Symposium no. 13
Paper no. 445
Presentation: poster
Effects of natural fallow on topsoil properties and
subsequent crop yields in a forest Oxisol
of Southern Cameroon
YEMEFACK Martin (1), NOUNAMO Laurent (2), NJOMGANG Rosaline (2) and
BILONG Paul (3)
(1) International Institute of Geo-Information Science and Earth Observation (ITC),
P.O. Box 6, 7500 AA Enschede, The Netherlands
(2) Institute of Agricultural Research for Development (IRAD) Nkolbisson, P.O. Box
2067 Yaounde, Cameroon
(3) University of Yaounde I, Faculty of Sciences, P.O. Box 812 Yaounde, Cameroon
Abstract
Studies were conducted on farmers' agricultural practices in the Tropenbos
Cameroon Programme situated in the humid forest zone of Southern Cameroon, in order
to provide information to be used in the management planning of the area. This paper
describes changes occurring in soil characteristics and soil productivity of Oxisols
during the natural fallow period within the shifting cultivation system. A synchronised
approach analyzing at the same time fallows of different duration was used. Soil
characteristics from fallow of various ages were statistically compared. Stepwise
analyses of multiple linear regressions were used to evaluate relationships between soil
properties’ variation and crop (maize, groundnut, cocoyam and cassava) yields from
mixed crop fields following each fallow type. The results showed that (i) ashes from
burned vegetation biomass at the beginning of the cropping period act like lime
fertilizer and significantly (p<0.05) increase soil properties such as pH, exchangeable
bases (especially calcium), and decrease exchangeable acidity; (ii) there is a slow but
almost irreversible decrease in topsoil clay content during the cropping period; and (iii)
at the age 7 to 9 year bush fallow, there is a morphological recovery of the topsoil thin
(3-5 cm) A1 organic horizon, which was destroyed by tillage during the cropping period.
For subsequent mixed cropping, crop yields were generally very low due to the low
quality of breeding varieties in use. Fallows of more than 15 years appeared to be not
suitable for groundnut cultivation. On the whole, the productivity index (LER) showed
higher crop productivity following bush fallow (7-9 years). The multiple linear
regression analyses showed that the dry biomass yields of maize and groundnut were
related to soil chemical properties while grain yields correlated mostly with soil
physical properties. Cocoyam yield correlated positively with soil CEC and organic
matter. For a sustainable management of the area, there are possibilities to intensify
crop production and productivity within a rotational bush fallow (BF) system using for
example, an integrated management approach combining application of mineral and
organic fertilizer.
Keywords: shifting cultivation, natural fallow, soil properties, Oxisols, crop yields
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Introduction
In the tropical rain forest area of southern Cameroon, agriculture is one of the main
factors causing deforestation, soil degradation and loss of species diversity (FAOUNEP, 1981; Kotto-Same et al., 1997; Oldeman, 1988, 1990; Tan, 1994; Waterloo et
al., 1997; Yemefack and Nounamo, 2000). From many other studies carried out in this
area (Gemerden and Hazeu, 1999; Ndjib and Tchienkoua, 1987; Yemefack and
Moukam, 1995), this agriculture is based on soils made up of Oxisols and Ultisols (Soil
Survey Staff, 1998). These are strongly weathered soils in which edaphic constraints
such as soil acidity, high exchangeable aluminium and correlatively low basic cation
saturation ratio, have been identified as the main limiting factors to sustained permanent
cropping systems. The Oxisol order represents 65 to 70% of these soils.
At the level of small-scale farmers, the agricultural land use system is based on
shifting cultivation practices for subsistence food crop production. This agricultural
system is characterized by two years of mixed food cropping without any fertilizer use,
followed by a fallow period of variable duration, ranging from 3 to more than 15 years
(Nounamo and Yemefack, 2000; Nye and Greenland, 1960; Warner, 1991). Many
factors might justify the abandonment of the piece of land after two years of cropping.
