Soil Water Balance in the SudanoSahelian Zone (Proceedings of the
Niamey Workshop, February 1991). IAHS Publ. no. 199,1991.
Physical and hydraulic properties of soils of
the Sudano-Sahelian regions of Nigeria
H. O. MADUAKOR
Federal University of Technology, Owerri, Nigeria
Abstract The inadequacy of rainfall and poor soil fertility in
the Sudano-Sahelian zone of West Africa are responsible
for the hunger, misery and desertification in the area. It is
therefore important to understand the soils and their
properties for effective management strategies. In Nigeria,
the soils are formed from aeolian, lacustrine and basement
complex parent materials and are classified mostly as
Entisols, Inceptisols and Vertisols. They are poorly
structured, mostly sandy except for the Vertisols, and easily
prone to surface sealing at the beginning of the season. The
other physical and hydraulic properties are discussed.
INTRODUCTION
Hunger and poverty are already endemic in sub-saharan Africa, especially in
the Sudano-Sahelian zone (SSZ). Added to these is the ongoing degradation
process - desertification - defined as the sustained decline of the biological
productivity of arid and semiarid land (Gorse & Steeds, 1985) which is
caused by a combination of climatic variations and over population pressure
on the meagre resources.
The soils of the SSZ zone are generally regarded as low in fertility
and productivity. This problem is compounded by the inadequacy of
rainfall. It is generally believed that moisture is the most limiting factor
to production in these zones (Gorse & Steeds, 1985; Jones & Wild, 1975;
Breman & de Wit, 1983). Therefore to increase productivity and arrest
the advancing desert, a sound knowledge of the climate and soil
properties especially those related to moisture storage and utilization is
necessary. This paper describes the physical and hydraulic properties of the
soils of the SSZ of Nigeria and attempts to relate these to soil
management.
THE SUDANO-SAHELIAN ZONE (SSZ) OF NIGERIA
In Nigeria, the SSZ lies roughly between latitudes 11° and 13°N and
longitudes 4° and 15CE (Kowal & Knabe, 1972) and comprises most of
Bornu, Kano and Sokoto States (Fig. 1). It occupies approximately
26 159 km2 or 28.3% of the area of Nigeria. The climatological and other
features of the zone are hereby described.
229
H. O. Maduakor
230
Fig. I Map of Nigeria showing the Sudano-Sahelian zone.
Source: Ojanuga (1987); Kowal & Knabe (1972).
Qimate
Rainfall In the Sudano-Sahelain zone of West Africa rains fall from May
to September with the rest of the year virtually dry. In Nigeria the mean annual
rainfall is between 1016 mm in the wettest part and less than 508 mm in the driest
part (Ojanuga, 1987). Figures 2 (a), (b) and (c) show the rainfall and some other
climatological data of three locations: Maiduguri, Kano and Sokoto, which are
situated within the zone. At the northern boundary, there is a 10% probability of
receiving 50 mm or less rainfall while at the southern border the probability of
receiving 400 mm or less is also 10% (Breman & de Wit, 1983). Arguing that the
delimitation of the SSZ on the basis of mean annual rainfall was inadequate,
Sivakumar (1989), and Sivakumar & Wallace (1991) proposed the use of growing
season length (GSL) to demarcate the zones. Accordingly, the SSZ was
described as the having a GSL of 60-150 days.
Temperature and évapotranspiration The mean annual temperature is
between 26 and 28°C (Fig. 2) in the SSZ of Nigeria (Ojanuga, 1987) but in
some countries of West Africa, the temperature at the time of sowing can
exceed 40°C (Sivakumar, 1989).
The potential évapotranspiration is high and can reach 3 to
îrt
£
K
&
a- <,'
H
1- I
S
£
0
e ^
1
1 fey
!
I
o
I
•I
1
o
•Sis
CN
«
H. O. Maduakor
232
5 mm day"1 during the rainy season (Breman & de Wit, 1983).
Evapotranspiration exceeds precipitation for about eight months of the year in
Nigeria (Figs 1(b) and (c)) resulting in a deficit for about 75% of the year.
Vegetation
In West Africa, vegetation zones follow rainfall zones. As a result of the low
rainfall and poor soil fertility, the natural vegetation of the SSZ is made up
of mainly thorn shrubs and trees dominated by grasses. Woody species make
up less than 5% of the area on soils formed from aeolian deposits but on
soils formed from sandstones and riverine deposits with temporary puddles
due to accumulation of runoff water, coverage can be up to 20% (Breman &
de Wit, 1983).
