FERTILIZATION RESPONSE AND POTASSIUM AVAI LABILITY FOR SOYBEAN IN OXISOL

The soil, when intensively cultivated with soybean-wheat double-croppi ng system during several growing seasons, has the available K (fixed a nd specifically adsorbed K) reserves depleted after some years of crop ping. This causes soil K deficiency for crops and yield reduction. The objective of this work was to study the soybean response to K fertili zation, and the residual effect on soybean yields, on leaf and soil K content. The experiment was run with soybean during the growing season s 1983/84 through 1992/93 in a 'Latossolo Roxo eutrofico' (Eutrorthox) in Londrina, Parana State, Brazil. In the first five years rates of z ero, 40, 80, 120, 160 and 200 kg/ha/year of K2O were applied. After te n years of experimentation it was concluded that in the 'Latossolos Ro xos eutroficos' of high fertility, even with high K availability the s oybean cannot be cultivated for more than two years without K fertiliz ation, with high probability to have limitations to obtain maximum yie lds. The K maintenance of 80 kg/ha/year of K2O is not enough and in fi ve years of double-cropping almost depleted the soil-K. The fertilizat ion should be at least 120 kg/ha of K2O for soybean and 160 kg/ha/year of K2O for the soybean-wheat double-cropping system to keep top yield s of 3,000 to 3,500 kg/ha. Soybean high yields, over 3,000 kg/ha, were associated with K in the leaf tissue higher than 17.1 g/kg.