Agroforestry for Soil Conservation - World Agroforestry Centre

32 Agroforestry for Control of Soil Erosion
levels, e.g. about 5 m apart on a 40% (22°) slope; this means that the
proportion of land taken is substantial unless the barriers are narrow.
A distinction should be made between impermeable and permeable barriers.
Impermeable barriers are those, such as ditch-and-bank structures,
which check all runoff, either by diversion or by causing infiltration. Permeable
barriers are those which allow some proportion of runoff to pass
through. In agroforestry, barriers are only impermeable in cases of trees
planted on earth structures. Where the barriers are purely biological, such
as hedges or grass strips with trees, they are partly permeable.
Most standard soil-conservation findings are based on the assumption
of impermeable barriers. Research is needed into the functioning of partly
permeable plant-based types of barrier.
Land cover has a large influence on rate of erosion. Whichever of the
predictive models is used, if the effects of the rainfall, erodibility and slope
factors alone are calculated, high rates of erosion usually result. For
example, a site in the subhumid zone (R typically 500), with a ferric luvisol
(K typically 0.3) on a 50 m, 10% (5.7°) slope (S = 1.7) will have a predicted
erosion of 255 t/ha/yr. Reducing the slope length to 10 m by barrier-type
works lowers erosion to 105 t/ha/yr. These apparently high values are
predictions, validated by experimental work, of the erosion to be expected
if land is left under bare fallow.
The cover factor can dramatically reduce predicted erosion rates (Table
7). For annual crops, the value varies substantially with growth and management.
A moderate-yielding cereal crop has a C value of about 0.4, a
late-planted, low-yielding one may be 0.8, whilst for a high-yielding crop
with mulching, a value as low as 0.1 has been obtained (N.W. Hudson,
personal communication). Intercropping generally gives greater cover than
monocropping. Perennial tree crops with cover crops beneath can reduce
erosion to between 0.1 and 0.01 of its rate on bare soil. There are large
differences according to whether residues are applied as surface mulch or
burned or buried.
In summary, the combined effects of rainfall, soil erodibility and slope
will frequently lead to predicted rates of erosion which are unacceptably
high, whilst cereal and root crops do not greatly reduce such rates. On the
other hand, any management system in which a substantial soil cover is
maintained during the period of erosive rains has the capacity to reduce
erosion to between a tenth and a hundredth of its value on bare soil.
Acceptable erosion
It is impossible to reduce the rate of soil loss to zero. Limits have to be
set as targets for the design of land-use systems. They need to be set low
enough such that there will not be a serious or progressive decline in crop
production, yet high enough to be realistically achievable.