year.
The contrast in the few significant effects of the tillage systems on the PSM under sorghum
(in which the CT and the NT indicated higher values in the first and second seasons,
respectively) suggests that rainfall distribution may be important in adjudging their relative
suitability for moisture conservation. The pattern of PSM storage in the sole soybean and in
the intercropped plots in both years generally indicated greater potential of enhancing soil
moisture storage with the CT than with the NT. However, significant advantage of the CT
over the NT, found in the sole sorghum and in the intercropped plots, was only on few
sampling dates in the first year. The higher PSM storage observed under the CT compared to
the NT were reported in similar studies elsewhere (Payne, 1999; Kovac et al., 2005; Ijaz and
Ali, 2007). It could be that the presence of large clods and larger surface area (due to
breaking apart of soil crusts and roughening of the soil surface), following CT, enhanced
rainwater infiltration into the soil, which led to the increased moisture storage (Aina, 1993;
Smith, 1993; Hatfield et al., 2001; Ali and Talukder, 2008).
76
The poor influence of the tillage systems on PSM storage was most apparent in the
sole soybean plot. As shown earlier, the soil moisture contents tended to be higher in the
plough zone and in the deeper layer, respectively with the CT and the NT in this plot. It was
possible that these differing effects of the two tillage treatments within and beneath the
plough zone offset each other in terms of storage in the entire profile. Generally, the poor
influence of the tillage systems on PSM storage may partly be as a result of the textural
attribute of the soil of the study location. It had been shown by Buschiazzo et al. (1998) that
the sandier a soil was, the more identical were its moisture contents due to tillage systems.
The view that the NT is usually less effective on soils characterized by poor aggregation and
weak structure (Aina, 1993), as the one studied, may further explain the relative poor PSM
storage under the NT under in the three experimental plots. Perhaps, there were also instances
when the CT induced evaporative moisture losses, which were of the magnitude to offset the
increased PSM due to the enhanced infiltration of water at the ploughed topsoil (Hatfield et
al., 2001). Since tillage moves moist soil to the surface (which predisposes the soil) and
breaks apart any crust (which provides greater surface area), Hatfield et al. (2001) contend
that CT sometimes leads to increased moisture losses by evaporation.
Reports from similar studies in semi-arid environments (Lawrence et al., 1994;
Chiroma et al., 2006), as well as in a sub-humid environment (Sarkar and Singh, 2007)
indicated that tillage systems had no effect on PSM storage. Derbeit et al. (1986) found that
precipitation storage efficiency (the amount of soil moisture stored in the upper 1.2 m relative
to the precipitation during the non-growing season) under wheat in the northern Great Plains
was comparable among tillage systems.
Surface application of mulch significantly enhanced the PSM storage in the three
experimental plots only on few sampling dates. This could generally be due to the view that
the potential benefits of mulch are less in the humid and sub-humid regions than in the arid
and semi-arid regions (Payne, 1999). The few cases of significant enhancement in the PSM
storage under mulch could be attributed to better interception and transitory detention of
rainwater, thereby ensuring more voluminous infiltration of this water into the soil.
Moderating of the soil temperature under mulch in this environment (Mbagwu, 1991) would
help to minimize losses in PSM due to desiccation. In the sole sorghum and the intercropped
plots, the positive effect of mulch was more pronounced in the second than in the first year,
probably due to the erratic nature of rainfall in the second year. Moitra et al. (1996) observed
that mulch was more effective in conserving soil moisture under a low rainfall situation.
77
Mulch offers protection to the wet soil from evaporative demand of the atmosphere (FAO,
1995b), which has been shown to be severer in a relatively dry year (Tolk et al., 1999).
The CTM almost always had the highest moisture storage, often followed by the NTB
or – only in the sole soybean plot – the NTM. In all the three plots, the CTM always had in its
profile more moisture than the CTB. This corroborates the findings of Jin et al. (2007) that, in
the Loess Plateau of China, precipitation storage efficiency was consistently enhanced under
the CTM over a 5-year period compared to the CTB. It appears that, with respect to moisture
storage, CT benefits more from mulch application than from no-mulch condition. The
improvement in the PSM storage under the CTM treatment could be partly attributed to the
surface modifying effect of CT (Hillel, 1982; Omer and Elamin, 1997), and partly to the
reduction of evaporation losses due to the presence of surface mulch and a larger leaf canopy.
Similar observation has been reported in some other parts of Nigeria (Alhassan et al., 1998;
Olasantan, 1999; Chiroma et al., 2006).
The enhanced PSM in the NTB to the point of being comparable with the CTM in the
sole sorghum and intercropped plots is inexplicable. On the other hand, the NTM was not just
inferior to the CTM always and to the NTB most of the time, but could not show a clear-cut
superiority over the CTB in these plots. In their study with a similar crop (wheat) on the
Loess Plateau of China, Su et al. (2007) found that the NTM was superior to the CTB, just as
the highest moisture storage in a 67-m depth profile was found under the NTM in a similar
study after wheat harvest in Germany (Dahiya et al., 2007). Though the reason for the present
observation is not clear, it should at least be recalled that Ksat under the NTM was the lowest
(Table 7); suggesting an unfavourable soil environment for good water relation. With the NT
system sometimes associated with poor water transmission down the profile (Guzha, 2004;
Lipiec et al., 2006; Anikwe and Ubochi, 2007; Martinez et al., 2008), the NTM treatment
probably required more than mere application of mulch in order to improve the moisture
retention capacity of the soil.
However, the above may not imply absolute inefficacy of the NTM for enhancing
moisture conservation on these coarse-textured soils, as its effect was remarkable in the
soybean field. It is, therefore, apparent that effect of the tillage-mulch practices could be
dependent on crop grown. According to Obi and Nnabude (1990), adequate groundcover is
required to enhance soil moisture storage. There was almost complete shading of the soil
under soybean by the well established canopy of the crop (see plate 1b), which was not
obtainable under sorghum. That might have contributed to the enhanced storage under the
NTM. A plausible explanation is that such a crop canopy structure was presumably needed to
78
encourage more infiltration and storage of the rainwater intercepted and retained at near the
surface by the applied mulch. There are many reports which imply that the wetter an
environment is, the more pronounced would be the beneficial effects of the NTM in soil
moisture conservation (Aina 1993; Babalola and Opara-Nadi 1993; Opara-Nadi 1993).
4.4.3 Soil water suction
The soil water suction in both the 0-30 and 30-60 cm depth zones of all the three plots
(Figs. 9, 10, and 11) did not indicate any serious water stress. In most of the cases, the gauge
readings were below 10 cb, the tension corresponding to the theoretical FC. Salako and Tian
(2003) had indicated that in a coarse-textured soil, suctions at 10 cb and below would
normally not be expected to generate water stress. In a few cases here, suction values
exceeded 10 cb, with varying margins. Such deviations from the monitored profile’s
theoretical FC (i.e. 150 mm 50-cm-1) under all the treatments implied short dry spell.
However, this was not of the magnitude to result in serious water stress, since such were
mostly evident in the sole-cropped sorghum and intercropped plots (Figs. 6 and 8
respectively) towards the end of the season, when the crops’ water utilization had drastically
dropped. The PWP of the soil of the experimental has been shown to be around 60 mm per 50
cm depth in an earlier study (Obalum, 2004). This was still below the least recorded moisture
storage of about 63 mm 50-cm-1 in the sole-cropped sorghum plot (Fig. 6).
Even if, at any growth stage, the crops were to be restricted to utilizing only the
readily available water (RAW), there was still no indication of water stress. The RAW is only
a proportion of total available water (TAW) that marks critical the limit below which the crop
roots may need to exert extra pressure in order to extract water from the soil. For the soil of
the study location, the RAW is about 70% of TAW (Mbagwu, 1985; 1987). However, the
value is a factor not only of the soil, but also of the crop, and has been shown to be lower for
sorghum and soybean – 55 and 50% respectively (Allen et al., 1998). Assuming the more
adverse situation during the growing seasons, the RAW in the monitored profile under
sorghum and soybean amounted to 49.5 and 45.0 mm respectively, based on the TAW of the
soil (90 mm 50-cm-1). Depletion of all the RAW would leave 40.5 and 45.0 mm of water
under sorghum and soybean respectively. These values are sufficiently below the least
recorded moisture storage in the soil profile (63 mm), implying that there was never a period
when all the RAW was depleted.
79
20
160
140
16
120
14
100
12
10
80
8
60
RAIN INPUT (mm)
SOIL WATER SUCTION (c'bars)
18
6
40
4
20
2
0
0
20
160
18
140
16
120
14
100
12
10
80
8
60
6
40
4
20
2
0
0
72
81
90
101
110
118
127
136
146
156
166
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
(b)
Fig. 9: Soil water suction in the (a) 0-30 and (b) 30-60 cm depth zones of the sole
sorghum plot in the second year
NTB: No-till and Bare
CTB: Conventional tillage and Bare
NTM: No-till + Mulch
CTM: Conventional tillage + Mulch
DAS: Counted as from 3 July, 2006 (a) and 7 June, 2007 (b)
RAIN INPUT (mm)
SOIL WATER SUCTION (c'bars)
(a)
80
160
20
140
16
120
14
100
12
10
80
8
60
RAIN INPUT (mm)
SOIL WATER SUCTION (c'bars)
18
6
40
4
20
2
0
0
20
160
18
140
16
120
14
100
12
80
10
8
60
6
40
4
20
2
0
0
72
81
90
101
110
118
127
136
146
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
(b)
Fig. 10: Soil water suction in the (a) 0-30 and (b) 30-60 cm depth zones of the sole
soybean plot in the second year
NTB: No-till and Bare
CTB: Conventional tillage and Bare
NTM: No-till + Mulch
CTM: Conventional tillage + Mulch
DAS: Counted as from 3 July, 2006 (a) and 7 June, 2007 (b)
RAIN INPUT (mm)
SOIL WATER SUCTION (c'bars)
(a)
81
20
160
140
16
120
14
100
12
10
80
8
60
RAIN INPUT (mm)
SOIL WATER SUCTION (c'bars)
18
6
40
4
20
2
0
0
(a)
160
18
140
16
120
14
12
100
10
80
8
60
6
40
4
20
2
0
0
72
81
90
101
110
118
127
136
146
156
166
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
RAIN INPUT
(b)
Fig. 11: Soil water suction in the (a) 0-30 and (b) 30-60 cm depth zones of the
intercropped plot in the second year
NTB: No-till and Bare
CTB: Conventional tillage and Bare
NTM: No-till + Mulch
CTM: Conventional tillage + Mulch
DAS: Counted as from 3 July, 2006 (a) and 7 June, 2007 (b)
RAIN INPUT (mm)
SOIL WATER SUCTION (c'bars)
20
82
4.5 Crop water use or evapotranspiration
In this study, water use (WU) was taken to comprise both moisture lost from the soil
surface by evaporation and water extracted by the test crops’ roots and actively transpired by
their vegetative parts in the field. These were considered as the key avenues of depletion of
rainwater stored in the soil profile. This WU term was referred to as evapotranspiration (ET).
4.5.1 Soil moisture depletion approach
Table 9 shows the total WU of the sole-cropped sorghum, the sole-cropped soybean,
and the intercropped sorghum-soybean, based on field measurements of the progressive soil
moisture depletion during the crops’ growing seasons in 2006 and 2007. In the intercropped
plot, the ET was that which took place with sorghum and soybean growing simultaneously. It
should be noted, however, that after the harvest of soybean, whatever ET estimated was only
for the sorghum component of the intercrop (which was still left in the field).