Geology and landform
The land of the SSZ of Nigeria is gently sloping with occasional outcrops of
rock. Aitchison et al. (1972) identified five major groups of rocks in the
area they studied covering 176 000 km2 in northeast Nigeria. Those rocks
include granites, gneiss, sandstones, clays shists, pyroclastic rocks and basalt,
ancient alluvium, lacustrine sands, lagoonal clays, aeolian sands, and recent
alluvium. The predominant parent material is made up of aeolian, alluvial and
lacustrine sediments deposited over older sediments or on basement complex
rock or sedimentary rocks (Ojanuga, 1987).
Aeolian deposits constitute the most extensive parent material in the
northern part of the region while in the southern part, saprolites form the
next most important parent material. Sedimentary rocks are sometimes
exposed in a small part of the region. Alluvial deposits are found mainly in
the valleys of the Rima-Sokoto, Yobe, Jamaare, Katanguru and Gana while
lacustrine deposits are confined to the eastern part of the Chad basin.
THE SOILS
Formed on the parent material described above are soils which Heinkenberg
& Higgins (1968), Higgins et al. (1960) using the legend of D'Hoore (1964)
identified as juvenile soils, Vertisols, brown and reddish brown soils and
Lithosols, ferruginous tropical soils, hydromorphic and halomorphic soils.
However Ojanuga (1987) employing the soil survey legend of USDA (1975)
has classified these soils as mainly Entisols and Inceptisols though Alfisols,
Vertisols as well as hydromorphic and halomorphic soils also occur in the
zone. The Inceptisols and Entisols are formed on aeolian deposits. Among
the Inceptisols, Typic Ustropepts are dominant. Typic Ustropsamments
represent the commonest Entisol on aoelian deposit and they occur on the
northeastern part of the country. Owing to their sandy nature, their profiles
are not well developed. In some areas, soils developed from aoelian deposits
233
Physical and hydraulic properties of soils
are underlain by latérite layer. Such soils are found in Kano, Watari, and
Gari areas of Kano State (Ojanuga, 1987).
Soils developed from basement complex rocks also occur in the SSZ and
are mapped as Alfisols when they occur on the upper to lower slope and as
Entisols and Inceptisols when they occur on escarpment and hilly areas.
Soils formed over sedimentary rocks are distinguished on the basis of
whether they are formed over sandstones or over shales. Those formed over
sandstones are inceptisols although in the humid part of the zone, Oxic
Paleustalfs can be found. The soils over shales are usually found in
depression and topographic flats and are mostly vertisols. Soils over alluvium
in flood plains, delta fans and bottom lands are commonly classified as typic
Fluvaquents and Vertic Fluvaquents. Halomorphic soils classified as Aquic
Natrustalts and Typic Natraqualfs are found in the flood plains of Hadejia,
Katagun, Jamaare, Yobe and Yederam rivers (Ojanuga, 1987). These are
saline or saline sodic soils. Soils formed from lacustrine deposits and classified
mainly as vertisols occur in areas south of Lake Chad. These soils exhibit
different physical and hydraulic properties depending on their positions in the
landscape.
Physical properties of the soils
Soil structure The SSZ soils have poorly developed weak structure
which deteriorates with cropping and compaction (Ojanuga, 1987; Jones &
Wild, 1975; Hoogmoed & Klaij, 1990; Ohu et al, 1989). The aggregates are
small and unstable in wet conditions with a tendency to pack early in the wet
season owing to the impact of raindrops and/or cultivation. Soils form crusts
which reduce infiltration thus causing runoff. This unstable structure may be
due to weak binding of the aggregates due to low organic matter content
especially in the soils formed from aeolian deposits.
Soil depth The soils are sufficiently deep for the growth of arable
crops but owing to their poor structure, root penetration may be restricted
even under sufficient moisture condition. The poor structure also limits the
amount of available water held in the soil volume since crusting on the
surface of these soils results in runoff. Soils formed from aeolian depots are
deeper than those formed from basement complex.
Soil texture Most of the soils of the SSZ are sandy, the texture
ranging from sandy to very fine sandy loam (Tables 1 and 2). The exception
are the Vertisols, hydromorphic and holomorphic soils of depressions and
bottom lands which have a higher percentage of clay fraction. In soils formed
from aeolian deposits, the fine sandy fractions dominate while in those
derived from basement complex, coarse fractions may be higher.
