Soil and Water Conservation under some Improved Fallow Plant Species in Vihiga District, Kenya.
Soils in parts of western Kenya are degraded due to continuous cultivation thus contributing to low crop yields. Alternative farming systems involving the use of short-term tree legume fallows are being encouraged instead of the conventional system where land is left under natural fallow after crop harvest. This study was carried out over a nine-month period to assess the effect of improved fallows on soil erosion and soil water conservation in a subhumid area in Western Kenya. Soil erosion was assessed in the field using a rainfall simulator. The following treatments were considered: (a) Continuous maize (b) Natural fallow cover (c) Improved fallow cover of Tephrosia candida, (d) Improved fallow no cover, (e) Natural fallow no cover ~illed,and (e) Improved falliw no cover tilled. Soil moisture storage was assessed on a weekly basis using a lO-cm interval up to a depth of 60 ern under: (a) Continuous maize, (b) Natural fallow, (c) Tephrosia candida, (d)Tephrosia candida + Macroptilium atropurpureum, (e) Crotalaria grahamiana, and (f) Crotalaria paulina. Runoff intensity, turbidity and soil losses were higher under natural fallow no cover tilled (47.6 mm h-l , W.87 g L-I , 3.02 t ha") compared to the improved fallows no cover tilled (42.4 mm h-I, 4.92 g L-I , 1.31 t ha"). There was no significant difference (p= 0.05) in terms of aggregate stability in the top lO-cm soil between the treatments. Fluctuations in soil moisture content were greater in the upper soil layers and reduced with depth but the variation among treatments increased with depth. There was a faster recharge of water stocks but a gradual decrease under the improved fallows compared to conventional farming practices. Rainfall events of less than 10 mm had very limited effect under the treatments where the cover was over 70%. Improved fallows performed better in improving soil physical conditions as shown by the decrease in soil detachability. Improved fallows offer better opportunity for controlling soil erosion and conserving soil moisture.