For the sole-cropped sorghum, the mean total WU value in the first year was 596.9
mm. A value close to this (604.5 mm) has been reported under similar treatments in semiarid tropical Ethiopia (Mesfine et al., 2005). From their studies in texturally-contrasting soils
in the United States, some other authors have reported such comparable values as 618 mm
(Tolk et al., 1997) and 584 mm (Tolk and Howell, 2008). For the sole-cropped soybean, the
mean values in the first and second year were 578.9 and 614.4 mm respectively. A somewhat
median value of 590 mm has been reported in the United States (Hattendorf et al., 1988).
Similar to these results, seasonal WU, measured using the soil moisture depletion in the
United States, was reported by Erie et al. (1981) to be 769.6 and 564.0 mm respectively for
sorghum and soybean.
4.5.2 Water balance approach
Tables 10, 11, and 12 show the total WU of the sole-cropped sorghum, the solecropped soybean, and the intercropped sorghum-soybean, based on field measurements of the
storage term, changes in PSM storage during the crops’ growing seasons in 2006 and 2007.
Where the values of ∆S were negative, it implied that the cumulative PSM storage on all
sampling dates was higher than the cumulative PSM storage on all successive sampling dates.
In other words, the ET was more than the effective rainfall while the crops lasted in the field,
hence the net negative PSM.
The values obtained with the depletion approach were generally lower than those
obtained using the water balance approach. It could be that, in the quantification of depletion,
83
Table 9: Total water use (mm) of the sole- and intercropped sorghum and soybean based
on soil moisture depletion under the tillage-mulch practices in the two years of the study
Year
2006
2007
Experimental
Plot
Treatment combinations
Mean
NTB
NTM
CTB
CTM
1. Sole sorghum
611.3
594.7
592.1
589.4
596.9
2. Sole soybean
585.1
575.8
578.3
576.3
578.9
3. Intercrop
635.2
623.9
614.5
618.9
623.1
1. Sole sorghum
715.6
699.6
705.5
681.5
700.6
2. Sole soybean
620.0
608.6
617.1
612.1
614.4
3. Intercrop
753.2
731.1
735.2
738.9
739.6
NTB = No-till & Bare, NTM = No-till + Mulch;
CTB = Conventional tillage & Bare, CTM = Conventional tillage + Mulch
84
Table 10: Total water use (mm) of the sole-cropped sorghum as computed from the other
components of the water balance equation in the two years of the study
Year
P
(mm)
D
(mm)
ET
(mm)
∆S
(mm)
Mulch practice
B
2006
909.8
151.6
Tillage
system
Mulch practice
M
Mean
−5.8
−17.4
NT 787.2 764.0 775.6
CT −34.6 −32.0 −33.3
CT 792.8 790.2 791.5
NT −29.1
Mean −31.8 −18.9
B
M
Mean
Mean 790.0 777.1
†LSD ns, ns, ns
2007
1112.5
255.8
Tillage
system
B
M
Mean
32.4
−12.3
10.0
NT 824.3 869.0 846.7
CT −11.5
18.0
3.2
CT 868.2 838.7 853.5
NT
Mean
10.4
2.8
B
Mean 846.3 853.9
†LSD ns, ns, ns
P = Precipitation during the growth period
D = Drainage or deep percolation below the root zone
∆S = Change in the profile soil moisture storage
ET = Evapotranspiration, computed as P – D – ∆S
†Given for tillage system, mulch practice, and tillage x mulch in that order
ns stands for not significant at P ≤ 0.05
M
Mean
85
Table 11: Total water use (mm) of sole-cropped soybean as computed from the other
components of the water balance equation in the two years of the study
Year
P
(mm)
D
(mm)
∆S
(mm)
ET
(mm)
Mulch practice
B
2006
836.2
128.4
M
Mulch practice
Mean
B
M
Mean
NT 12.1 47.6
29.8
NT 695.7 660.2 678.0
CT 20.8 36.3
28.5
CT 687.0 671.5 679.3
Tillage
system
Mean 16.4 41.9
Mean 691.4 665.9
†LSD ns, 20.9*, ns
B
2007
922.5
217.2
M
Mean
B
M
Mean
NT 23.4 39.4
31.4
NT 681.9 665.9 673.9
CT 27.4 46.0
36.7
CT 677.9 659.3 668.6
Tillage
system
Mean 25.4 42.7
Mean 679.9 662.6
†LSD ns, 16.5*, ns
P = Precipitation during the growth period
D = Drainage or deep percolation below the root zone
∆S = Change in the profile soil moisture storage
ET = Evapotranspiration, computed as P – D – ∆S
†Given for tillage system, mulch practice, and tillage x mulch in that order
ns stands for not significant at P ≤ 0.05
* denotes significance at P ≤ 0.05 level of probability
86
Table 12: Total water use (mm) of the intercropped sorghum-soybean as computed from the
other components of the water balance equation in the two years of the study
Year
P
(mm)
D
(mm)
∆S
(mm)
ET
(mm)
Mulch practice
B
2006
909.8
151.6
Tillage
system
NT 64.2
M
Mean
7.9
36.0
NT 694.0 750.3 722.2
78.0
CT 720.5 639.8 680.2
CT 37.7 118.4
Mean 50.9
Mulch practice
63.1
B
M
Mean
Mean 707.3 695.1
†LSD 33.9*, ns, 48.0**
B
2007
1112.5
255.8
Tillage
system
M
Mean
31.3
29.9
NT 828.2 825.4 826.8
CT 25.2 128.9
77.1
CT 831.5 727.8 779.7
NT 28.5
Mean 26.9
80.1
B
M
Mean
Mean 829.9 776.6
†LSD 28.8*, 28.8**, 40.8*
P = Precipitation during the growth period
D = Drainage or deep percolation below the root zone
∆S = Change in the profile soil moisture storage
ET = Evapotranspiration, computed as P – D – ∆S
†Given for tillage system, mulch practice, and tillage x mulch in that order
ns stands for not significant at P ≤ 0.05
* and ** denote significance at P ≤ 0.05 and 0.01 levels of probability respectively
87
the regular rainwater inputs were not adequately accounted for by the notional water budget
equation used for the purpose. Alternatively, the actual FC of the soil might have been below
the theoretical FC used to estimate the drainage component of the water balance equation.
4.5.3 Water use estimated with Blaney-Criddle Equation
The value of seasonal ET obtained using the Blaney and Criddle’s (1950) estimation
model in this environment is 809.9 mm for the sole sorghum and 502.2 mm for the sole
soybean. In comparison with the measured values in both years, the value obtained for
sorghum was a fairly reliable estimate of the crop’s actual WU determined using the water
balance approach in the first year (Fig. 12a). Quantitatively, it was about 136 and 116% of
the measured values in the first and second year respectively, using the moisture depletion
approach; and about 102 and 95% of the measured values in the first and second year
respectively using the water balance approach. In the case of soybean, the estimated value
was quite below the measured values. The estimated value was closest to the value
determined in the first year using the moisture depletion approach (Fig. 12b). Quantitatively,
it was about 87 and 82% of the measured values in the first and second year respectively,
using the moisture depletion approach; and about 74 and 75% of the measured values in the
first and second year respectively using the water balance approach.
According to FAO (1995), sorghum production in West Africa requires around 400500 mm of rain. However, the relative maturity range of a selected variety has the most
impact on seasonal WU (Klocke et al., 1996). Food Security Department (2003) specifically
indicated that the water requirement of the long growth cycle variety is within the range of
950-1100 mm. Therefore, the relatively higher WU values of sorghum, especially with the
water balance method, would partly be associated with the long growth cycle of the latematuring sorghum cultivar used in this study. The implication appears to be that the WU of
this variety of sorghum in this environment would be expected to exceed 500 mm, even
though it may not reach 1000 mm. In the light of the above and the fact that the WU of
sorghum in the first year was not unnecessarily profuse, the mean WU value of 791.4 mm for
sorghum, determined from the components of water balance would be understandable. With
the estimated value of 809.9 mm, the Blaney-Criddle model seems appropriate for the
estimation of WU of sorghum under the local conditions in this environment.
As regards soybean, its water requirement has been shown to be within the range,
450-700 mm (Dupriez and Leener, 1992; Al-Kaisi, 2000; Kranz et al., 2005; FAO AGL,
2006). With the early-maturing soybean cultivar, lower value of total WU would be expected.
88
900
800
SEASONAL ETcrop (mm)
700
600
500
400
300
200
100
MOISTURE
DEPLETION
2006
MOISTURE
DEPLETION
2007
WATER
BALANCE
2006
WATER
BALANCE
2007
BLANEYCRIDDLE
WATER
BALANCE
2007
BLANEYCRIDDLE
SOURCE OF DATA
(a) Sole-cropped sorghum
800
700
SEASONAL ETcrop (mm)
600
500
400
300
200
100
0
MOISTURE
DEPLETION
2006
MOISTURE
DEPLETION
2007
WATER
BALANCE
2006
SOURCE OF DATA
(b) Sole-cropped soybean
Fig. 12: Estimated water use of the sole-cropped sorghum (a) and soybean (b) with
Blaney-Criddle formula in comparison with values obtained using moisture depletion and
water balance approaches
89
Therefore, despite the all-encompassing nature of the above range, values closer to the lower
limit would be more realistic for the early-maturing cultivar used in this study. The estimated
value (502.2 mm) is about 87% of such least measured value (578.9 mm), obtained using the
soil moisture depletion in the first year. Based on the above relationship, the total WU of
soybean could not be considered reliably estimated by the Blaney-Criddle model.
4.4.4 Variations in the crop water use
Sole-cropped sorghum: In the first year, mulch practice significantly (P ≤ 0.05)
influenced WU at 46 DAS, when more water was used under the mulch-applied than the bare
treatments. The greater utilization of soil water under the mulch-applied treatments could be
attributed to deeper and denser rooting (Sow et al., 1997). In the second year, significant (P ≤
0.05) differences in WU at 72 and 101 DAS due to the effect of the mulch treatments
indicated higher values under the bare treatments. These differences in WU on some
sampling dates may have been due to a combination of physical and physiological factors
(Hulugalle and Lal, 1986). In both years, differences in the total WU between the two tillage
systems and between the two mulch practices were, however, not appreciable. Similar
observation as regards the effect of the NT and the CT on the total WU has been reported
elsewhere for sorghum (Mesfine et al., 2005) and for sunflower (Aboudrare et al., 2006), just
as Tolk et al. (1999) reported that mulch had no appreciable influence on the total WU of
maize. However, in the comparatively dry environment of their study, Mesfine et al. (2005)
found that total WU of sorghum was lower under mulch than under bare soil surface.
Effect of the tillage x mulch was evident in the second year. The WU was
significantly (P ≤ 0.05) lower in the CTM than in the rest of the combinations at 72 DAS. As
part of its WU characteristics, sorghum extracts more water as from booting till grain-filling,
and this usually falls in-between 60 and 75 DAS (Laryea and Unger, 1995; Laddha and
Totawat, 1997). This result suggests, therefore, that the sampling at 72 DAS coincided with
the growth stage when the crop was actively transpiring, the extent of which was influenced
by the treatments’ interaction. The differences in WU among the treatment combinations
during this period reflected in the total WU which, though not significant, indicated lower
values in the CTM and NTB than in the CTB and NTM.
Sole-cropped soybean: In both the first and the second year, the tillage systems had
no significant influence on the crop WU at any stage during the growing seasons.
