Differential response of sweet potato (ipomoea batata) to green manuring with elephant grass (pennisetum purpureiuri) and spear grass imperata cylindrical on a ferralitic soil in Sierra Leone

Two separate field studies were conducted to evaluate the response of sweet potato to green manuring with elephant grass (Pennisetum purpnreitm) and spear grass ( Imperata cylindricd). The yield and yield components of sweet potato increased consistently with increasing quantities of elephant grass before levelling up between 20, and 301 ha"1. This suggests the suitability of elephant grass as green manure in sweet potato production. Yield and yield components of sweet potato increased with green manure rates of spear grass up to 10 t ha"1 but progressively declined beyond this rate. This may be due to low N availability resulting from high C:N and consequent poor mineralization. Moreover phytotoxicity of spear grass at higher rates of application beyond 10 t ha"1 could be responssible for the low yield. Thus for higher yield, manure rates of spear grass above 10 t ha-1 can not be recommended. Further studies to separate phytotoxic effects of decomposing speargrass residues from poor minerilzation effects are needed.