SOYBEAN RESPONSE TO FERTILIZATION AND POT ASSIUM AVAILABILITY IN AN OXISOL (HAPLORTHOX)

With the objective of studying the best technique for corrective K fer tilization and the residual effect of K fertilization, a long term exp eriment was run in an Oxisol (Haplorthox), in Maua da Serra, PR, Brazi l. In the first five growing seasons (1983 to 1987), rates of potassiu m chloride of 0, 40, 80, 120 and 200 kg of K2O/ha were applied every y ear in the seed furrow and by broadcasting. In the next five growing s easons, soybean was cultivated without K fertilization. Treatment effe cts were evaluated on exchangeable-K in the soil, and K content in the leaves of the upper third of the plant and in the seeds, and on soybe an yield. It was concluded that soybean can not achieve high yields wh en cultivated in an Oxisol (Latossolo Roxo distrofico -LRd) low in exc hangeable K (0,10 cmol(c) dm(-3)) without K soil correction with 150 t o 200 kg of K2O/ha plus a yearly maintenance application of 80 kg of K 2O/ha. Soybean leaves of the upper third with less than 12.5 g kg(-1) of K content show K deficiency symptoms. High yields are always associ ated with K content in leaves higher than 17.1 g kg(-1).