Significantly (P ≤ 0.05) higher WU occurred under the bare compared to the mulch-applied
treatments only once in the second year – at 72 DAS. In both years, the total WU was
90
significantly (P ≤ 0.05) higher under the bare treatments compared to the mulch-applied
treatments. The higher WU under the bare treatments early in the season in this plot was
probably not compensated for by an increase in ET following better vegetative growth under
mulch at a later stage, hence the significant differences in the total WU (Verplancke, 1985).
Intercropped sorghum-soybean: In both the first and the second year, total WU was
significantly (P ≤ 0.05) higher with the NT compared to the CT. Similarly, higher WU with
NT treatment has been reported in semi-arid environments for such other cereal crops as
wheat (Triticum aestivum) (Lawrence et al., 1994) and maize (Zea mays) (Gill et al., 1996).
The mulch practices indicated significantly (P ≤ 0.01) higher total WU under the bare
treatments compared to the mulch-applied treatments only in the second year. In both years,
significant tillage x mulch interaction on the total WU indicated the least value in the CTM,
followed by the NTB in the first year, and the NTM in the second year.
From the foregoing, the tillage systems had appreciable influence on the crops’ WU
only in the intercropped plot. The applied mulch reduced ET in the sole soybean and the
intercropped plots, and was more pronounced in the second year of erratic rainfall. Adequate
surface coverage was the likely reason for the less ET under mulch-applied treatments. The
reduction in evaporation from soil surface when fully and effectively covered by organic
mulch (Klocke et al., 1996; Allen et al., 1998) implies reduction in ET.
Generally, differences in total WU, regardless of the method of computation, were
more between the two years than among the treatments, particularly under the sole sorghum
and the intercrop. The relative wetness of the soil in the two years of the study substantiates
these results. Indeed, the number of times the soil was nearer FC at the time of sampling was
more in the first than in the second year (Figs. 6-8). This suggests that the ET was higher in
the second than in the first year. Hattendorf et al. (1988) indicated that seasonal ET could be
altered not only by cultural practices, but also by weather conditions and variety selection;
and so warned that values from studies of this nature should not be viewed as absolute for the
test crops, but as providing a relative comparison among treatments. Yearly variations in crop
WU are mainly due to changes in climatic variables (Collinson, 1996; Scherer et al., 1996).
As noted earlier, rainfall was poorly distributed in the second compared to the first year (refer
to Table 3), and could be one of the probable reasons for the generally higher WU values in
the former. Jin et al. (2007) found, as confirmed in this case, that rainfall variations had larger
influence on the magnitude of water balance components than did soil management practices.
Moreover, the sorghum cultivar used in this study was sensitive to photoperiod. The
crops were sown a month earlier in the second than in the first year. Craufurd and Qi (2001)
91
remarked that this locally adapted cultivar flowers at the end of the rains irrespective of
sowing date. Consequently, the second year crops stayed longer in the field and had longer
vegetative growth period (including an elongated inductive phase – for the sorghum crop).
This delay in attainment of maturity resulted in profuse ET in the second year.
4.4.5 Cumulative water use
Figs. 13, 14, and 15 show the cumulative values of WU in the sole-cropped sorghum,
sole-cropped soybean, and intercropped sorghum-soybean plots under the tillage x mulch
interaction in the two years of the study. As could be seen from the graphs (Figs. 13-15), the
patterns of WU in both years were alike among the treatment combinations, on one hand, and
between the two approaches used in the field measurement of WU, on the other hand.
The results pictorially show the pattern of steady rise in WU with time in both years.
Across the sampling dates, WU based on water balance approach exhibited a kind of
distinctness among the different treatments in the intercropped plot in both years. However,
in each of the two years of the study, values of the total WU under the sole sorghum and
intercropped plots were comparable. This concurs with the report by Hulugalle and Lal
(1986) that cropping systems had little effect on total WU, based on observation made for
maize and cowpea in western Nigeria. It appears that, provided the PSM storage is not
significantly reduced in the intercropped plot, transpiration by the second crop largely
substitutes for otherwise unproductive soil evaporation (Grema and Hess, 1994). In this
study, the PSM storage in the sole sorghum and intercropped plots indicated comparable
values (Figs. 6 vs. 8). Therefore, since there was no evidence of water stress up to soybean
harvest, around 136 DAS (Fig. 11), the dryland intercropping of sorghum with soybean could
be considered a judicious option in water resource utilization in this environment.
4.6 Components of yield
4.6.1 Plant height and girth: sorghum
The height and girth of the sorghum crop at specific stages of development in the
second growing season are shown on Table 13. In both the sole- and intercropped sorghum,
the tillage and mulch treatments had no significant influence on the plant height and girth. In
the sole sorghum plot, values of the plant height and girth indicated non-significant tillage x
mulch interaction. Similar observation had been reported for corn (Jones et al., 1969). In the
intercropped plot, interaction resulted in significantly (P ≤ 0.05) taller plants at both 81 and
101 DAS in the CTM and/or NTB than in the CTB and/or NTM.
92
800
700
CUM WU (mm)
600
500
Moisture depletion
400
Water balance
300
200
100
0
0
36 46 56 65 73 81 91 101 110 120 130
0
36 46 56 65 73 81 91 101 110 120 130
DAYS AFTER SOWING (DAS)
(a)
900
800
CUM. WU (mm)
700
600
Moisture depletion
500
Water balance
400
300
200
100
0
0
72 81 90 101 110 118 127 136 146 156 166
0
72 81 90 101 110 118 127 136 146 156 166
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
(b)
Fig. 13: Cumulative water use of sorghum under the different treatment
combinations in the 2006 (a) and 2007 (b) growing seasons
DAS: Counted as from 3 July, 2006 (a) and 7 June, 2007 (b)
93
700
600
CUM. WU (mm)
500
400
Moisture depletion
Water balance
300
200
100
0
0
36
46
56
65
73
81
91
101 110
0
36
46
56
65
73
81
91 101 110
DAYS AFTER SOWING (DAS)
(a)
700
CUM. WU (mm)
600
Moisture depletion
500
Water balance
400
300
200
100
0
0
72
81
90
101 110 118 127 136
0
72
81
90
101 110 118 127 136
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
(b)
Fig. 14: Cumulative water use of soybean under the different treatment
combinations in the 2006 (a) and 2007 (b) growing seasons
DAS: Counted as from 3 July, 2006 (a) and 7 June, 2007 (b)
94
800
700
CUM. WU (mm)
600
500
Moisture depletion
400
Water balance
300
200
100
0
0
36 46 56 65 73 81 91 101 110 120 130
0
36 46 56 65 73 81 91 101 110 120 130
DAYS AFTER SOWING (DAS)
(a)
900
800
CUM. WU (mm)
700
600
Moisture depletion
Water balance
500
400
300
200
100
0
0
72 81 90 101 110 118 127 136 146 156 166
0
72 81 90 101 110 118 127 136 146 156 166
DAYS AFTER SOWING (DAS)
NTB
NTM
CTB
CTM
(b)
Fig. 15: Cumulative water use in the intercropped plot under the different
treatment combinations in the 2006 (a) and 2007 (b) growing seasons
DAS: Counted as from 3 July, 2006 (a) and 7 June, 2007 (b)
95
Table 13: Some components of yield of the sorghum crop under the tillage-mulch treatments
at specific stages of development in the second season: plant height and girth
‡Plant height (81 DAS)
‡Plant height (101 DAS)
‡Plant girth (136 DAS)
Sole-cropped sorghum
Treatments
Mulch practice
B
M
Mean
B
M
Mean
B
M
Mean
NT 182.8 165.5 174.1
NT 351.4 354.3 352.8
NT 6.60 6.23 6.42
CT 178.8 195.1 186.9
CT 327.4 363.4 345.4
CT 6.27 6.62 6.45
Tillage
system
Mean
180.8 180.3
Mean
†LSD ns, ns, ns
339.4 358.8
†LSD ns, ns, ns
Mean
6.44 6.43
†LSD ns, ns, ns
Mean
Intercropped sorghum
B
M
Mean
B
NT 186.6 140.7 163.7
NT 316.6 294.4 305.5
NT 7.02 5.53
6.27
CT 162.2 186.6 174.4
CT 286.8 346.1 316.4
CT 5.80 6.00
5.94
B
Mean
M
174.4 163.7
†LSD ns, ns, 22.9**
‡Expressed
Mean
301.7 320.2
†LSD ns, ns, 45.9*
Mean
M
Mean
6.45 5.77
†LSD ns, ns, ns
in centimeters
DAS stands for days after sowing
Notations of the treatments are as explained in the preceding Tables
†Given for tillage system, mulch practice, and tillage x mulch in that order
ns stands for not significant at P ≤ 0.05; * and ** denote significance at P ≤ 0.05 and 0.01
levels of probability respectively.
96
4.6.2 Number of leaves and leaf area: sorghum
Table 14 shows the number of leaves and leaf area of sorghum in the second growing
season. In the sole sorghum plot, leaf area was significantly (P ≤ 0.05) larger with the NT
treatment. Agbede et al. (2008) reported similar observation for sorghum in southwestern
Nigeria. Significant variations existed in the leaf number at 81 DAS and in the leaf area at
136 DAS (P ≤ 0.05 in each case) due to interaction. In both cases, there was a general trend
of decrease in performance in the order, CTM→NTB→CTB→NTM. In the intercropped
plot, neither the tillage systems nor the mulch practices significantly influenced the two
indicators (leaf number and area) of the crop’s performance. However, differences were due
to interaction, in which the overall trend was similar to that on the sole-cropped plot.
In contrast to the above results, maize and groundnut plants have been reported to be
taller with larger leaf area under CT than under NT at the study site (Chinaka, 1998; Anikwe,
2000). On the other hand, maize and cowpea plants were taller with mulch than without
mulch at the study site (Mbagwu, 1991). Likewise, reports from studies conducted elsewhere
indicated that surface-applied mulch resulted in taller plants and larger leaf areas of maize
(Jones et al., 1969) and, specifically, sorghum (Gill et al., 1996; Chiroma et al., 2006).
4.6.3 Plant height and number of leaves: soybean
The plant height and number leaves of the sole- and intercropped soybean indicated
no significant variations due to the tillage and mulch management practices (Table 15). With
respect to tillage, similar results had been reported for soybean outside Nigeria (Kramer and
Alberts, 1988; Rodrigues et al., 2009) and for groundnut in the study location (Obi, 1989).
The interaction was not significant either, except for the plant height at 72 DAS in the
intercropped plot. As with sorghum, plant height on this occasion under the CTM and NTB
was remarkably (P ≤ 0.001) different from that under the CTB and NTM combinations.
Soils under NT had been reported to have cooler temperatures than soils under CT
(Ojeniyi and Adekayode, 1999; Fabrizzi et al., 2005; Dahiya et al., 2007; Sarkar et al., 2007).
Such soils become cooler with surface residue, and often result in slower crop growth during
the early season (Hatfield et al., 2001). This coupled with the tendency of such soil to
become warmer at the later stage (Dalmago et al., 2004), would explain the poor performance
of the intercropped soybean under the NTM treatment combination.
