Fertilization response and potassium availability for soybean in oxisol

The soil, when intensively cultivated with soybean-wheat double-cropping system during several growing seasons, has the available K (fixed and specifically adsorbed K) reserves depleted after some years of cropping. This causes soil K deficiency for crops and yield reduction. The objective of this work was to study the soybean response to K fertilization, and the residual effect on soybean yields, on leaf and soil K content. The experiment was run with soybean during the growing seasons 1983/84 through 1992/93 in a 'Latossolo Roxo eutrofico' (Eutrorthox) in Londrina, Parana State, Brazil. In the first five years rates of zero, 40, 80, 120, 160 and 200 kg/ha/year of K2O were applied. After ten years of experimentation it was concluded that in the 'Latossolos Roxos eutroficos' of high fertility, even with high K availability the soybean cannot be cultivated for more than two years without K fertilization, with high probability to have limitations to obtain maximum yields. The K maintenance of 80 kg/ha/year of K2O is not enough and in five years of double-cropping almost depleted the soil-K. The fertilization 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 yields 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.