Residual and annual potassic fertilization for soybeans

There is a lack of long-term research on potassium fertilization for soybea n in the tropical and subtropical soils of Brazil and the residual effect o f the fertilizer has generally not been considered. An experiment was condu cted in plots that had been fertilized with 0, 40, 80, 160 and 240 kg ha(-1 ) of K2O for 3 years on a Dark Red Latosol, loamy sand (Acrortox). The effe cts of annual or residual fertilization with those rates of K were studied for three additional years. Potassium was supplied as potassium chloride or potassium sulphate. Soil and leaf samples were taken annually. There was a residual effect of the nutrient, which provided for high yields up to the 3rd year with the highest rate of K. In order to maintain the K contents of the soil in the medium range and obtain at least 90% of the maximum grain yield, an annual application of 80 kg ha(-1) of K2O was necessary, irrespec tively of the source. The highest soybean yields were always associated wit h K levels above 15 mg kg(-1) in the leaves and 1.2 mmol(c) dm(-3) in the s oil.