However, abundant research in the humid Tropics has identified the soil conditions to
be of dominant influence (Nye and Greenland, 1960; Sanchez, 1976; Volkoff et al.,
1989).
Indeed, during the cropping period, soil chemical and physical properties undergo
rapid changes and land becomes less productive after one cropping cycle. The role of
the fallow phase is to facilitate the regeneration of soil productivity. The original plot of
land is returned to, and re-used, after a suitable fallow period. Nowadays, with the
increasing demand on land due to population pressure and the influence of market
economy for food crop in the area, farmers are more and more relying on shorter
fallows for food crop production. This in turn, may lead to continuous degradation of
soils, increasing poor yields, and irreversible deforestation processes. This study was
carried out within the shifting cultivation system in farmers’ fields, with the objectives
to quantify changes occurring in soil during the natural fallow period, and to evaluate
the relative soil productivity after a natural fallow plot is re-used for food crop
production.
Materials and Methods
The research area and the study design
The study was carried out at the Tropenbos-Cameroon research area situated in
Southern Cameroon, between 2° 47'-3° 14' N and 10° 24'-10° 51' E. The landscape
consists of a rolling relief with hills ranging from 450 to 900 m asl. The climate is of the
equatorial type (Köppen, 1936), with two rainy seasons (March-June and SeptemberNovember) and two dry seasons (December-February and July-August). The mean
annual rainfall amounts to 1,800 mm and mean temperature is 25°C. The climax
vegetation is classified as biafran atlantic forest of mid altitude (Letouzey, 1985).
Upland soils are derived from the “calco-magnesian complex of Ntem” composed
mainly of leuco-mesocratic gneiss and old granites with pyroxene (Champetier de Ribes
and Aubague, 1956). These soils are of Oxisol order (Soil Survey Staff, 1998), falling
into two groups: Typic Kandiudox and Typic Hapludox (Alumic Acrisols, Xanthic
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Ferralsols and Acri-xanthic Ferralsols classes of the FAO classification system (1998)).
They are found on flattish crests and gentle backslopes, and occupy about 70% of the
total land area. This study was restricted to these Oxisol groups.
In order to quickly estimate the effects of natural fallow across a longer cycle of
cropping and fallowing, a synchronic approach was used for the study. This approach
consisted of analyzing simultaneously in space fallows of various durations. The study
design was a Randomized Complete Block Design with three villages as blocks, three
farmers per village as replications and five selected plot types per farmer as treatments.
In total, forty-five plots from a fallow chronosequence of increasing duration, ranging
from 0 year (cropped land) to more than 50 years (virgin forest) were studied. Based on
natural fallow typology characterization by Nounamo and Yemefack (2000), treatments
were chosen which comprised three fallow plot types, a groundnut-maize-cassava plot
(afub wondo) established following the clearing of fallow age 3 to 9 years, and a virgin
forest as control (Table 1). In this cropping system, no chemical fertilizer and no other
amendment (except ash from burned biomass materials) are used.
Table 1 Characteristics of treatments.
Treatments
Fallow
age
(years)
Average plot
surface (ha)*
Vegetation
type/crop type
Major crops
association in the
cropped plot
Number of
plots
CL
(Cropped Land)
0
0.5
Groundnut-maize- Groundnut-maizecassava association cassava - cocoyam
(afub wondo)
9
CF
(Chromolaena Fallow)
3-5
0.5
Dominated by
Chromolaena
odorata shrub
Groundnut-maizecassava – cocoyam
9
BF
(Bush Fallow)
7-9
1
Pioneer species and Groundnut-maizeyoung forest trees cassava-cocoyamplantain-cucumber
9
FF
(Forest Fallow)
12-15
2
Forest species of
secondary forest
Maize-cocoyamcucumber-plantain
9
FV
(Virgin Forest)
>50
many
Tropical rainforest
species
Maize-cocoyamcucumber-plantain
9
Total
45
* From Nounamo (1997)
The cropping pattern is made of crop associations where up to more than 10 crop
species can be found. However, major crops in the associations are represented in
higher density. These major food crops are: cassava, cocoyam, groundnut, maize,
banana-plantain, and cucumber (Cucumeropsis manii). The associations of major food
crops in a plot are linked to the type of preceding fallow as shown in Table 1. From
these crop association types, crop densities and yields were measured in farmers’ fields
for the major food crops, excepted for banana-plantain. All these crops are of local
varieties, which are commonly grown in the area for local consumption. They are
generally low yielding.
