EFFECT OF CASSIA-SIAMEA HEDGEROW BARRIERS ON SOIL PHYSICAL-PROPERTIES

Hedgerows of Cassia siamea were planted 4 m apart in a maize/cowpea rotation on a 14% slope of a Lixisol/Alfisol in 1988 at Machakos, Kenya. Infiltration rates were measured in situ across hedgerow barriers with a drip infiltrometer. During the dry season the steady infiltration rate under the hedgerows was 135 mm h(-1) while on the alleys in between it was 41-49 mm h(-1) and on the control plot 39-48 mm h(-1). Corresponding values for the wet season were 69 mm h(-1) for the hedge and 8-11 mm h(-1) for the alley. Removal of the hedge plants revealed that much of the increase is due to significantly more macropores in the topsoil beneath the hedgerow than in the topsoil beneath the alley. Differences in pore size distribution in the subsoil were small and not significant. Total increase in average infiltration rate of the hedgerow barrier system in the dry season is 30% and in the wet season is 94% as compared to the alley. The combined effect of above- and below-ground changes makes hedgerow barriers valuable for runoff control.