Soil consistency Because of their sandy nature, the consistency of the
Sudano-Sahelian soils is non sticky and non plastic when wet and friable to
very friable when dry (Ohu et al, 1989, Jones & Wild, 1975). Exceptions,
234
H. O. Maduakor
Table 1 Surface texture of some soils in the Sudano-Sahelian region
of Nigeria
State
Soils
Kama Namoda
Shin Kafe
Sokoto
Argungu
Birnin Kebbi
Kadawa
Kano
Dambaaa
Giimel
Hadejia
Potskun
Damaturu
Maiduguri
Bama
Gajiram
Sokoto
Sokoto
Sokoto
Sokoto
Sokoto
Kano
Kano
Kano
Kano
Kano
Bornu
Bornu
Bornu
Bornu
Bornu
Particle size analysis:
%Sand
% Silt
%Clay
79
70
90
86
94
85
87
92
94
92
70
82
78
76
92
8
6
4
4
4
4
4
4
3
4
15
5
7
5
5
13
24
5
10
2
11
9
4
3
4
15
13
15
19
3
Source: Iwuafor et al. (1980).
Table 2 Soil textures of the soil associations in the Dange areas of
Sokoto State
Horizon
Depth
(cm)
Particle size analysis(%):
Coarse sand
Fine sand
Dange series (summit and shoulder) Typic Paleustalf
A
0-15
30
45
AB
15-40
27
32
Btl
40-55
30
31
Bt2
55-78
28
33
2C
78-150
31
29
Zafanadi: (Back slope) Typic Haplustalf
API
0-10
33
52
AP2
10-18
28
S3
Bt
18-38
23
47
2B
38-58
35
38
Merina (foot slope) Fluventic Ustropept
AP
0-26
38
50
BW1
26-54
32
54
BW2
54-96
30
57
BW3
96-135
40
51
26
135-180
27
49
Total sand
Silt
Clay
75
59
61
61
60
5
9
7
8
10
20
32
32
31
30
85
81
70
73
6
6
7
5
9
13
23
22
88
86
87
91
76
5
4
4
3
4
7
10
9
6
20
Source: Agbu et al. (1989).
however, include highly plastic and sticky Vertisols which occur on localized
positions in the zone. The liquid and plastic limits of a sandy loam and loamy
sand in Bornu State are presented in Table 3 which shows that the plasticity
index is higher for a sandy loam than a loamy sand as a result of more fine
materials in the former as compared with the latter. There is hardening of
soil with a decrease of soil moisture commencing when about 50% of the
Physical and hydraulic properties of soils
235
Table 3 The particle size analysis, liquid and plastic limits of loamy
sand and sandy loam soils in Bornu State of Nigeria
Soil type
Loamy sand
Sand loam
%Sand
90
70
% Silt
5
19
%Clay
5
U
Liquid limit
Plastic limit
(%)
(%)
16.20
30.08
12.11
16.00
Source: Ohu et al. (1989).
field capacity moisture is lost and continuing thereafter (Jones & Wild,
1975).
The clay type is mainly kaolinitic and sometimes associated with free
iron and aluminium oxides. Vertisols, however, contain predominantly
montmorillonite and related minerals.
Soil bulk density The bulk density of the soils of the zone is high
ranging from 1.5 to 1.8 g cm"3. Ahmed & Maurya (1989) showed that over
one season, cultivation did not significantly alter the bulk density (Table 4)
however compaction increased it as shown for a sandy loam in Bornu State
(Ohu et ai, 1989). In some soils, plinthite with bulk densities of up to
2.00 g cm"3 may occur at some depth and may restrict root penetration and
growth.
Table 4 The effect of conventional tillage on the bulk density of a
loam soil with iron pan at 120-150 cm depth
Depth
(cm)
0-10
10-20
20-30
30-45
Bulk density (g cm" )
1984/1985
1985/1986
1.50
1.71
1.65
1.57
1.52
1.70
1.63
1.59
Source: Ahmed & Maurya (1989).
Total porosity Total porosity values give the amount of space
occupied by air and water and may indicate the degree of compaction.
Total porosity values for the SSZ soils of Nigeria are not available but
values of 34-46% have been reported for a Guinea savanna soil in
Nigeria (Kowal, 1968) compared with values of 32-41% corresponding to
bulk densities of 1.8 and 1.5 g cm"3 respectively which have been reported
in Senegal (Jones & Wild, 1975). Compaction reduced total porosity as
demonstrated for a sandy loam in the north eastern region of Nigeria
(Ohu et al., 1989).