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Table 2 Changes in soil properties within the shifting cultivation system.
Soil slice (cm)
Soil slice (cm)
Soil slice (cm)
0-10 10-20 30-50
0-10 10-20 30-50
0-10 10-20 30-50
Organic carbon (%)
C/N ratio
P av. (mg.kg-1)
CL
2.6
1.4
0.8
14.0 12.2 11.0
17.1
3.4
1.6
CF
3.1
1.6
1.0
10.0 10.4
9.1
7.8
3.7
1.3
BF
3.3
1.7
0.9
10.6
9.5
7.9
6.7
3.7
1.4
FF
3.6
2.0
0.9
13.7 12.0
9.4
6.8
3.9
1.8
FV
3.7
1.9
1.1
16.3 14.4 13.0
8.1
3.0
1.3
se
0.17 0.22 0.08
0.76 0.90 0.98
0.86 0.47
0.26
p
>0.05 >0.05 >0.05
<0.05 <0.05 <0.05
<0.01 >0.05 >0.05
pH water
Total acidity
Sum bases
(cmol kg-1)
(cmol kg-1)
CL
4.9
4.3
4.3
0.87 2.83 3.27
5.19 1.90
1.26
CF
4.5
4.3
4.5
1.87 3.46 3.52
3.86 1.21
0.84
BF
3.9
3.9
4.4
3.70 4.22 3.69
2.00 0.87
0.74
FF
3.8
3.9
4.5
5.18 4.66 4.13
1.63 0.76
0.56
FV
4.0
4.2
4.7
5.53 4.87 4.17
1.81 1.25
0.82
se
0.11 0.10 0.08
0.59 0.47 0.30
0.34 0.15
0.11
p
<0.01 <0.05 >0.05
<0.01 <0.01 <0.05
<0.01 <0.05 >0.05
CEC (pH7) (cmol kg-1)
Bases saturation
Sand content (%)
(%)
CL
9.3
7.6
7.6
81
35
25
45
35
29
CF
9.9
7.4
8.4
54
18
13
39
34
28
BF
10.0
8.9
7.9
28
12
13
39
30
26
FF
14.8
8.9
8.3
16
9
8
34
30
23
FV
14.3 10.1
8.9
15
14
11
29
28
21
se
2.32 1.11 1.36
4.70 1.89 1.99
4.06 3.20
2.50
p
<0.01 <0.05 >0.05
<0.01 <0.01 <0.05
<0.05 >0.05 >0.05
Bulk density (kg m-3)
Aggregate stability
Clay content
(%)
(n° water drops)
CL
1.10 1.24
144
28
36
46
CF
1.11 1.22
133
33
39
48
BF
1.05 1.19
142
33
40
49
FF
0.96 1.18
182
38
43
53
FV
0.89 1.13
164
41
46
54
se
0.05 0.05
18
3.03 3.87
2.99
p
<0.05 >0.05
<0.05
<0.05 >0.05 >0.05
Keys: CL= Crop Land (‘afub wondo’ based on groundnut-maize-cassava association); CF = Chromolaena
o. fallow (3 to 5 year old); BF = Bush fallow (7 to 9 year old); FF = Forest fallow (more than 15
year old); FV = Virgin Forest (control); se = standard error; p = probability of difference between
CL/CF and FV (p<0.01 = highly significant difference, p<0.05 = significant difference, p>0.05 =
non significant difference).
