Potassium in Two Humid Tropical Ultisols under a Corn and Soybean Cropping System: II. Dynamics

Abstract

AbstractUnderstanding 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 (Zeu 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 20‐cm depth intervals to 60 cm in the loam and 80 em 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.