RESIDUAL EFFECT OF POTASSIUM FERTILIZATIO N APPLIED TO SOYBEAN IN 3 DIFFERENT OXISOLS

Sunflower and corn are two new crop options to be considered in the so ybean-wheat double-cropping system. However, there is little informati on on mineral nutrition for these two crops. With the objective of stu dying the residual effect of potassium fertilization applied for soybe an, on the K-content of sunflower and corn leaves and their yield, thr ee experiments were run on three Oxisols: Latossolo Roxo distrofico (L Rd), Latossolo Roxo alico (LRa) and Latossolo Roxo eutrofico (LRe). Al l these three soils had the exchangeable-K depleted by soybean-wheat d ouble cropping for ten years. In the first five years they were K-fert ilized by zero, 40, 80, 120, 160 and 200 kg/ha/year of K2O, and in the next five years no K fertilizer was applied. It was concluded that K absorption by plants will be restricted and the leaf content will be l ess than 18.8 g kg(-1) of K. This will restrain sunflower yield in Oxi sol soils with less than 0.12 cmol(c) dm(-3). For corn, K-leaf content less than 15.5 g kg(-1) of K decreases yield. Thus, in order to obtai n maximum corn yield, the available K (Mehlich-1) should be higher tha n 0.15 cmol(c) dm(-3) and the K-leaf content higher than 15.5 g kg(-1) .