97
Table 14: Some components of yield of the sorghum crop under the tillage-mulch treatments at specific stages of development in the
second season: number of leaves and leaf area
Number of leaves (81 DAS)
Number of leaves (136 DAS)
‡Leaf area (101 DAS)
‡Leaf area (136 DAS)
Sole-cropped sorghum
Mulch practice
Treatments
B
M
Mean
Tillage
system
B
M
Mean
B
M
Mean
B
M
Mean
NT
9.54
7.94
8.74
NT
17.90
16.75
17.32
NT
616
540
578
NT
538
474
506
CT
8.10
9.06
8.58
CT
15.19
17.54
16.36
CT
539
601
570
CT
518
565
541
Mean
8.82
8.50
Mean
16.54
17.15
Mean
578
570
Mean
528
519
†LSD ns, ns, 1.41*
†LSD ns, ns, ns
Intercropped sorghum
M
Mean
B
M
Mean
NT
7.95
6.81
7.38
NT
18.39
15.52
16.95
CT
7.16
7.77
7.46
CT
16.56
17.89
17.23
Mean
7.55
7.29
Mean
17.48
16.70
†LSD ns, ns, 1.06**
B
†LSD 6.7*, ns, ns
†LSD ns, ns, 1.71***
†LSD ns, ns, 85.7*
B
M
Mean
NT
564
453
509
CT
520
575
548
Mean
542
514
†LSD ns, ns, 96.7*
B
M
Mean
NT
560
528
544
CT
509
539
524
Mean
535
534
†LSD ns, ns, ns
‡Expressed in squared centimeters
DAS stands for days after sowing
Notations of the treatments are as explained in the preceding Tables
†Given for tillage system, mulch practice, and tillage x mulch in that order
ns stands for not significant at P ≤ 0.05; *, **, and *** denote significance at P ≤ 0.05, 0.01 and 0.001 levels of probability respectively.
98
Table 15: Some components of yield of the soybean crop under the tillage-mulch
treatments at specific stages of development in the second growing season
‡Plant height (72 DAS)
‡Plant height (101 DAS)
Number of leaves (81 DAS)
Sole-cropped soybean
Treatments
Mulch practice
B
M
Mean
B
M
Mean
B
M
Mean
NT 67.9 72.8
70.4
NT 52.3 53.4
52.9
NT 53.8 53.8 53.8
CT 66.3 72.9
69.6
CT 48.1 55.9
52.0
CT 47.9 55.7 51.8
Tillage
system
Mean
67.1 72.8
Mean
†LSD ns, ns, ns
50.2 54.7
Mean
†LSD ns, ns, ns
50.8 54.7
†LSD ns, ns, ns
Mean
Intercropped soybean
B
M Mean
NT 82.8 66.2
74.5
NT 55.1 56.3
55.7
NT 42.4 44.9
43.7
CT 69.0 83.4
76.2
CT 53.5 57.1
55.3
CT 46.0 46.7
46.3
B
Mean
M
75.9 74.8
†LSD ns, ns, 16.7***
Mean
54.3 56.7
†LSD ns, ns, ns
B
Mean
M
Mean
44.2 45.8
†LSD ns, ns, ns
‡Expressed in centimeters
DAS stands for days after sowing
Notations of the treatments are as explained in the preceding Tables
†Given for tillage system, mulch practice, and tillage x mulch in that order
ns stands for not significant at P ≤ 0.05; *** denotes significance at P ≤ 0.001 level of
probability.
99
4.7 Seed yield (SDY) and total dry matter (TDM)
4.7.1 Sole- and intercropped sorghum
Sorghum SDY and TDM, as well as the harvest index (HI), as affected by the tillage
and mulch treatments in both the sole- and the intercropped plots, are shown on Table 16. As
with total WU, variations in yield were more between the two years than among the
treatments. The ranges for SDY in the sole crop were 0.87-1.41 and 0.32-0.70 Mg ha-1 in the
first and second year respectively. Mean values of 0.88 Mg ha-1 from the study site (Amana,
2008) and 1.18 Mg ha-1 from the forest savanna transition zone (Agbede et al., 2008) have
recently been reported for this crop. In the core sorghum-growing savanna zones of Nigeria,
the commonly obtained mean SDY include 1.12-1.15 Mg ha-1 (Andrews, 1975; Chiroma et
al., 2006). Others in the semi-arid tropics outside Nigeria include a mean value of 1.57 Mg
ha-1 in India (Patil and Sheelavantar, 2006), ranges of 0.6-2.5 Mg ha-1 from a sandy loam in
Burkina Faso (Zougmore et al., 2004) and 2.2-4.7 Mg ha-1 in Ethiopia (Mesfine et al., 2005).
Sole-cropped sorghum: In the first year, the SDY was not appreciably influenced by the
tillage systems. The SDY was significantly (P ≤ 0.05) higher under the mulch than under the
bare treatments. In the second year, significantly (P ≤ 0.05) higher SDY and HI were
obtained with the NT compared with the CT. This result is in good agreement with many
previously reported cases of higher SDY of sorghum with the NT in semi-arid environments
(Allen et al., 1980; Christensen et al., 1994; Norwood, 1994; Laryea and Unger, 1995) and
from another sandy loam soil in western Nigeria (Agbede et al., 2008). It, however, contrasts
with other responses of sorghum SDY to tillage trials in other sandy loam soils in western
India (Laddha and Totawat, 1997) and Tanzania (Guzha, 2004). In interaction, there was
significantly (P ≤ 0.01) lower SDY in the CTB than in the rest of the combinations.
Intercropped sorghum: Main effects of the tillage systems and mulch practices were not
significant. Variations were significant (P ≤ 0.01) due to the interaction, with the SDY and
TDM declining in the order, CTM→NTB→NTM→CTB. The better yield under the NTM in
comparison with the CTB corroborates the results of earlier findings from similar studies
conducted elsewhere with maize (Lal et al., 1978; Lal, 1995) and wheat (Li et al., 2007; Su et
al., 2007). In the second year, the positive effect of mulch was evident (P ≤ 0.05) on the SDY
and HI. Results of the interaction indicated that the highest and lowest yields were obtained
in the CTM and CTB respectively. Mesfine et al. (2005) found that the CTM and NTM were
statistically superior to the NTB and CTB. However, yields in the NTM and NTB of the
present study were comparable, as also found by Agbede and Ojeniyi (2009) in a similar
work with sorghum as the test crop in southwestern Nigeria.
100
Table 16: Seed yield, total dry matter, and harvest index of sole and intercropped sorghum under
the tillage-mulch treatments in the first and second growing seasons
SDY (Mg ha-1)
Year
TDM (Mg ha-1)
HI
2006
Sole-cropped sorghum
Treatments
Tillage
system
Mulch practice
B
M Mean
B
M
Mean
B
M
Mean
NT 1.14 1.21
1.17
NT 19.20 19.00 19.10
NT 0.06 0.07
0.06
CT 0.87 1.41
1.14
CT 15.90 22.20 19.00
CT 0.06 0.06
0.06
Mean 1.01 1.31
Mean 17.50 20.60
Mean 0.06 0.06
†LSD ns, 0.27*, ns
†LSD ns, ns, ns
†LSD ns, ns, ns
Intercropped sorghum
B
M
B
Mean
M
Mean
B
M
Mean
NT 1.34 0.98
1.16
NT 18.14 12.88 15.51
NT 0.07 0.08
0.07
CT 0.87 1.59
1.23
CT 11.71 19.74 15.72
CT 0.07 0.08
0.08
Mean 1.11 1.28
Mean 14.92 16.31
Mean 0.07 0.08
†LSD ns, ns, 0.77**
†LSD ns, ns, 7.91***
†LSD ns, ns, ns
2007
Mean
Sole-cropped sorghum
B
M
Mean
B
NT 0.70 0.63
0.66
NT 12.73 12.78 12.76
NT 0.06 0.05
0.05
CT 0.32 0.58
0.45
CT
CT 0.03 0.04
0.04
B
M
9.52
13.20 11.36
M
Mean 0.51 0.61
Mean 11.13 12.99
Mean 0.04 0.05
†LSD 0.17*, ns, 0.16**
†LSD ns, ns, ns
†LSD 0.01*, ns, ns
Mean
Intercropped sorghum
B
M
Mean
B
M
Mean
B
M
Mean
NT 0.36 0.38
0.37
NT
7.34
5.59
6.47
NT 0.05 0.07
0.06
CT 0.26 0.64
0.45
CT
5.90
8.78
7.34
CT 0.05 0.07
0.06
Mean 0.31 0.51
Mean
6.62
7.19
†LSD ns, 0.14*, 0.22*
†LSD ns, ns, 2.0**
Mean 0.05 0.07
†LSD ns, 0.02*, ns
SDY, TDM, and HI stand for, seed yield, total dry matter, and harvest index respectively
Notations of the treatments are as explained in the preceding Tables
†Given for tillage system, mulch practice, and tillage x mulch in that order
ns stands for not significant at P ≤ 0.05; *, **, and *** denote significance at P ≤ 0.05, 0.01
and 0.001 levels of probability respectively.
101
Yields were reported higher with NT for other cereals in especially the semi-arid
environments (Lawrence et al., 1994; Liang and Wang, 2002; Barzegar et al., 2003) and
elsewhere (Jones et al., 1969; Lal, 1995). In such environments, Buschiazzo et al. 1998)
noted that, with adequate rainfall, no differences in yield would normally occur between NT
and CT. It appears therefore that, as found Dao and Nguyen (1989), the NT system shows the
greatest response in growth and yield of crops under unfavourable growing conditions. In the
present study, the yield advantage of the NT over the CT was substantial only in the second
year – when rainfall distribution was rather erratic. Similarly, some other authors (Al-Darby
and Lowery, 1984; Azooz and Arshad, 1998) found yields to be significantly higher with NT
compared to CT only in a comparatively dry year. In their study with barley in semi-arid
Spain, Bescansa et al. (2006) found, similar to this case, that the NT out-yielded the CT only
in the driest year out of a five-year period. The enhanced yield under the NT in the second
year might, therefore, be due to the ability of the system to provide protection against short
duration droughts by contributing to more resourceful utilization of moisture through deeper
rooting in the soil (Jones et al., 1969; Blevins et al., 1971; Webber et al., 1987). Improved
yield with NT has also been linked with greater moisture accretion deeper in the profile
(Norwood, 1994), alongside lower soil water evaporation and the associated maintenance of
better plant water status in the field (Lindwall et al., 1994; Radford et al., 1995; Rathore et
al., 1998; Moroke et al., 2005; De Vita et al., 2007). The lower soil temperatures commonly
associated with the NT early in the season (Dalmago et al., 2004; Anikwe and Ubochi, 2007)
must have contributed to minimizing desiccation to the benefit of the crop.
The only observed enhancement in PSM storage with the NT occurred in this sole
sorghum plot in the second year (Fig. 6), and was due mainly to the fact that the moisture
advantage of the NT over the CT was not limited to the deeper soil layer. Indications were
that this trend equally obtained in the crop’s entire root zone early in the season, when PSM
monitoring had not commenced; and this must have contributed to the greater yield with the
NT. In deed, NT encourages moisture storage at the beginning of the growing seasons
(Hamblin and Tennant, 1981; Opara-Nadi, 1993; Lopez et al., 1996; Watts et al., 1996). At
that stage, the crops were still too tender for the roots to have extended beyond 30 cm depth,
and so relied more on the moisture retained within the topsoil region. The sampling period
marked when the rhizosphere was no longer confined within the 0-30 cm depth zone and
when, incidentally, moisture content within the 30-50 cm layer tended to be higher with the
NT treatments. The period was inclusive of the critical growth stage when the positive effect
of NT on PSM had been found pronounced (Fabrizzi et al., 2005), and too the later part of the
102
season when sorghum roots were found to grow deeper in the profile due to favourable soil
moisture under NT (Zaongo et al., 1994; Moroke et al., 2005). In that case, more water and
nutrients extraction may also have taken place during the sampling period.
