PRODUCTIVITY OF WHEAT FOLLOWING NON-FERTI LIZED SOYBEAN IN A DYSTROPHIC DUSKY RED LATOSOL

This research evaluates the soil fertility potential for wheat of a dy strophic Dusky Red Latosol of the state of Parana Brazil, and the tech nical and economical yield response of wheat, double cropped wheat-soy bean, as a function of phosphorus and potassium rates applied to wheat and/or soybean. The experiment was carried out at National Soybean Re search Center/Embrapa, Londrina, PR, from 1990 to 1995. The treatments were six rates of P and K fertilization arranged in the following way : 1) no fertilization for both soybeans or wheat; 2) 30 kg/ha of K2O f or wheat; 3) 50 kg/ha of P2O5 for wheat; 4) 50 kg/ha of P2O5 and 30 kg /ha of K2O for wheat; 5) 50 kg/ha of P2O5 and 30 kg/ha of K2O for whea t, 30 kg/ha of P2O5 and 50 kg/ha of K2O for soybeans; 6) 50 kg/ha of P 2O5 and 30 kg/ha of K2O5 for wheat, 60 kg/ha of P2O5 and 100 kg/ha of K2O for soybeans. Every year, the fertilizers were mechanically applie d in the sowing furrow. The soil maintained its originally high fertil ity level during the three first years, even when no fertilizer was ap plied for soybeans. Application exclusively for wheat crop of 50 kg/ha of P2O5 was enough to maintain the P content in the soil above medium levels during the six years. When P and K contents in the soil approa ched the critical levels, the effect of the fertilization applied for soybeans was detected in the wheat crop. The net income from wheat cro p fertilization became bigger as soil P and K contents dropped approac hing the critical levels.