Nitrogen fertilizer replacement value of legumes with residues incorporated in the guinea savanna zone of Nigeria
Crop rotation with legumes can help reduce the inorganic nitrogen fertilizer need of the following maize as a result of increased nitrogen availability in the soil. The Nitrogen Fertilizer Replacement Value (NFRV) method was used to estimate the nitrogen contribution of grain legumes (soybean, cowpea) and an herbaceous legume (Centrosema pascuorum), to a succeeding maize crop. The legumes and fallow fields were established in 2001 followed by a test crop of maize in 2002 with inorganic fertilizer rate of 0, 20, 40 and 60 kg N ha-1. The soil inorganic nitrogen was significantly (P< 0.05) increased by 100 and 150 % by soybean and Centrosema residue incorporation respectively while fallow residue incorporation did not change the level. Maize grain yields were significantly (P< 0.05) higher after the legumes than that after fallow. Succeeding maize crops had lower response after soybean than after Centrosema while the fallow had a high response to inorganic nitrogen fertilization. Soybean had the greatest NFRV of 55 kg N ha-1 followed by Centrosema, 34 kg N ha-1 and cowpea, 10 kg N ha-1. The effects of the legumes on the grain yield and nitrogen needs of the succeeding maize were considered to be due to the improved soil nitrogen status after the legumes. Integration of legumes into the cropping systems of the zone is highly recommended to enhance crop productivity.
Key words: Guinea savanna, incorporation, legume residues, N fertilizer replacement value.