Chemistry Reference
In-Depth Information
and release moisture), depth and inherent soil fertility. Most of the tropical
soils have serious constraints to crop production; among them, extended
periods / seasonal moisture stress, perhaps the overriding constraint in much of
African soils (about 14% of Africa is relatively free of moisture stress), salinity,
sodicity, acidity, drainage, shallow rooting depth and fertility problems. For
agricultural planning, it is therefore essential that the distribution, extent,
limitations and potential of these different soil types be appreciated. The
general occurrence, characteristics, use and management of the major soils
found in Sub-Saharan Africa are summarised in Table 1.
Tables 2 and 3 show the physical and chemical characteristics of the major
soil types of Kenya and for some west African countries (Liberia, Nigeria,
Ghana, Togo, Burkina Faso, Benin, Niger, Mali and Gambia). The two tables
show that most of the soils are dominated by Lixisols, Acrisols, Luvisols,
Nitisols, Alisols and Ferralsols. In particular, the Kenyan soils represent the
major soil types occurring in east and central Africa regions. Table 2 shows
soil physical and chemical properties for 45 fertiliser trails in the high and
medium potential areas of Kenya for a wide range of major soil types found in
the Kenyan highlands. The soils were selected for fertiliser recommendation
for different agroecological zones in Kenya. It is noted in Table 2 that even
within the same soil group, the soil properties can vary greatly. For instance,
soils for Mumias and Chepkumia are both Acrisols, yet the texture, organic
carbon and total nitrogen vary greatly in both sites (sandy loam texture, 5 g
kg
21
organic carbon (OC) and 0.6 g kg
21
N for Mumias and clay loam texture,
28 g kg
21
OC and 4.5 g kg
21
N). Such variations in soil properties do re-
emphasise the need for specific fertiliser recommendations rather than blanket
recommendations as is mostly the case in Sub-Saharan Africa. Most of these
tropical soils have undergone ferrugination and ferralitisation processes, an
indication of soils that have undergone intense chemical weathering. As a
result, these soils are of low inherent fertility. Coupled with low fertiliser
inputs, on-farm nutrient balance is, in most cases, negative. For instance,
nutrient balance calculations revealed that annual nutrient depletion in Kisii
(Kenya) was 112, 2.5 and 70 kg ha
21
for N, P and potassium (K), respectively,
3
whereas in southern Mali the values were 25, 0 and 20 kg ha
21
for N, P and K,
respectively.
4
In Kisii, removal of nutrients in the harvested product was the
strongest contributor to the negative balance, followed by run-off and erosion.
Work carried out in Kenya on the effect of erosion on soil fertility supports
these findings.
5
Changes in soil pH (regression coefficient, r 5 0.77), OC (r 5
0.59) and total nitrogen (TN) (r 5 0.71) were highly and positively correlated
with soil loss, while maize grain yield was highly and negatively correlated
with soil loss (r 520.91). In the same study, sediment from the plots was 247%
to 936% richer in P than the soil from which it originated. The data indicate
that nutrient loss due to erosion is one of the major causes of soil fertility
depletion of Sub-Saharan African soils. Soil degradation of arable land,
through loss of soil organic matter (SOM) and soil structural stability also
results from soil tillage and the removal of plant biomass. In many tropical
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