Chemical and mineralogical characteristics of some strongly weathered Fijian soils: Fertility implications

Abstract

Chemical and mineralogical characteristics of six soils, representing areas of Fiji with potential for agricultural development, were investigated with a view to identifying possible fertility constraints. The soils were highly weathered, with clay mineral assemblages consisting of hydrous oxides (gibbsite/goethite) and kaolinite, and all were strongly acid (Al saturation of 29–81%). Solute concentrations in saturation extracts of the soils were unusually low. Ion-activity fraction data suggest that several of the soils are likely to be Ca deficient. Because exchangeable Ca was not related to either the Ca concentration in solution or to the Ca activity fraction, this parameter is unlikely to be a reliable indicator of available Ca status. The soils are known to be severely P deficient, and several other nutrients (S, K, Mg, and Si) are present in very low concentrations. Although Al was the dominant exchangeable cation, a toxicity threshold Al 3+ activity of 4 × 10 −6 M was exceeded in only two of the soils. The concentration of Al in the saturation extracts was not related to either exchangeable Al or percent Al saturation. however, there was a close linear relationship between the negative logarithm of Al 3+ activity and saturation extract pH. The possibility of exploiting this relationship for the identification of Al-toxic soils is suggested.