Sampling strategies and measurement methods
A minipit (60 cm depth) was dug in each plot type and soil morphological
characteristics (horizon thickness, colour, structure, consistence, porosity, etc.) were
described according to the FAO guidelines for soil profile description (FAO, 1990).
Composite soil samples were collected by auger at 0-10, 10-20 and 30-50 cm depth in
each fallow, forest, and newly cropped plot prior to planting. The soil samples were airdried before grinding and sieving, then used for routine laboratory analyses using
methods described in Anderson and Ingram (1993). Aggregate stability was determined
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by Water-Drop Impact method (Imeson and Vis, 1984). Crops densities and data
collected at harvest time from sub-sample plots in farmers’ fields were used to compute
various crops yields and Land Equivalent Ratio (LER).
Analyses of variance (ANOVA) were performed for soil property changes under
various land use types using Systat software, and the least significant difference (LSD)
method was used for means separation. Stepwise analyses of multiple linear regressions
were used to evaluate relationships between soil properties variation and crop (maize,
groundnut, cocoyam and cassava) yields from mixed crop fields following each fallow
type.
Results and Discussion
Changes in soil during the fallow period
During the fallow period the dynamics of the vegetation regrowth affect the
behaviour of soils and their properties change from the morphological to the physicochemical viewpoints.
Topsoil morphology
The most important change in soil morphology during the natural fallow regrowth
was observed on the topsoil thin (3-5 cm) A1 organic horizon. This shallow surface
horizon enriched in humus is dominant under the virgin forest and is known in moist
tropical regions as resulting from high mineralization rates of litter production (FA0,
1998). In shifting cultivation systems, this horizon is destroyed by tillage. During the
natural fallow regrowth, the horizon is recovered at the age 7 to 9 years bush fallow
(Figure 1).
Keys:CL= Crop Land (‘afub wondo’ based on groundnut-maize-cassava association); CF =
Chromolaena o. fallow (3 to 5 year old); BF = Bush fallow (7 to 9 years old); FF = Forest
fallow (more than 15 years old); FV = Virgin Forest (control), (After Yemefack and
Nounamo, 2000).
Figure 1 Development of the topsoil layer with the age of the natural fallow.
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Under the bush fallow and forest fallow, the A1 horizon was observed for all the
soils. In fact, with the development of the shrub vegetation cover at the beginning of the
fallow period, biological activities and the amount litter production and decomposition
are intensified between the new vegetation and the soil surface. The humus content of
the topsoil increases and seeds of pioneer tree species germinate. The tree vegetation
population develops a dense fine root criss-crossing system to better ensure their
nutrition. This process results in the A1 formation. This topsoil horizon is humic, very
rich in fine roots, very friable, very porous, with granular subangular structure induced
by the fine rooting system. It is within this A1 horizon that farmers plant with zerotillage, cucumber (Cucumeropsis mannii) and maize (Zea mays) seeds when they clear
an old forest plot.
Soil chemical properties
Some selected soil chemical properties showed considerable changes under fallow
of various durations. These changes are illustrated in Table 3, at three soil depths (0-10,
10-20 and 30-50 cm).
Table 3 Multiple Linear Regression Equations of various crop yields as functions of
physico-chemical soil properties in 0-20 cm soil slice (at p<0.05).
r²
n
Crop Yield Regression Equation (t ha-1)
Mbio = 0.51CEC + 0.55Ca +0.55C/N – 3.16OC
0.89
37
Mgr = 1.32Bd + 0.04Clay – 0.54OC
0.92
37
0.93
25
Gnbio = 0.05Pav + 2.87Bd – 3.16∆pH
0.91
25
Gngr = 1.63Bd – 0.02clay
0.84
32
Coyam = 0.44CEC + 0.87C/N – 0.82Ca
Keys:
Mbio = Maize biomass yields; Mgr = Maize grain yields; Gnbio = Groundnut biomass yields;
Gngr = Groundnut grain yields; Coyam = Cocoyam tuber yields. Pav. = Available phosphorus;
Ca = Calcium; OC = Organic Carbon; Bd = Bulk density; C/N = C/N ratio; CEC = Cation
Excheange Capacity; n = number of sample.
