Page 107 - Soil Degradation, Conservation and Remediation

94 3 Soil Erosion by Water

Fig. 3.16 Alley cropping of cowpea with Leucaena (Photo courtesy of IITA)
interfering for light and moisture with the crops between the rows (Fig. 3.16 ). When
light-demanding crops like corn (maize) will be grown, the alleyways need to be
wide enough to let in plenty of light even when the trees have matured. Alternatively,
the cropping sequence can be planned to change as the trees grow. For instance,
soybeans or corn could be grown when the trees are very small; as the tree canopy

closes, forages could be harvested for hay; and finally, when the trees are fully
grown and the ground is more shaded, grazing livestock, or shade-tolerant crops like
mushrooms or ornamental ferns could occupy the alleyways. For alley cropping,
generally fast-growing multipurpose leguminous tree species including white lead
tree ( Leucaena leucocephala ), Kassod tree ( Senna siamea ), Erythrina ( Erythrina
poeppigiana ), and Gliricidia ( Gliricidia sepium ) are selected. Between these tree
rows, corn (maize), rice, soybean, etc. are grown as alley crops.

3.4.11 Buffer Strips

Buffer strips are zones of permanent vegetation – trees, shrubs, and grasses – used
for different purposes including reduction of erosion. Buffers reduce runoff by
obstructing its way and cutting downslopes, filter sediments, and remove sediment-

borne chemicals and dissolved nutrients and agrochemicals. Buffer strips are gener-
ally established between agricultural lands and streams, rivers, and lakes. When
placed perpendicular to the direction of water flow, buffers are effective measures

for reducing sediment fluxes. Buffers are commonly used in sloping lands of devel-

oping regions where access to heavy equipment and construction of mechanical