POTASSIUM AVAILABILITY IN ACID SULFATE SOIL LANDSCAPES UNDER SUGARCANE CULTIVATION, EASTERN AUSTRALIA

The potassium (K) supply characteristics were determined for sugarcane in the acid sulfate soils (ASS) that characterise many estuarine floodplains. The K+ potential was used to determine the likely availability of K for adequate growth of sugarcane. Changes in soil K availability do not occur independently of other cations; therefore, the K+ potential, expressed as a chemical activity ratio of K+ relative to Ca2+ plus Mg2+, can be used to compare the K status of soils. Results indicate a K deficiency in the floodplain surface soils above the depth associated with pyrite oxidation and acidification of ASS. The problem of K deficiency is associated with the formation of the sulfide mineral oxidation product, jarosite, which acts as a sink for K in the upper sulfuric horizon, and reduces the amount of K that is readily available for sugarcane growth. Conventional exchangeable K measurements describe the quantity of K in the soil, but do not describe the availability of K, as defined by the K+ potential, which describes how tightly the K is bound to the soil. The K adsorption curve, relating exchangeable K to K+ potential, can be used in predicting the critical amount of K needed to provide an adequate supply of available K to the sugarcane crop over the growing season.