Agriculture Reference
In-Depth Information
Erosion is normally a slow process, generally in equilibrium with the creation of
soil through pedogenesis. For example, Leneuf (1959) estimated natural erosion in
forested areas of the Côte d'Ivoire to be 0.15 Mg
i.e.,
1 m in 100,000 years.
The period for complete alteration of 1m of granite in the same area was considered
to be between 20,000 to 100,000 years. In natural humid tropical ecosystems, soil loss
estimates of 0.05 to
ca.
6 Mg have been reported (Smith and Stamey, 1965;
Roose and Lelong, 1976; Capelin and Prove, 1983). Lombardi Neto and Bertoni
(1975) consider that values as high as 4.2 to 15 Mg are the limits of acceptable
soil loss tolerance within which the functioning of these soils would not be signifi-
cantly impaired.
In contrast, extremely large annual soil losses have been reported from sugarcane
agro-ecosystems in northern Queensland, due to inappropriate land use practices.
Average values under a cultural regime of burning the crop prior to harvest and any
post-harvest residues and cultivating the soil was 148 Mg with extreme values
exceeding 500 Mg The mean soil losses under regimes involving no-tillage,
no pre-harvest burning of the crops or of post-harvest residues reduced this to approxi-
mately 21 Mg In the United States, rates of total erosional soil losses
considered tolerable in terms of maintaining long-term agricultural production range
from 5 to 11 Mg (Brady and Weil, 1996).
Depending on its intensity, natural erosion slowly alters the nature of the soil profile
(Roose, 1980; Lal, 1984). Under conditions of low slope gradients and thick vegetation
cover, sheet erosion and sub-surface lateral translocation may remove fine particles from
the upper horizons. The soil fauna may partially offset such losses by depositing fine
materials derived from deeper horizons on the soil surface.
When the rates of soil loss by sheet or creep erosion become greater than those of
the formation of humified surface soil, the soil profile may be altered to form either an AC
or a truncated (BC) profile. In the first case, the soil, although shallow, may be relatively
fertile due to the constant input of nutrients from newly-weathered parent material.
Conversely, in the second case, extended soil weathering may lead to the formation of
biologically-exhausted infertile horizons. Eroded material accumulates in low-lying
areas as footslope and valley floor deposits and a continuing flux of new material from
upper parts of the slope or the watershed may be an important feature of the evolution
of landscapes and soils.