Organic matter
Organic carbon increased slightly, but gradually with the age of natural fallow.
Kotto-Same et al. (1997) reported this relative slow increase of the soil carbon under
various vegetation cover types in the eastern part of the study area. Although this
increase of organic carbon was not statistically significant at 95% confidence interval,
the difference between the cropped land and virgin forest was around 30%. This results
from rain forest of Cameroon can be correlated with the results of a study in southern
Nigeria (Mulongoy et al., 1992; Kirchhof and Salako, 2000), which showed that fallow
length tended to increase both organic matter and humic acid contents of soils.
However, C/N ratio showed two phases in its evolution. In recently cropped fields (CL)
where the slashed vegetation provided many vegetal organic particles in the tilled soil,
the C/N ratio was as high as from the virgin forest. During the cropping period and the
beginning of the fallow phase, these vegetal particles decayed rapidly and the C/N ratio
diminished significantly (p<0.05) compared to the control FV. During the fallow years,
the humic organic matter was re-built under the forest fallow (FF) and the ratio
increased towards values obtained under the virgin forest.
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Available phosphorous
The available phosphorous was generally very low in the soils of the study area. It
was around 5 mg kg-1 in the first 20 cm of the soil profile. However, the use of these
soils for the traditional agriculture increased very significantly (p<0.01) its availability
in 0-10 cm of the surface layer. The values obtained from the virgin forest (FV) were
three times higher in the cropped land (CL) after burning of the vegetal material. The
increasing P availability in these conditions should probably be seen as a result of the
increase in pH due to the liming effect of ash (Ulery et al., 1993). The ageing of the
fallow reduced this availability of phosphorus. This is with no doubt, due to P
absorption by plants and storage in vegetation biomass. After about eight (8) years of
natural fallow, the P situation became similar to that of the virgin forest.
Cation exchange capacity (CEC), base saturation percentage (BSP)
The CEC was low throughout the area (< 12 cmol kg-1). At the soil depth of 0-10,
the CEC value increased slightly from CL up to fallow age eight of BF, then increased
significantly (p<0.01) with fallow age to reach optimum value as land goes back to
forest fallow at age sixteen and above. In the 10-20 cm and 30-50 cm soil depths, fallow
age had little influence on the evolution of CEC. Trend in BSP evolution was similar to
that of the sum of exchangeable bases. BSP was high under cropped land (CL), and then
decreased significantly (p<0.01) with increase in fallow age to reach lower value at
fallow age eight (BF) and above (FF and FV). It is important to note here, the similarity
that was found between changes in organic carbon and CEC. Changes in CEC are
probably related to changes in soil organic carbon content. This is due the fact that these
soils have low activity clays (1:1 clay minerals) and soil fertility status is often
associated with soil organic carbon rather only clay content (Voundi Nkana et al.,
1997).
pH and exchangeable bases
pH water was very significantly higher (p<0.01) when land was taken into
agricultural use, compared to the virgin forest (FV). After the cropping period, pH water
and exchangeable bases decreased progressively with the fallow age. In bush fallow
(BF) and forest fallow (FF), their values were similar to those obtained in the virgin
forest. ECEC was very significantly high only in the 0-10 cm depth, when land was
taken into cropping. It decreased as land goes back to forest fallow. This increase in pH
and Exchangeable Bases by cropping is ascribed to the effect of ashes from burned
vegetation biomass at the beginning of the cropping period which act as lime fertiliser
(Ulery et al., 1993; Tulaphitak et al., 1985). This liming effect of ashes is reduced by
crop harvest, sheet erosion on bare soil and leaching of cations from cropping to fallow.
