RESIDUAL NITROGEN FROM LEGUMES AND ITS EFFECT ON THE SUCCEEDING CROP OF SWEET POTATO

Abstract

Three legumes (mungbean, bushbean, and soybean) and sweet potato (control) were planted during the first cropping. After harvesting each crop, the residues except those of sweet potato were plowed under and incorporated into the soil. The final planting of sweet potato was done subsequently after the legume residues had been decomposed. Results showed that there was a significant increase in the weight and length of vines of the three legumes. Weight and number of marketable tubers, weight of non-marketable tubers, and total tuber yield greatly increased in plots previously planted to bushbeans and soybeans while the said parameters had no effect on mungbeans. Tuber yield and other agronomic characters were positively influenced by the different fertilizer levels. Fertilizer level of 0-30-60 kg/ha increased considerably the tuber yield of sweet potato planted after bushbeans or soybeans. Cost and returns analysis revealed that, regardless of the level of fertilizer applied, higher net profits were obtained from plots previously planted to bushbeans and soybeans. The best result was noted in the continuous sweet potato cropping treated with 60-60-60 kg/ha on both croppings. Considering net returns per unit time, however, alternate cropping of sweet potato with bushbeans or soybeans gave greater output in addition to the improved physical conditions of the soil.