236
H. O. Maduakor
Hydraulic properties
Hydraulic conductivity Very little information exists on the hydraulic
conductivities of the soils of SSZ in Nigeria. Ohu et al. (1989) showed that
the saturated hydraulic conductivity of a sandy laom and a loamy sandy soil
decreased with increases in compaction and bulk density (Table 5). A similar
Table 5 The saturated hydraulic conductivity (K) of loamy sand
and a sandy loam at various compaction levels
Compaction levels
(no. of blows)
5
10
15
Sandy loams:
K-value
Mean density
Loamy sand:
K-value
x 20"4 ™
(Mgm~3)
x 10'4 mm
33.47
27.54
23.63
1.64
1.67
1.69
34.12
29.62
27.59
Mean density
(Mgm~3)
1.66
1.69
1.73
Source: Ohu et al. (1989).
result was obtained on a coarse sandy soil in Niamey (Hoogmoed & Klaij,
1990). Work on Vertisols of Lake Chad basin shows that sodium drastically
reduced hydraulic conductivity while calcium and magnesium increased it
(Adeoye et al, 1988). This is understandable since sodium disperses the soil
particles which may then block the conducting pores. Calcium and
magnesium on the other hand, floculate the particles and thus increase the
total porosity and hence the saturated hydraulic conductivity. Hydraulic
conductivity measured on 0-15 cm surface soil layer at Samaru ranged from
1.50 cm h"1 for bare follow to 12.75 cm h"1 for a cotton bearing soil with an
annual application of 5 t ha"1 farm manure (Kowal, 1968).
Infiltration and runoff The infiltration rate of a soil is one of the
major factors influencing runoff. Whenever the rate of infiltration falls below
the rate at which water is supplied to the surface, runoff may occur if the
soil is positioned on a slope. Infiltration, however, is greatly influenced by
the conditions on the soils of the SSZ (Ojanuga, 1987; Jonas & Wild, 1975;
Wilkinson, 1975). Data on infiltration rates of the region are scarce. Kowal &
Knabe (1972) reported that the infiltration rate of an Argiustoll of sandy
loam texture was moderate to rapid (2-25 cm h"1), while those of a Vertisol
of topographic depression (Grumustert) and a Vertisolic hydromorphic soil
(Mollic Ochraqualf) were slow (0.1-0.5 cm h"1) and moderately slow
(0.5-2 cm h"1) respectively. The infiltration rates of topsoil in Gari and
Watari valleys of Kano State as reported by Anon (1981a,b) are presented in
Table 6 and show that infiltration decreases as the texture gets finer. Data
237
Physical and hydraulic properties of soils
reported by Virmani & Swindale (1984) show that infiltration rate in a
Vertisol in India decreased with time from an initial rate of 76 mm h"1 to
0.21 ± 0.1 mm h"1 after 144 h. Figure 3 also shows a similar trend for a soil
under conventional tillage in Kadawa, Kano (Ahmed & Maurya, 1989).
Table 6 Infiltration rates in some soils of the Gari and Watari
Valleys
Soil faction
Surface texture
Infiltration rate
(cm h"X)
Gari Valley
Fine sand
Loamy fine sand
Fine sandy loam
Sandy clay
Loamy very fine sand
Fine sandy loam
Clay loam
14.7
6.5
4.8
0.9
34
26
4
Watari Valley
Source: Anon (1981a, b).
E100--
e
,s so
100
200
Time in minutes
Fig. 3 Average soil infiltration rate under conventional tillage in
Kadawa, Kano. Source: Ahmed & Maurya (1989).
Published runoff values for the SSZ in Nigeria are not available but
runoff is reported to be commonly low on upland sandy soils of aeolian origin
probably because of their higher infiltration rates. These soils, when disturbed
by cultivation and tillage, can be subjected to runoff as a result of surface
sealing which reduces the infiltration rate.
Moisture retention characteristics The moisture retention characteristics
of soils are important because they determine the amount of moisture that is
available for extraction by plants. There is a dearth of published information
on the moisture retention characteristics of soils of SSZ in Nigeria. However,
because of the fineness of their particles, these soils are likely to hold an
238
H. O. Maduakor
appreciable amount of moisture between field capacity and wilting point.