The contribution of Calcium in these changes of pH and Exchangeable Bases was about
85%. As reported by Ulery et al. (1993), the potassium is by far the most abundant
water-extractable cation in fresh ash. Consequently, it is the most readily leached cation
by rainwater during the fallow period.
Total acidity and exchangeable aluminium
Total acidity and exchangeable aluminium were below 1 cmol.kg-1 of soil in
cropped land where pH values were around 5. But, in the fallow plots with a pH value
below 5 they occurred and increased six times with the fallow age. The difference
between the control FV and cropped land (CL) or younger fallow CF) was highly
significant (p<0.01). The contribution of exchangeable aluminium to these variations of
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the total acidity was of 75%. However, stepwise multiple linear regressions did not
show any relationship between crop yields and exchangeable aluminium or pH.
Soil physical properties
Some soil physical properties were also assessed under the natural fallow dynamics.
Changes observed in these properties are shown in Table 3. They also varied with the
fallow age. However, their variation rate was somewhat less than that of soil chemical
properties. Nevertheless, clay particles content was significantly (p<0.05) lower in CL
and CF than in FV. Conversely, sand fraction was higher in CL and CF. These changes
in particles size distribution explained the high bulk density and low aggregate stability
in the CL and CF during the same period. As the fallow duration increased, clay content
and aggregate stability tended to increase, while the bulk density decreased. But, the
difference between CL or CF and FV was not statistically significant.
During the two years cropping period, soil porosity was reduced by 15 to 20% due
to increase in bulk density. During the fallow period, the morphological development of
the A1 horizon certainly influenced soil structure making aggregate stability and
porosity the most variable soil physical properties in the study area. The fine rooting
system in this A1 horizon changes the subangular blocky structure of the eroded topsoil
into a porous granular structure. The high inverse relationship between aggregate
stability and bulk density in 0-10 cm slice reflects the development of this A1 horizon.
However, in the process of natural reforestation, a number of soil physical properties
seemed not to benefit from these changes. This implies that the degradation of soil
physical properties such as clay particle and bulk density during the cropping period is
irreversible or hardly reversible during the fallow period. This is a critical problem since
clay particles and the organic matter are the main soil adsorption complex acting as soil
nutrient reservoir in theses Oxisols (Voundi Nkana et al., 1997; Schwertmann et al.,
1992). The similarity between the increase of aggregate stability and organic carbon
content as shown in Table 3 probably denote the effect of organic matter acting as a
stabilising agent of soil structural units.
Crop yields from field plots derived from various natural fallow types
Yield levels obtained in this study were very low, but justified by the crop breeding
varieties, which were of local ones commonly grown in the area for local consumption.
In fertilized conditions, the maximum grain yield harvested for these maize-breeding
materials was 5-6 times less than that of genetically improved materials (IRAD’s Maize
Breeding Unit, pers. com.). In addition to the quality of crop varieties, the crop densities
in the associations were very low compared to density in mono-cropping system.
However, the relative variation of these yields due to fallow age was what concerned in
this study.
No significant difference was found in crop yields between the different fallow
types. However, maize yields tended to be higher after bush fallow. But, the reverse was
showed by groundnut yields. According to farmers, forest fallow and virgin forest were
not suitable preceding fallows for growing groundnut. This local knowledge was then,
confirm by this study.
The land Equivalent Ratio (LER) was estimated only for the major crops, except for
banana-plantain. Figure 2 shows the pooled LER of each field plot type. This pooled
LER was thus, underestimated for BF and FF since banana-plantain was not included.
