Agriculture Reference
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earthworm cast material, a CrT formed by material transported by termites, and a CrG of
accumulated gravel. CrT varies from 10 to 60 cm depending on the degree of maturity;
it is underlain by a 1.25-2 cm CrW horizon with a high humus content. Nye considered
that the CrT derived from the CrG and S, and CrW derived from CrT. The gravel-free
horizon formed by the fauna promotes soil creep down-slope and plants constantly
acquire minerals from the continuously decomposing parent material. The total amount
of soil brought to the surface by termites and accumulated in the CrT was estimated to
be 30 cm in 12,000 years ( i.e., 0.025 mm and up to 0.10 mm in some situations).
This soil would clearly qualify as a biomantle (Johnson, 1990).
In another tropical forest of Côte d'Ivoire, Nooren et al. (1995) hypothesised that
selective erosion of earthworm casts rich in clay was responsible for the creation of
sandy surface soils.
3.4
Erosion
The removal of solid particles from surface soils through erosion is a normal geomor-
phological process and a dominant factor in landform evolution through the movement
of soil materials to lower parts of the landscape. In contrast, accelerated erosion occurring
as a consequence of poor land use practices is a major problem in the management of
a wide range of agricultural soils and in the urban environment.
The factors controlling soil loss through rainfall erosion are given by the revised
universal soil-loss equation (Renard et al., 1994):
A = RKLSCP where A, the predicted soil loss, is the product of
R = climatic erosivity (rainfall and runoff);
K = soil erodibility;
L = slope length;
S = slope gradient or steepness;
C = cover and management;
P = erosion control practices.
Wind may be just as important an agent of erosion as water, notably in drier areas.
Predicted losses due to wind are described by the following interacting factors (Brady
and Weil, 1996):
where:
E = the predicted wind erosion;
I = soil erodibility factor, including soil properties and slope;
C = the climatic factor which includes wind velocity, soil temperature
and precipitation (through its effect on soil moisture);
K = the soil-ridge-roughness factor which includes the size distribution
of the soil surface unit (clods in cultivated soils), the vegetation
cover and whether ridges are present;
L = is the width of the field in the down-wind direction; and
V = is the vegetation cover factor which describes the characteristics of
the vegetation cover present.
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