Table 7 shows the soil moisture storage capacity for available water in the
various textural classes of soils in the zone and appears to confirm the earlier
observation. Figure 4 shows the moisture characteristics curves of three soils
in Sokoto (Rima) river basin in northwestern Nigeria and further illustrates
that finer textured soils (Vertisols and vertisolic hydromorphic soils) retain
more moisture than coarse textured ones at the same moisture tension.
Similar trends have been reported elsewhere on Vertisols (Virmani &
Swindale, 1984; Eswaran & Cooke, 1988).
Table 7 Soil storage capacity for available water (cm/per 30 cm depth of soil)
for textural classes of soils in the Sudano-Sahelian zone of Nigeria
Textural class
Coarse sand (< 10%
clay and silt)
Fine sand (<10%
clay and silt)
Fine loamy sand
(10-29% clay and silt)
Sandy clay
(30-50% clay and silt)
Clay loam
(45-60% clay and silt)
Sandy clay
(60-70% clay and silt)
Vertisol
Field capacity:
Range
Mean
Wilting point:
Range
Mean
Available water:
Range
Mean
2.3-4
3.1
1-1.5
1.3
1.3-2.5
1.9
3.5-5
4.3
1.3-2
1.8
2.3-3
2.5
5-7.5
6.3
1.5-3
2.3
3.5-4.5
4.0
7-S
7.5
2.5-3.5
3.0
4.5
4.5
9-10
9.5
3.5-4.0
3.8
5.5-6
5.8
10.5-12.3 11.3
6.5-7.0
6.8
4-5.3
4.5
12.5
5-7.5
6.3
15-22.5
18.8
10-15
Source: Kowal & Knabe (1972).
GENERAL DISCUSSION
Moisture is probably the most important factor limiting yield in the SSZ. The
supply of moisture to the crops on the other hand is determined by physical
characteristics of the soil and other climate factors especially rainfall. Soil
properties such as infiltration, hydraulic conductivity, moisture retention
characteristics and available water holding capacity are influenced by texture,
structure, porosity and soil surface characteristics such as surface sealing
which occur frequently in the SSZ during the early part of the season (Jones
& Wild, 1975).
Published information on the above soil properties in relation to
moisture supply to crops in the region is very scarce. Much more work needs
to be done in this direction especially in obtaining basic information on the
physical and hydraulic properties of the soils. Research should be directed in
the following areas:
(a) determination of the physical and hydraulic properties of the different
soil types in the region such as particle sizes, bulk densities, soil strength
Physical and hydraulic properties of soils
239
50
X
X Aquic to aridic Argiustoll:
Sandy loam
Vertisol: Grumustert:
1.0
2.0
3.0
4.0
45
pF
Fig. 4 Moisture characteristic curves of some soil in Sokoto Rima
basin. Source: FAO (1969).
and soil structure.
(b) determination of infiltration rates, hydraulic conductivities, moisture
retention characteristics, compactivity at different moisture contents and
root penetration into the soils.
(c) The effect of cultural and improved management practices on these
various properties should be investigated.
With a thorough knowledge of these properties, effective management
packages can be designed to improve the soils' ability to support arable and
tree crops as well as pasture. Such management packages should aim at
improving the soil structure and preventing surface sealing through use of
organic matter and mulches. Improved infiltration and reduced runoff should
be encouraged. These will help alleviate hunger and reduce desertification.
REFERENCES
Agbu, P. A., Ojanuga, A. G. & Olson, K. R. (1989) Soil-landscape relationship in
Sokoto-Rima basin in Nigeria. Soil Sci. 148 (2), 132-139.
Adeoye, K. B., Folorunso, A. O. & Mohamed Salem, M. A. (1988) Effects of adsorbed cations
on the physical properties of Vertisols in the Lake Chad basin of northeast Nigeria. In:
Proc. Conf. held atlLCA, Addis Ababa, Ethiopia. (August-September 1987), 127-128.
H. O. Maduakor
240
Ahmed, A. & Maurya, P. R. (1989) The effects of chiselling subsoiling and irrigation frequency
on wheat production at Kadawa, Nigeria. Samaru J. Agric. Res. 6, 17-22.
Aitchison, P. J., Bawden, M. G., Carrol, D. M., Glover, P. G., Klienkenberg, K. & de Leeuw, P. N.
(1972) Land Resources of North East Nigeria, vol. I: The Environment. LRD Study no. 9,
LRD, Surbiton, Surrey, UK.