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Land Equivalent Ratio (LER)
Cucumber
Cassava
Cocoyam
Groundnut
Maize
LER (ratio)
1.8
1.3
0.8
0.3
-0.2
CF
BF
Fallow type
FF
Keys: CF = Chromolaena o. fallow (3 to 5 year old); BF = Bush fallow (7 to 9 year old); FF = Forest
fallow (more than 15 year old); FV = Virgin Forest (control).
Figure 2 Crop productivity from the mixed cropping fields following each fallow type
However, we assumed that this might not change significantly the actual pattern of
land productivity index. From this pattern, old fallows tended to show higher land
productivity compared to the Chromolaena Fallow (BF). Bush Fallow field showed the
relative high LER. The inclusion of banana-plantain LER might reduce the difference
between BF and FF, but, not between CF and BF. It is important to note that, although
the effects of fallow duration were controversial from individual crop yields, pooled
productivity index (LER) showed that the optimum fallow duration in the area should
be around the Bush Fallow. That is approximately 10 years.
Relationship between crop yields and soil properties
Because of the absence of any significant difference between 0-10 cm and 10-20
cm slices, results of these analyses are presented for the top 0-20 cm soil layer.
Regression coefficients and equations of soil properties explaining around 90% of crop
yield variations of are shown in this Table 4. These results showed that, in the area, the
biomass yields of maize and groundnut were mostly related to soil chemical properties
such as CEC and organic matter. While grain yields were controlled by soil physical
properties. Despite the effects of ash materials on groundnut yields, stepwise multiple
linear regressions highlighted bulk density as mean factor related to groundnut grain
yields. Cocoyam yield correlated positively with soil CEC and the organic matter; but
negatively with the ash effect measured by the soil calcium content (Table 4). Cocoyam
yields are then, highly related to less mineralized organic matter, which decomposition
may proceed and release nutrient elements during the crop growth. Cassava yield did
not show any relationship with any soil properties. This in fact, justified the rustic
adaptability of cassava crop to poor soils of the area. Despite the absence of the
regression equation, cucumber growth is more suitable in A1 horizon. Its yield was then
mostly related to the soil morphology. These results are of utmost importance in making
decision regarding the type of fertilizer or amendment to be used when agricultural
intensification is to be considered with fertilisation inputs.
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Conclusion
From clearing a piece of forest land for cropping to the formation of a secondary
forest during the fallow period, soil constituents undergo important changes. But the
magnitude of these changes varies from one soil property to another. These variations
affect mostly soil layers above 20 cm depth; indicating that management practices in
shifting cultivation systems do not directly influence subsoil layers. The following soil
properties appeared to require a particular attention: Organic Carbon, Carbon/Nitrogen
ratio, available Phosphorus, pH, Calcium, Cations Exchange Capacity, Bulk density,
Clay content. They change significantly during the natural fallow period, and correlate
also significantly with crop yields. Ashes from burned vegetation biomass at the
beginning of the cropping period act as lime fertiliser and significantly influence soil
chemical properties. Besides, soil physical properties undergo slow, but hardly
reversible changes. Topsoil structure and porosity are the most affected soil physical
properties as they are more influenced by the development of the topsoil A1 horizon.
The study showed finally that all processes taking place in soils under natural fallow
were directed towards the climax equilibrium existing under primary forests. For that,
the re-use of a fallow plot for crop production was much valuable from the bush fallow
type. It is important to note that crop yield levels obtained by farmers were very low
compared to the standard level. But, they are not aware of possibilities to increase their
productivity. For a sustainable management of the forest zone of southern Cameroon,
this study showed that there are possibilities to intensify crop production and
productivity within a rotational Bush Fallow (BF) system. This intensification can be
achieved by prolonging the cropping period without complete degradation of soil
fertility using for example, the Integrated Nutrient Management methods described by
Janssen (1993), based on combined application of mineral and organic fertilisers.
Acknowledgements
The International Tropical Timber Organization (ITTO) within the framework of
Project PD 26/92 financed this study, which was part of the Tropenbos-Cameroon
Programme.
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