The positive effect of mulch manifested in the SDY of sorghum. Several authors have
reported similar increases in yield with mulch for sorghum (Bhaska, 1985; Eagleton et al.,
1991; Mesfine et al., 2005; Chiroma et al., 2006) and for other cereals (Jones et al., 1969;
Mbagwu, 1991; Lal, 1995; Tolk et al. 1999; Ghuman and Sur, 2001). Overall, higher yields
were obtained in the CTM compared to the CTB, with intermediate values in the NTB/NTM.
This reflected the moisture storage patterns under these treatments (Fig. 6). The observed
enhancement of yields with mulch would explain the divergence in yield between the two CT
treatments (CTM and CTB). On the other hand, the yields under the two NT treatments
(NTM and NTB) were comparable. These results imply that use of the CT for the growing of
sorghum in this environment requires that mulch be applied, so as to boost the yield; but with
the NT, mulch application may not be necessary.
4.7.2 Sole- and intercropped soybean
The SDY, TDM, and HI of sole- and intercropped soybean under the tillage-mulch
treatments are shown on Table 17. Variations were, as with sorghum, greater between the two
years than among the treatments. The SDY ranged from 0.71 to 0.81 Mg ha-1 in the first year
and from 1.22 to 1.91 Mg ha-1 in the second year. The range in the first year approximated to
the mean (1 Mg ha-1) in the semi-arid tropics (Wani et al., 2007) and a range of 0.7-0.9 Mg
ha-1 recently reported from a sandy loam soil in southwestern Nigeria (Lasisi and Aluko,
2009). The second year yields approximated to the world average (1.4 Mg ha-1) (Jain, 1988)
and are comparable with the SDY results (range, 1-2 Mg ha-1) reported from a trial involving
26 varieties in the southern guinea savanna of Nigeria (Akande et al., 2007). Tillage system
had no significant effect on the yield in either year. Soybean has similarly been reported to be
non-responsive to tillage systems (Kramer and Alberts, 1988; Wilhelm and Wortmann, 2004;
Alvarez and Steinbach, 2009; Koga and Tsuji, 2009; Rodrigues et al., 2009). Kramer and
Alberts (1988) concluded, after a six-year study, that tillage systems had no significant effect
on SDY of soybean. However, this result differs from that reported for soybean by Lasisi and
Aluko (2009), the similarity in soil textural and climatic attributes of the present location with
theirs notwithstanding. The differences are attributed to the fact that, unlike the manual CT in
this study which rarely exceeded 20 cm, Lasisi and Aluko (2009) used tractor-mounted
implements to plough and harrow to a greater soil depth.
103
Table 17: Seed yield, total dry matter, and harvest index of sole and intercropped soybean
under the tillage-mulch treatments in the first and second growing seasons
SDY (Mg ha-1)
Year
TDM (Mg ha-1)
HI
2006
Sole-cropped soybean
Treatments
Tillage
system
Mulch practice
B
M Mean
B
M
Mean
B
M
Mean
NT 0.71 0.81
0.76
NT 2.23 2.38
2.30
NT 0.32 0.34
0.33
CT 0.71 0.81
0.76
CT 2.22 2.49
2.35
CT 0.32 0.32
0.32
Mean 0.71 0.81
Mean 2.22 2.43
Mean 0.32 0.33
†LSD ns, ns, ns
†LSD ns, ns, ns
†LSD ns, ns, ns
Intercropped soybean
Mean
B
NT 0.31 0.27
0.29
CT 0.23 0.26
0.25
B
M
M
Mean
B
M
NT 1.02 0.93
0.98
NT 0.30 0.30
0.30
CT 0.72 1.03
0.87
CT 0.32 0.27
0.30
Mean 0.27 0.26
Mean 0.87 0.98
Mean 0.31 0.29
†LSD ns, ns, ns
†LSD ns, ns, ns
†LSD ns, ns, ns
Mean
2007
Mean
Sole-cropped soybean
B
M Mean
NT 1.54 1.91
1.72
NT 3.07 3.47
3.27
NT 0.50 0.55
0.52
CT 1.22 1.73
1.48
CT 2.37 3.53
2.95
CT 0.51 0.49
0.50
B
M
B
M
Mean 1.38 1.82
Mean 2.72 3.50
Mean 0.51 0.52
†LSD ns, ns, ns
†LSD ns, 0.61*, ns
†LSD ns, ns, ns
Mean
Intercropped soybean
B
M
Mean
B
NT 0.68 0.65
0.66
CT 0.58 0.71
0.64
M
Mean
B
M
NT 1.39 1.39
1.39
NT 0.51 0.50
0.51
CT 1.04 1.69
1.36
CT 0.56 0.45
0.51
Mean 0.63 0.68
Mean 1.22 1.54
Mean 0.54 0.48
†LSD ns, ns, ns
†LSD ns, 0.27*, 0.47*
†LSD ns, ns, ns
SDY, TDM, and HI stand for, seed yield, total dry matter, and harvest index respectively
Notations of the treatments are as explained in the preceding Tables
†Given for tillage system, mulch practice, and tillage x mulch in that order
ns stands for not significant at P ≤ 0.05; * denotes significance at P ≤ 0.05 level of probability.
Mean
104
In this plot, tillage-induced significant differences in moisture contents within the
monitored profile indicated higher values under the CT than under the NT system in the 0-20
cm plough depth zone, and vice versa in the 30-50 cm layer. These differences offset each
other when the entire monitored profile was considered, resulting in nominal variations in
moisture storage on all sampling dates. This, coupled with the shallow-rooting nature of
soybean, was presumably the reason why the effect of tillage systems on soil moisture could
not reflect in its yield. Hakansson and von Polgar (1984), who observed similar situation,
showed how those differences could eliminate variations in yield responses on seedbeds.
Some previous studies (Kanwar, 1989; Tessier et al., 1990; Aboudrare et al., 2006; Anikwe
and Ubochi, 2007) indicated that changes in soil moisture content in response to tillage were
not of the magnitude to influence crop yield.
The positive effect of mulch was significant (P ≤ 0.05) only on the TDM in both the
sole-cropped and intercropped plots in the second year. Mbagwu (1989) reported higher dry
SDY of cowpea with mulch in each year of a two-year study in this environment. The TDM
from the intercropped plot was significantly influenced by the tillage x mulch interaction in
the second year. The highest and lowest yields were from the CTM and CTB respectively.
Notably, yield from the NTM approximated to that under the CTM and NTB probably due to
the high moisture storage under the NTM in this plot (Fig. 7). Similar enhanced storage under
the NTM was shown to be in the topsoil region of a similar-textured (sandy loam) soil and
was instrumental to better performance of maize (Gicheru et al., 2004).
4.7.3 Overall trend in yield and the allied factors
In the overall analysis, the average yields of sorghum and soybean could be said to
have been enhanced with the NT, even though cases of values being statistically the same
with the CT were noted. Similar observations had been reported not only for sorghum and/or
soybean (Lal et al., 1989; Thiagalingam et al., 1996; Buschiazzo et al., 1998; Norwood,
1999), but also for other cereals and legumes (Moreno et al., 1997; Ojeniyi and Adekayode,
1999; De Vita et al., 2007; Martinez et al., 2008). The significantly higher yield of sorghum
under the NT could be linked to the tendency for compensatory growth of the roots later in
the season in response to moisture depletion (Moroke et al., 2005), since soil moisture tended
to increase with depth under the NT. This may not be evident with soybean, since the crop
was harvested before the cessation of rains.
The tillage x mulch interaction indicated that the CTM, NTM, and NTB generally
enhanced the yields of sorghum and soybean over the CTB. However, the CTM gave the best
105
yield of sorghum in the intercrop, followed by the NTB and/or NTM. The yield advantage of
the CTM concurs with some previous results obtained elsewhere for corn (Jones et al., 1969;
Gill et al., 1996), sorghum (Mesfine et al. 2005), and wheat (Jin et al., 2007). The positive
response to the CTM could be linked to the favourable interplay between rainwater supply
and evaporative demand during the growing season (Gill et al., 1996). In general, the
enhanced SDY under the two NT treatments with and without mulch (NTM and NTB,
respectively) is an indication of the superiority of NT over CT – which enhanced the crops`
yield only with mulch. Higher yield with the NTM could be related primarily to increased
moisture storage and lower evaporation rates due to moderate surface soil temperatures. This
has been reported specifically for soybean (Doran et al., 1984) and other crops (Hill, 1990;
Baumhardt and Jones, 2002a). The enhanced SDY under the NTM supports some previous
trials with different crops in both the present location (Mbagwu, 1990; Obi and Nnabude,
1990) and other locations (Lal, 1983; Kamara, 1986; Alhassan et al., 1998).
The enhanced sorghum SDY under the NTB has been postulated to result from two
independent, but desirable, attributes of the crop; first, its preferential tolerance of moisture
stress and conditions of excessive temperature at the soil surface (Doran et al., 1984), and,
second, improved access to the moisture stored deeper in the profile and reduced evaporation
near the surface (Moroke et al., 2005). Such a condition of increased soil moisture deeper in
the profile – as well as in the entire storage – under the NTB was applicable to sorghum in
this study (Fig. 6). In relative terms, the enhanced SDY of soybean under the NTB defies any
plausible explanation, more so as there was no similar increased storage of moisture in the
profile under the NTB of the soybean plot (Fig. 7). On the other hand, the very poor yield in
the CTB is consistent with some other earlier findings in the tropics (Lal, 1983; Kamara,
1986), and specifically in this location with the same set of treatments (Mbagwu, 1990).
Some other factors might help to explain the yield benefit of the NT over the CT. One
of such is the density of earthworm casts under these two tillage treatments. The NT system
encouraged better earthworm activities compared to the CT (Table 6). Earthworms, through
their burrowing activities, leave behind durable pathways in the soil, which ensure effective
infiltration of rainwater and better conductivity (Lal et al., 1980; Moreno et al., 1997), as well
as enhanced moisture storage (Hangen et al., 2002). This could partly account for the yield
advantage of the NT over the CT. Furthermore, it had been shown that the nutrients content
of worm casts was much higher than those of the original soil (ATTRA, 1999), being
enriched in exchangeable bases, organic matter, base saturation and CEC (Oyedele et al.,
106
2006). Probably connected to the better fertility status, Oyedele et al. (2006) found that the
casts had higher silt and clay contents and Fe-Al oxyhydroxide than the topsoil.
The fact that the tillage x mulch interaction produced, as stated earlier, remarkably
high yields in the NTM in the sole-cropped soybean plot may find explanation in the positive
contribution of earthworm casts to crop yields. The highest and the lowest densities of worm
casts were found on the sole soybean and sole sorghum plots respectively (Table 6). ATTRA
(1999) found that the number of worms on a soybean plot had been between six and seven
times larger than that on a maize plot. Additionally, increase in earthworm activity commonly
observed under mulch cover (Lal et al., 1980; Mbagwu, 1990; Birkas et al., 2004; Brevault et
al., 2007) manifested only in the sole-cropped soybean plot. The preference of soybean plot
by earthworms and the additional role of mulch on earthworm activity imply that the NTM
(NT with mulch) has much greater potentials for enhancing soybean yield.
