POTASSIUM IN 2 HUMID TROPICAL ULTISOLS UNDER A CORN AND SOYBEAN CROPPING SYSTEM .2. DYNAMICS

Understanding the dynamics of soil K is important for the formulation of a sound fertilizer management strategy. Two experiments were conducted to determine the behavior of K in the soil after fertilization. Five rates of K (0 to 120 kg ha-1) were applied to two Typic Paleudults, a loam and a sandy loam, in the Peruvian Amazon Basin. A three-crop rotation of corn (Zea mays L.), corn, and soybean [Glycine max (L.) Merr.] was planted at each site. Three additional crops of corn were grown at the loam site. Potassium rates were applied only to the first crop of each three-crop cycle. The corn stover was returned, while the soybean stover was removed. Soil samples were taken at germination, flowering, and harvest of each crop at 2 cm depth intervals to 60 cm in the loam and 80 cm in the sandy loam. Soil K was extracted with NaHCO3, NH4OAc, and concentrated H2SO4. Topsoil exchangeable K increased markedly with K addition, either as fertilizer or as stover, but rapidly decreased during cropping. Changes in the exchangeable pool were characterized with an exponential model. Increases in exchangeable K below 20 cm were of minor significance for the loam. For the sandy loam, however, subsoil exchangeable K was significantly increased to 60 cm. Evidences of K fixation and release were detected in the loam where the topsoil non-exchangeable pool peaked after K additions and decreased during cropping. There was no evidence of fixation in the sandy loam. These differences suggest that higher K rates can be applied to the finer textured soils and there is less risk of loss of K from the rooting zone.