Effects of cowpea, crotalaria and sorghum crops and phosphorus fertilizers on maize productivity in semi-arid West Africa

abonos fosfatados abonos nitrogenados abonos verdes application rates crotalaria dose d'application efectos residuales engrais vert fertilidad del suelo fosfato mineral green manures mais maize nitrogen fertilizers phosphate fertilizers phosphate naturel residual effects rock phosphate rotation culturale rotational cropping soil fertility sorghum bicolor superfosfato superphosphate vigna unguiculata zea mays