Higher yields with mulch application, as observed in the sole-cropped plots, appear a
common agronomic result for many crops in the tropics (Verplancke, 1985; Mbagwu, 1990;
Lal, 1995; Moitra et al., 1996; Sharma and Acharya, 2000; Nwokocha et al., 2007),
particularly in the semi-arid sub-region (Adetunji, 1990; Gill et al., 1996; Sow et al., 1997;
Ramalan and Nwokeocha, 2000; Gicheru et al., 2004; Li et al., 2005; Mesfine et al., 2005;
Chiroma et al., 2006; Chakraborty et al., 2008). It has also been reported outside the tropics
(Cook et al., 2006; Sarkar and Singh, 2007; Sarkar et al., 2007). In the present study, the well
established positive influence of mulch on the moisture status of soils was evident. In fact,
the higher yield of the less drought-tolerant soybean in the second year – when rainfall
distribution was more erratic – than in the first year could be a manifestation of the effect of
better moisture conservation with mulch. As also attested to by other workers (Rathore et al.,
1998; Anikwe and Ubochi, 2007), the moisture-conserving benefit of mulch was more
pronounced in the profiles of soils under the treatment during the early growth period when
deterioration of the mulch materials had not set in. Rathore et al. (1998) observed that the
subsequent release of this conserved moisture ameliorated and regulated the soil
hydrothermal regimes, thereby improving the soil conditions for better plant water status and
the associated favourable root growth. In this present study, the most likely mechanisms
behind the higher yields with mulch were moisture-enhanced proliferation of roots and the
associated availability of nutrients (Arora et al., 1991; Gill et al., 1996; Selvaraju et al., 1999;
Sarkar and Singh, 2007). Since diversification of the root zone helps crop plants to use water
and nutrients from the subsoil more efficiently (Arora et al., 1991), better crop development
and yield under mulch would be understandable.
107
4.7.4 Yield under sole- and intercropping
Notably, SDY of sorghum in the sole-cropped plot was generally lower than that in
the intercropped plot in the first year and vice versa in the second year (Table 16). This lends
credence to the observation that crop yields may be affected by other soil and cropping
factors more than the changes in PSM due to tillage (Agriculture and Food, 2004). In
Ethiopian highlands, Astatke et al. (1995) similarly reported that the grain yields of wheat
and maize were higher when intercropped with legumes than as sole crops, and attributed the
observation to the absence of external N input. In the same manner, the present study was
conducted without any external source of N. Such other factors as low P availability have
also been implicated (Grema and Hess, 1994). This may have been the case here, even if
partly, especially with soybean (whose yield was not appreciably depressed) being highly
dependent on P (Buschiazzo et al., 1998). According to Akunda (2001) who obtained similar
results with sorghum and soybean, the latter appeared to be comparatively more amenable to
intercropping than the former.
The first year was the year of comparatively higher yield of sorghum. This manifested
more in the intercrop, probably due to sharing in the nitrogen fixed by the soybean
component, as well as the height advantage of the sorghum component (see Plate 2b), which
accorded the crop more direct access to solar radiation. The order was, however, reversed in
the second year, when the yield of soybean was comparatively high. The performance of
soybean in the intercrop was poor, despite its population advantage in the intercropped plot
and the residual nodulation effect. Similar observation was attributed to decreased intensity
of solar radiation due to partial shading of the crop, when intercropped with maize (Dalal,
1977) and sorghum (Mohta and De, 1980). More recently, Gilbert et al. (2003) demonstrated
substantial reduction in transmission of photosynthetically active radiation to cowpea
intercropped with tall sorghum.
4.7.5 Instability of yield in the two cropping seasons
The data on Tables 16 and 17 clearly depict that yield of sorghum was much higher in
the first than in the second year and vice versa for soybean. Averaged over the treatments,
there were about 52 and 66% reduction in the sole- and intercropped sorghum SDY
respectively, in the second year compared to the first year; and about 53 and 59% increment
in the sole- and intercropped soybean SDY respectively, in the second year compared to the
first year. Since the crop management was the same in both years, the observed variations in
crop yields may be the result of differences in environmental conditions during critical
108
growth stages (Laddha and Totawat, 1997). Above all, the better rainfall distribution in the
first compared to the second year, earlier stated (Table 4), could largely explain the difference
in sorghum yield between the two years. The critical grain-filling stage (the stage of peak
water demand which falls in the mid season) coincided with the short break in rainfall in this
area in the second, but not in the first year (Table 4). This resulted in short dry spell and,
consequently, drastic yield reduction. Lack of rain at flowering and grain filling had similarly
been shown to decrease SDY of sorghum grown in a dryland environment (Tewolde et al.,
1993). Some other workers (Mesfine et al., 2005; Stone and Schlegel, 2006) have found that
sorghum yields vary considerably between years and show a close dependence on rainfall.
The closeness of the HI values (0.06-0.08) in the sole and intercropped sorghum in the first
year, compared to that (0.03-0.07) in the second year is an indication of less moisture stress
in the former than in the latter (Oluwasemire et al., 2002).
The lower sorghum yield in the second year could be partly due to planting earlier
that year. It has already been noted that planting earlier than normal time resulted in longer
vegetative growth period in the second year; a phenomenon that has been acknowledged to
reduce yield (Laddha and Totawat, 1997). Besides, it has been documented that the most
successful production of sorghum is assured only on a fertile land that has not been cropped
with it the previous year (ICS-Nigeria, 2003a). So, the drastic reduction in yield could be
accounted for partly by the plot having been cropped to sorghum the previous year. This
result shows that, as remarked in Agriculture and Food (2004), such other factors as cropping
history may mask the effect of tillage-induced soil moisture on crop yields.
Another factor that might have contributed to the better yield in the first year was
more favourable soil reaction. Zaongo et al. (1994) reported that root penetration of sorghum
was controlled by soil pH. The mean soil pH was 6.6 before cropping in the first year and 5.4
immediately after the study. Based on the recommended 6.5 for optimum yields of sorghum
(Hagan, 2008), the soil reaction was favourable to the crop in the first year.
The reverse was the case with soybean. Specifically, yield was higher in the second
than in the first year. Soybean performs poorly in the first year of establishment on a plot not
cropped to it for some three years, due to the absence of residual effect of nodulation
(Kratochvil et al., 2006). It could be, therefore, that the yield benefit of this effect in the
second year compensated for the seemingly unfavourable climatic factor, which has also been
reported to have less influence on soybean yield (Wilhelm and Wortmann, 2004). Besides,
soybean received more rainfall during pod-filling (the stage of peak water demand which
falls in the crop development stage) in the second than in the first year (Table 4). In addition,
109
the time of planting in the second year seemed more suitable for the crop in the study area. In
terms of soil reaction, the desirable soil pH range for optimum soybean yields is 5-6 (FAO
AGL, 2006; Kratochvil et al., 2006). Therefore, the pH of the soil (which indicated to be 6.6
before cropping in the first year and 5.3 immediately after the study) was more favourable for
the crop in the second year. These considerations notwithstanding, crop yields are normally
determined by many other factors.
4.8 Yield-water use relationship: water use efficiency
Table 18 shows the WUE of sole-cropped sorghum and soybean in the two growing
seasons. In the first year, significantly (P ≤ 0.05) higher value was obtained under the mulchapplied treatments (CTM and NTM) than the bare treatments (NTB and CTB) in the sole
sorghum plot. Similar observations have been reported for some crops in the humid tropics
(Verplancke, 1985), and from other sandy loam soils in both sub-humid (Moitra et al., 1996;
Sarkar and Singh, 2007) and semi-arid (Chiroma et al., 2006) environments. Tolk et al.
(1999) attributed the positive effect of mulch on WUE to more beneficial utilization of soil
water for crop growth. Chiroma et al. (2006) attributed similar observation to the cumulative
effects of improved soil physical conditions and greater storage of rainwater in the profile,
which created an environment conducive for sorghum growth. In the second year, WUE was
influenced by the tillage systems in the sole sorghum plot. The NT treatments (NTB and
NTM) gave significantly (P ≤ 0.01) higher values than the NT treatments (CTM and CTB).
The higher WUE with NT stemmed from the protection it had provided against short duration
droughts (Jones et al., 1969; Blevins et al., 1971), probably due to the observed increase in
soil moisture with depth under the treatment. Radford et al. (1995) noted that this often
obtains under appropriate soil and crop management practices.
In interaction, the WUEs decreased in the order, NTB→NTM→CTM→CTB; only
that under the CTB was statistically (P ≤ 0.01) different. Higher WUE under the NTM than
under the CTB, as in this case, has also been reported by other workers (Lal et al., 1978;
Mesfine et al., 2005; Li et al., 2007; Sarkar et al., 2007; Su et al., 2007). The WUEs under
the NT treatments (NTB and NTM) were not appreciably different from the values under
CTM, probably due to the effect of mulch. The CTM was superior to the NTB in a similar
work with sorghum under a semi-arid condition in Ethiopia (Mesfine et al., 2005), a pointer to
the enormous potentials of the combination of CT with mulch in enhancing WUE of the crop.
In the sole soybean plot, tillage-induced differences in WUE were not significant.
Similar results have been reported for yellow sarson (Brassica rapa L.) (Moitra et al., 1996)
110
Table 18: Water use efficiency of sorghum and soybean under the tillage-mulch treatments in the 2006 and 2007 growing seasons
Sole-cropped sorghum
Year
WUE (kg ha-1 mm-1)
Sole-cropped soybean
Intercropped sorghum and soybean
2006
Treatments
Tillage
system
Mulch practice
B
M
Mean
Mulch practice
B
M
Mean
NT
1.45
1.57
1.51
NT
1.02
1.23
1.13
NT
2.36
1.66
2.01
CT
1.10
1.78
1.44
CT
1.03
1.20
1.11
CT
1.54
2.89
2.21
Mean
1.27
1.68
1.48
Mean
1.03
1.21
1.12
Mean
1.95
2.27
2.11
†LSD ns, 0.32*, ns
2007
Treatments
Tillage
system
Mulch practice
B
M
Mean
†LSD ns, ns, ns
B
M
Mean
NT
0.85
0.72
0.78
CT
0.37
0.69
Mean
0.61
0.71
†LSD 0.13**, ns, 0.18**
†LSD ns, ns, 0.84**
B
M
Mean
NT
2.25
2.87
2.56
0.53
CT
1.81
2.62
0.66
Mean
2.03
2.74
†LSD ns, 0.57*, ns
B
M
Mean
NT
1.26
1.25
1.25
2.22
CT
1.01
1.86
1.44
2.39
Mean
1.14
1.55
1.35
†LSD ns, 027**, 0.38**
WUE: Water use efficiency
Notations of the treatments are as explained in the preceding tables
†Given for tillage system, mulch practice, and tillage x mulch in that order
ns stands for not significant at P ≤ 0.05; * and ** denote significance at P ≤ 0.05 and 0.01 levels of probability respectively.
111
and barley (Hordium vulgare L.) (Sarkar and Singh, 2007). In the first year, the effect of
mulch was not significant. The dense canopy structure of the crop (see plate 2a), which
provided adequate ground cover that seemingly compensated for the absence of mulch in the
bare treatments, may help to explain this observation. The role of mulch in the enhancement
of WUE was appreciable (P ≤ 0.05) in the second year. In the intercropped plot, the mulchapplied treatments (CTM and NTM) resulted in significantly (P ≤ 0.01) higher WUEs than
the bare treatments (NTB and CTB) in the second year. Variations due to the interaction
indicated higher values in the CTM and/or NTB than in the NTM and/or CTB in both years.
The range of WUE values for sorghum was 0.37-1.78 kg ha-1 mm-1, with the lowest
and highest values obtained respectively in the CTB in the second year and in the CTM in the
first year. This was quite below a range of 1.95-2.74 kg ha-1 mm-1 reported by Chiroma et al.
