Potassium Studies on Some Sugarcane Growing Soils in Fiji

Abstract

This study investigated the potassium (K) status of sugarcane growing soils in Fiji, using samples taken from fields under cultivation and also from sites that had not been cultivated for at least 30 years. Five sites were on highly weathered oxyhydroxide soils where the total K contents and K retention capacities were generally low. The two less weathered soils containing significant amounts of 2:1 expanding clay minerals had much higher total K and non-exchangeable K. Exchangeable K and soil solution K contents varied significantly, but were generally low for most soils. Exchangeable K contents were lower in the regularly cultivated fields at five sites out of the eight, were higher at 2 sites, while for one site there was no change. These variations may be due to different levels of K input, uptake by crop and losses due to leaching and erosion. The two less weathered soils (Sigatoka and Nawaicoba) had lower contents of both non-exchangeable and exchangeable K in the cultivated soil when compared with the fallow sites suggesting net loss of K due to cultivation. These soils, however, also had high surface negative charge and thus a better capacity to retain K. Since the K requirement of sugarcane is characteristically high and large amounts of K are removed in the harvested crop, K fertilizer practices need to be scrutinized carefully in Fiji.