Anon. 1981(a) Gari Irrigation Scheme: Annexes, Feasibility Studies, vol. 11, December 1981.
MANR, Kano State, Nigeria.
Anon. 1981(b) Watari River Project: Annexes, Feasibility Study, vol. II, December 1981. MANR,
Kano State, Nigeria.
Breman, H. & de Wit, C. T. (1983) Rangeland productiity and exploitation in the Sahel. Science
221, 1341-1347.
D'Hoore, J. I. (1964) Soil Map of Africa Scale: 1 to 5 000 000. Explanatory Monograph. Publ.
no. 93, Commission for Technical Cooperation in Africa, Lagos.
Eswaran, H. & Cook, T. (1988) Classification and management related properties of Vertisols.
In: Proc. Conf. held at ILCA, Addis Ababa, Ethiopia (August-September 1987), 64-84.
Gorse, J. E. & Steeds, D. R. (1985) Desertification in the Sahelian and Sudanian zones of West
Africa. World Bank Tech. Pap. no. 81, Washington, DC
Higgins, G. M., Ramsay, D. M., Pullman, R. A. & De Leeuw, P. N. (1960) Report of the
reconnaissance and semi-detailed soil survey undertaken in north-east Bornu. Soil Survey
Bull. 14, Institute for Agricultural Research, Samaru, Nigeria.
Hoogmoed, W. B. & Klaii, M. C. (1990) Soil management for crop production in the West
Africa Sahel. 1. Soil and climate parameters. Soil Till. Res. 16, 85-109.
Iwuafor, E. N. O., Balasubramanian, V. & Mokwunye, A. U. (1980) Potassium status and
availability in the Sudano Savanna zone of Nigeria. Proc. Potassium Workshop (Ibadan,
Nigeria, October 1980), 97-209.
Jones, M. J. & Wild, A. (1975) Soils of the West African Savannah. Tech. Commun. 55,
Commonwealth Bureau of Soils, Harpenden, UK
Klinkenberg, K. & Higgins, G. M. (1968) An outline of northern Nigeria Soils. Nigeria J. Sci. 2,
91-115.
Kowal, J. (1968) Some physical properties of soil at Samaru, Zaria, Nigeria. Storage of water
and its use by crops. I. Physical status of Soils. Nigeria Agric. J. 5, 23-20.
Kowal, J. M. & Knabe, T. (1972) An Agroclimatological Atlas of Northern States of Nigeria with
Explanatory Notes. Ahmadu Bello University, Samaru, Zaria.
Ohu, J. O., Ayotamuno, M. S. & Folorunso, O. A. (1988) Characteristics of prominent
agricultural soils in Bornu State of Nigeria. Agric. Int. 40 (11), 242-243.
Ojanuga, A. G. (1987) Characteristics of soils of the semi-arid region of Nigeria. In:
Ecological Disasters in Nigeria: Drought and Desertification (Proc. Nat. Workshop, Kano,
December 1985), 64-77. Federal Ministry of Science and Techology, Lagos, Nigeria.
Sivakumar, M. V. K. (1989) Agroclimatic aspects of rainfed agriculture in the Sudano-Sahelian
Zone. In: Soil Crop and Water Management Systems for Rainfed Agriculture in the SudanoSahelian Zone (Proc. Int Workshop, January 1987), 17-38. ICRISAT Sahelian Center,
Niamey, Niger.
Sivakumar, M. V. K. & Wallace, J. S. (1991) Soil water balance in the Sudano-Sahelian zone:
need, relevance and objectives of the workshop. In:
Soil Water Balance in the
Sudano-Sahelian Zone (Proc. Niamey Workshop, February 1991) (ed. by M. V. K.
Sivakumar, J. S. Wallace, C. Renard & C. Giroux), 3-10. IAHS Publ. no. 199.
Soil Survey Staff (1975) Soil taxonomy. A basic system of soil classification for making and
interpreting soil survey. Agricultural Handbook 436 Soil Conservation Service, USDA
Government Printing Office, Washington, DC.
Virmani, S. M. & Swindale, L. D. (1984) Soil taxonomy; an aid to the transfer of improved
farming systems for Vertisols in semi-arid tropics. Soil Taxonomy News 7, 3-9.
Wilkinson, G. E. (1975) Effects of grass fallow rotation on the infiltration of water into a
savanna zone soil of northern Nigeria. Trop. Agric. Trinidad52, 97-103.