(2006) from northeast Nigeria, in which the lowest and highest values were obtained from the
equivalents of the CTB and CTM combinations respectively. The range of WUE values for
soybean in the second year (when yield of the crop was higher) was 1.81-2.87 kg ha-1 mm-1.
This compares favourably with the range (2.00-2.35 kg ha-1 mm-1) reported for the crop under
similar management and dryland conditions elsewhere (Varvel, 1995; Norwood, 1999).
Generally, the WUE values were quite low and variable between the two years of the
study. According to Gibson et al. (1992), low WUE values could be due to low soil moisture
storage at early growth stage. On the other hand, variability could be due to difference in
rainfall between the two years of study (Mesfine et al., 2005). The variations were greater for
sorghum than for soybean. The variable nature of the result is a common problem with WUE
studies (Hatfield et al., 2001), especially with sorghum (Unger, 1991). In this case, the ET
values under the treatments were comparable. This suggests that higher values of water than
actually used consumptively by the crops was used in the computation of WUE; or else, other
yield-limiting factors, besides water, were responsible for these variations. Modification of
the soil surface (through the tillage and mulch application) appears to have much greater
impact on profile moisture storage than on fertility status of the soil. According to Tilander
and Bonzi (1997), enhancement of moisture storage in the soil does not always guarantee
high yield, particularly under conditions of nutrients being more limiting than moisture. As a
consequence, combination of moisture conservation practices with nutrient management
practices could be an interesting approach to improving water retention in the soil and WUE
of crop plants. Hatfield et al. (2001) remarked that moisture and nutrient management could
increase WUE by 25-40 and 15-25% respectively. Increased sorghum WUE with integration
of the two practices (more when N source was organic and less when inorganic) has been
112
reported under semi-arid condition (Oluwasemire et al., 2002; Zougmore et al., 2003; Patil
and Sheelavantar, 2006). The improved moisture availability with organic source of N under
such condition (Gicheru et al., 2004) would, under the enhanced fertility platform, increase
WUE appreciably.
4.9 Responses to intercropping
A comparison of the WUE in the intercrop, obtained using Morris and Garrity’s
(1993) guideline, with that in the sole crops (Table 18), would answer the question of
whether intercropping sorghum and soybean was beneficial with respect to water resource
utilization. The information on Table 18 shows that in the first year, the WUE in the intercrop
was generally higher than that in each of the sole crops. One of the benefits of intercropping,
especially a cereal and a legume, is higher WUE in relation to sole cropping, provided soil
moisture is not limiting (Kurt, 1984; Hulugalle and Lal, 1986; Morrity and Garrity, 1993;
Grema and Hess, 1994; Ibeawuchi, 2007). According to Hulugalle and Lal (1986),
productivity is increased in intercropping due to phase difference in periods of peak demand
for natural resources by the component crops. Increased crop yield in intercrop has also been
attributed to efficient use of solar radiation and potential compensatory growth from vagaries
of the environment, among others (Akunda, 2001).
In the second year, WUE with intercropping was higher than that under sole sorghum,
but lower than that under sole soybean. In part, this could be due to the observation that the
total ET in the intercropped plot was comparable to that in the sole sorghum plot, but higher
than that in the sole soybean plot. The variations in yield between the two years of the study
and between sole- and intercropping systems, earlier stated, may also partly explain the
generally lower WUEs under the intercrop compared with the sole soybean. However, when
the mean of the WUEs of sorghum and soybean in the sole crops was considered as their
combined WUE, the values became comparable with those in the intercrop; only CTM
treatment still resulted in higher WUE with intercropping. This is, with respect to efficiency
in the use of soil water resource in this environment, a pointer to the superiority of the CTM
treatment over the rest of the tillage-mulch combinations.
Intercrop productivity evaluated based on LER (a measure of the relative yield
advantage in terms of land space) showed that the values under the NTB, NTM, CTB, and
CTM were respectively 1.61, 1.14, 1.32, and 1.45 in the first year and 0.96, 0.94, 1.29, and
1.51 in the second year. The respective values based on ATER (a modification of LER which
takes time into consideration) were 1.49, 1.05, 1.23, and 1.36 in the first year and 0.97, 0.98,
113
1.33, and 1.61 in the second year. For both approaches, any value ≥ 1 signals yield advantage,
but a value of < 1 is yield disadvantage (Kurt, 1984; ATTRA, 1998). Apart from the NT
treatments (NTB and NTM) in the second year when the values were below 1, values
obtained from both approaches indicate consistency and reveal overall more benefit with the
system. The failure of intercropping to be justified under the NT in the second year was due
to the generally higher SDY with the NT in the sole-cropped sorghum and soybean plots.
This positive effect of NT on the second year SDY was significant in the case of the solecropped sorghum, but was probably negated by some other factors in the intercrop (Table
12). Consequently, the LER and ATER values were lower than one with the NT treatments in
the second year, implying that no benefits were due to intercropping on the NT that year.
However, taking the average of the four treatment combinations, the mean LER
values were 1.38 and 1.18 in the first and second year respectively. This implies that each of
the sole-cropped sorghum and soybean requires 38 and 18% more land than the intercrop to
produce equal yields in the first and second year respectively, an indication of greater land
use efficiency of intercrops than sole crops. Similar cases of land use advantage in terms of
LER (range, 1.04-1.57) have been reported upon intercropping soybean with sorghum
(Olufajo, 1995; Akunda, 2001) or with sunflower (Olowe et al., 2006) in different parts of
Nigeria. Yield advantage of the intercrop over the sole-crops has been attributed to the spatial
and temporal complementary use of water and nutrient sources by the intercrop components
(Willey, 1990; Agegnehu et al., 2006). The observation that intercropping assessment by the
LER approach was in good agreement with that by the ATER approach, which places much
emphasis on the value of time lag between harvest of intercrop component crops, is an
indication of the suitability of intercropping sorghum and soybean in this locality.
114
CHAPTER FIVE
SUMMARY, CONCLUSIONS AND RECOMMENDATIONS
An investigation was carried out to evaluate the increasingly popular NT with the
more commonly practised CT, each with surface bareness and mulch, as regards moisture
conservation, WU, growth, and yield of two test crops (sorghum and soybean) in Nsukka
agroecological zone. This was with the objective of establishing the effects of these tillagemulch practices on the soil physicochemical properties (including PSM storage) and the most
suitable management combination for enhancing the WUE of these crops in the zone.
In the entire monitored profile, there was a general tendency of loss of the moisture
advantage of the CT over the NT – and eventually higher value under the latter compared to
the former – with depth. This was most prominent in the sole soybean plot, where the CT and
the NT systems resulted in higher moisture contents in the 0-20 cm and in the 30-50 cm depth
zones respectively. Consequently, no differences in PSM storage were found between the two
tillage systems on many – and, in the case of the sole soybean plot, none – of the sampling
dates. The moisture-conserving attribute of mulch was most evident in the plough depth zone.
The effect more pronounced in the second year, in which rainfall distribution was
comparatively erratic. In the interaction, the CTM consistently enhanced PSM storage. The
values under the CTM were comparable to those under the NTB (in the sole sorghum plot)
and under the NTM (in the sole soybean plot). The NTB and NTM were fairly equal to each
other with respect to PSM storage in the intercropped plot.
The values of total WU measured from the components of water balance were
generally higher than those determined by soil moisture depletion. The values indicated
higher consumptive use of water by sorghum than by soybean, and in the second than in the
first year, regardless of method of determination. Total WU was influenced by the tillage
systems only in the intercropped plot, where the value was lower with the CT in both years.
Mulch lowered the total WU of soybean (in both years) and the intercrop (in the second year).
Significant interaction in the intercropped plot indicated that lower total WU occurred in the
CTM in both years. It appeared that rainfall variations had an overriding influence on the soil
management practices, with respect to their effects on the other components of water balance,
including ET. The mean total WU of the sole sorghum and the intercrop were comparable.
The Blaney-Criddle model reliably estimated the total WU of sorghum, as determined from
the components of water balance during the first growing season. For soybean, the estimated
total WU was lower than the measured values. Since fairly reliable estimate of the ETcrop of
sorghum was obtained, the model seems appropriate for the crop in this environment.
115
Yields of sorghum and soybean varied appreciably in the two years. The NT was, no
doubt, superior to CT with respect to the yields of sorghum but not soybean. Likewise, yields
were apparently higher with mulch than with bare soil. For all the measured yield parameters,
the interaction indicated a trend similar to that in PSM storage. However, even though the
NTB had more moisture storage than the NTM in the sorghum plot, and vice versa in the
soybean plot, neither of the two NT treatments was necessarily superior to the other in terms
of grain yield. The grain yields of both sorghum and soybean under the CTM, NTB and NTM
were comparable; only for the intercropped sorghum was the CTM superior to the rest. In
most of the cases, the CTB gave the poorest yield of the crops. As with yield, NT enhanced
the WUE of sorghum but not soybean. The applied mulch enhanced the WUE of sorghum in
the first year and of soybean in the second year. Based on WUE consideration, the CTM and
the CTB proved to be the most and least sustainable combinations, particularly in the
intercrop. The soil conditions in the sole- and intercropped plots were generally comparable.
Yet, intercropping proved to be economical in the utilization of land and water resources. It
is, thus, a profitably attractive cropping system for the test crops in this agroecological zone.
On the coarse-textured and structurally fragile soils common in this and similar
environments, the CT system should always be combined with mulch (the CTM), as this has
been found to maximize rainwater resource to enhance yield and WUE. However, the
combination is obviously burdensome, considering the cost of CT and that of procurement
and transport of mulch material. So, other good soil and water conservation practices that
could produce similar effects as the CTM may be preferred. In particular, the NTB and the
NTM are viable and yet promising options for conserving soil moisture under sorghum and
soybean respectively. The advocacy for NT is informed by the observation, that some key
agronomic variables (including earthworm activity, moisture contents below the plough
depth, and yields) were enhanced with the system. These practices have an additional appeal
to the farmer in that they entail very low land preparation cost – and this makes it irresistible.
It is, therefore, recommended that the NTB be adopted for growing not just sorghum and
soybean, but also other related cereals and legumes. The NTM may be used only when it is
absolutely desirable to enhance moisture storage under soybean, in which case the additional
task of mulch application involved with the system becomes justified. In sorghum-soybean
intercropping, however, the CTM may be more appropriate, especially if the sorghum
component of the system is the farmer`s base crop. Long term studies may be further needed
to ascertain the sustainability of NT, especially without mulch, for enhancing moisture
conservation, yield and WUE in this environment.
