SOYBEAN RESPONSE TO FERTILIZATION AND POT ASSIUM AVAILABILITY IN LATOSSOLO ROXO-ALICO (OXISOL)

Double-cropping soybean-wheat has been cultivated in the Latossolo Rox o Blico (Haplorthox) soils of Parana State, Brazil, since the 70's. Du e to credit restrictions to agriculture. since then most farmers have not used fertilizers for soybean or applied less potassium than that r emoved by the seeds. This system leads to low soil fertility and as a consequence low yields. With the objective to study the best technolog y for potassium fertilization and its residual effects on the soybean, a long term experiment was run in a Latossolo Roxo alico soil, in Cam po Mourao County. In the first five growing seasons, 1983/84 to 1987/8 8, it was applied, once a year, the doses of zero, 40, 80, 120, 160 an d 200 kg of K2O/ha in the sowing row and broadcasted. In the next five growing seasons, soybean was cultivated on the residual effect of the potassium fertilization and also it was studied its effects on K-plan t tissue and soybean yield. It was concluded that Latossolo Roxo Blico can not be cultivated for more than one year with the double-cropping soybean-wheat system without replace the K-removed by the seed.