116
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138
Appendix I
1800
1600
C u m . ra in fa llL (m m )
1400
1200
1000
800
600
400
200
0
Jan Feb Mar
Apr May June July Aug Sept Oct Nov Dec
Month of the year
2006
2007
Cumulative amounts of monthly rainfall in 2006 and 2007
139
Appendix II
The procedure for calculating the amount (in mm) of depleted moisture between two
successive soil moisture monitoring dates: an example with data collected on 31st
October and 9th November 2006 in the NTB treatment combination in the sorghum plot
U
md
θd
Soil
m2
Ρ
t
=
m1
=
layer
=
-1
-1
-3
(g g )
(mm)
(g cm )
(g g )
θd*t
(cm)
md*ρ
m1−m2
(mm)
0-10
0.1399
0.0825
0.0575
1.44
0.0827
100
8.27
10-20
0.1543
0.0908
0.0636
1.51
0.0960
100
9.60
20-30
0.1460
0.1089
0.0371
1.53
0.0567
100
5.67
30-50
0.1382
0.1172
0.0210
1.57
0.0330
200
6.59
Total
30.13
m1 = the gravimetric moisture content on a given sampling date, 31st Oct. 2006 in this case
m2 = the gravimetric moisture content on the following date, 9th Nov. 2006 in this case
md = the difference between m1 and m2
ρ = the bulk density of each sampled soil layer
θd = the equivalent of md on volumetric basis
t = the thickness of each sampled soil layer
U = the depth of soil moisture depleted
140
Appendix III
The procedure for deriving and using ∆S in the water balance equation to compute ET
(in mm) between two successive soil moisture monitoring dates: an example with data
collected on 31st Oct. and 9th Nov. 2006 under the NTB treatment in the sorghum plot
The formula states:
ET = P − D − ∆S;
Where P = precipitation (mm)
D = drainage (mm)
∆S = change in storage (mm)
The total value of P between the two sampling dates was 21.8 mm
The corresponding total value of D was 10.4 mm
The change in storage, ∆S, was obtained thus:
Soil
layer
(cm)
m2
(g g-1)
m1
(g g-1)
ρ
(g cm-3)
Θ2
=
m2*ρ
Θ1
=
m1*ρ
T
(mm)
S2
=
θ2*t
(mm)
S1
=
θ1*t
(mm)
∆S
=
S2−S1
(mm)
0-10
0.0825
0.1399
1.44
0.1188
0.2015
100
11.88
20.15
−8.27
10-20
0.0908
0.1543
1.51
0.1371
0.2033
100
13.71
20.33
−6.62
20-30
0.1089
0.1460
1.53
0.1666
0.2234
100
22.34
22.34
−5.68
30-50
0.1172
0.1382
1.57
0.1840
0.2170
200
21.70
43.40
−6.60
Total
79.05
106.22
−27.17
ET = (21.8 − 10.4 − [−27.17]) mm = (11.4 − [−27.17]) mm = (11.4 + 27.17) mm = 38.57 mm
m2 = the gravimetric moisture content on a given sampling date, 9th Nov. 2006 in this case
m1 = the gravimetric moisture content on a preceding date, 31st Oct. 2006 in this case
ρ = the bulk density of each sampled soil layer
θ2 = the equivalent of m2 on volumetric basis
θ1 = the equivalent of m1 on volumetric basis
t = the thickness of each sampled soil layer
S2 = the depth of moisture storage on a given sampling date
S1 = the depth of moisture storage on a preceding date
141
Appendix IV
Procedure for the computation of crop WU using the Blaney-Criddle model
The formula states:
U = 0.46Kp (t+18);
Where U = crop WU or ET (mm),
K = crop coefficient (known as K factor),
p = mean monthly percentage of daytime hours of Nsukka (06° 52´ N), and
t = the mean daily temperature (°C).
(A) For sorghum: The value of K for sorghum, as given by the USDA, is 0.87
The consumptive use factors, p and t, were obtained thus:
No. of
Month
Month
P
T
days
July
28
7.89
July
24.98
Aug
31
8.65
Aug
24.54
Sept
30
8.24
Sept
24.79
Oct
31
8.39
Oct
25.37
Nov
30
8.25
Nov
25.61
Dec
20
5.49
Dec
25.55
Sum
170
Mean
46.91
25.14
LJ U = (0.46 × 0.87 × 46.91 × [25.14 + 18]) mm
= (0.46 × 0.87 × 46.91 × 43.14) mm
= 809.8837 mm ≈ 809.9 mm
(B) For soybean: The value of K for soybean, as given by the USDA, is 0.68
The consumptive use factors, p and t, were obtained thus:
No. of
Month
Month
P
T
days
July
28
7.89
July
24.98
Aug
31
8.65
Aug
24.54
Sept
30
8.24
Sept
24.79
Oct
31
8.39
Oct
25.37
Nov
15
4.13
Nov
25.61
Sum
135
Mean
37.30
25.06
LJ U = (0.46 × 0.68 × 37.30 × [25.06 + 18]) mm
= (0.46 × 0.87 × 46.91 × 43.06) mm
= 502.4 mm
142
Appendix V
Estimation of the WU of sorghum and soybean from the product of
reference ET (based on the USDA model) and K factor (given by the FAO)
The equation states:
ETcrop = ETo × Kc;
where ETcrop = crop WU (mm), ETo = reference crop ET (mm), and Kc = crop coefficient.
The ETo was computed from the formula, p (0.46T + 8), based on Blaney and
Criddle’s (1950) USDA model; with p as the mean daily percentage annual daytime hours of
Nsukka (06° 52´ N) given by the USDA (Erie et al., 1981), and T as the mean daily
temperature (°C). The Kc values for the crops’ four growth stages (initial, crop development,
mid-season, and late season) were given by the FAO† respectively as 0.40, 0.72, 1.00, 0.52
for sorghum and 0.35, 0.75, 1.10, 0.45 for soybean (Doorenbos and Kassam, 1986*). The four
growth stages respectively corresponded to 25, 45, 65, and 35 (amounting to a growth period
of 170) days for sorghum; and 20, 35, 55, and 25 (amounting to a growth period of 135) days
for soybean. Using the procedure outlined by Chukwu (1999), monthly crop ET (ETcrop
month-1) was first obtained as a product of ETo and Kc month-1. The Kc value of each growth
stage was converted to monthly Kc, calculated thus, Kc month-1 = (Kc growth stage × n) ⁄ 30;
with n as the number of days the growth stage lasted in each month assumed to have 30 days.
No.
of
p
days
(A) For sorghum
July
28
7.89
Aug
31
8.65
Sept
30
8.24
Oct
31
8.39
Nov
30
8.25
Dec
20
5.49
Total
170
(B) For soybean
July
28
7.89
Aug
31
8.65
Sept
30
8.24
Oct
31
8.39
Nov
15
4.13
Total
135
Month
T
ETo
(mm)
24.98
24.54
24.79
25.37
25.61
25.55
153.79
166.85
159.87
165.01
163.19
108.44
Growth stages
I
25
II
III IV
3
31
11 19
31
15
25 45 65
24.98
24.54
24.79
25.37
25.61
153.79
166.85
159.87
165.01
81.60
20
8
27
4
30
21
20 35 55
15
20
35
10
15
25
Kc values
I
II
III
IV
Kc
ETcrop
(mm)
0.33 0.07
0.74
0.26 0.63
1.03
0.50 0.26
0.35
0.41
0.74
0.90
1.03
0.76
0.35
0.23 0.20
0.68 0.15
1.10
0.77 0.15
0.23
0.43 66.64
0.82 137.10
1.10 175.85
0.92 151.81
0.23 18.36
549.77‡
62.34
124.14
143.45
170.51
124.02
37.59
662.06‡
I – initial, II – Development, III – Mid-season, IV – Late-season
‡Compare these values with those in the preceding appendix, obtained using the USDA K factors
†Within the ranges given by Allen et al. (1998) in FAO Irrigation and Drainage Paper, 56.
*Not referenced in the main body of this dissertation: Doorenbos, J. and Kassam, A.H.
(1986). Yield Response to Water. Food and Agricultural Organization, Rome, Italy.
143
Appendix VI
Preliminary and final ANOVA tables format for a split-plot design in RCBD
Preliminary ANOVA table format for a split-plot design in RCBD
Source
d.f. (general)
d.f. (specific)
Replications
r–1
3
Treatments
Ab – 1
3
Error
(ab-1) (r – 1)
9
Total
abr − 1
15
Final ANOVA table format for a split-plot design in RCBD
Source
d.f. (general)
d.f. (specific)
Replications
r−1
3
Factor A
A−1
Error (a)
(r − 1) (a − 1)
3
ra − 1
7
Main plot
Total
1
Sub-plot
Factor B
B−1
1
Interaction AB
(a − 1) (b − 1)
1
Error (b)
a (r − 1) (b − 1)
6
Total
ar (b − 1)
8
Overall total
abr − 1
15
A: Tillage system = 2
B: Mulch practice = 2
r = replications = 4
144
Appendix VII
SOILLAYER(cm)
0-10
10-20
(a)
20-30
30-50
0
10
20
30
40
50
SOILLAYER(cm)
0-10
10-20
(b)
20-30
30-50
0
10
20
30
40
50
SOIL LAYER (cm)
0-10
10-20
(c)
the
20-30
30-50
0
10
20
30
40
50
MOISTURE DEPLETED (%)
NTB
NTM
CTB
CTM
Percentage depletion of soil moisture in each of the four sampled layers under (a)
sorghum, (b) soybean, and (c) intercrop in the 2006 growing season
145
Appendix VIII
SOILLAYER(cm
)
0-10
(a)
10-20
20-30
30-50
0
5
10
15
20
25
30
35
40
45
50
SOILLAYER(cm)
0-10
10-20
(b)
20-30
30-50
0
10
20
30
40
50
SOIL LAYER (cm)
0-10
10-20
(c)
20-30
30-50
0
10
20
30
40
50
MOISTURE DEPLETED (%)
NTB
NTM
CTB
CTM
Percentage depletion of soil moisture in each of the four sampled layers under (a)
sorghum, (b) soybean, and (c) intercrop in the 2007 growing season
146
Appendix IX
(a)
DAYS AFTER SOWING (DAS)
1
7
13
19
25
31
37
43
49
55
61
67
73
79
85
91
97 103 109 115 121 127 133 139 145 151 157 163 169
1
7
13
19
25
31
37
43
49
55
61
67
73
79
85
91
97 103 109 115 121 127 133 139 145 151 157 163 169
0
RAINFALL (mm) (2006)
5
10
15
20
25
30
35
40
45
50
SORGHUM WU (mm) 2006
14
12
10
8
6
4
2
0
(b)
DAYS AFTER SOWING (DAS)
1
7
13
19
25
31
37
43
49
55
61
67
73
79
85
91
97 103 109 115 121 127 133 139 145 151 157 163 169
1
7
13
19
25
31
37
43
49
55
61
67
73
79
85
91
97 103 109 115 121 127 133 139 145 151 157 163 169
RAINFALL (mm) 2007
0
5
10
15
20
25
30
35
40
45
50
55
SORGHUM WU (mm) 2007
14
12
10
8
6
4
2
0
Daily rainfall distribution (mm) and the corresponding daily water use (mm)
of sorghum in the (a) 2006 and (b) 2007 growing seasons
147
Appendix X
(a)
DAYS AFTER SOWING (DAS)
1
7
13
19
25
31
37
43
49
55
61
67
73
79
85
91
97 103 109 115 121 127 133 139 145 151 157 163 169
1
7
13
19
25
31
37
43
49
55
61
67
73
79
85
91
97 103 109 115 121 127 133 139 145 151 157 163 169
0
RAINFALL (mm) (2006)
5
10
15
20
25
30
35
40
45
50
SOYBEAN WU (mm) 2006
14
12
10
8
6
4
2
0
(b)
RAINFALL (mm) 2007
DAYS AFTER SOWING (DAS)
1
7
13
19
25
31
37
43
49
55
61
67
73
79
85
91
97 103 109 115 121 127 133 139 145 151 157 163 169
1
7
13
19
25
31
37
43
49
55
61
67
73
79
85
91
97 103 109 115 121 127 133 139 145 151 157 163 169
0
5
10
15
20
25
30
35
40
45
50
55
SOYBEAN WU (mm) 2007
14
12
10
8
6
4
2
0
Daily rainfall distribution (mm) and the corresponding daily water use (mm)
of soybean in the (a) 2006 and (b) 2007 growing seasons