Quantifying the available soil phosphorus pool with the acid ammonium oxalate method

Measurement of the available soil P pool is required for making P fertilization recommendations. Extractants capable of estimating this P pool are needed. The objective of this study was to explore the utility of the acid ammonium oxalate method for estimating available soil P pools in diverse soils. Oxalate extraction in the dark dissolves mainly active noncrystalline Fe and Al in the soil. Oxalate extractable P (P(ox)) has rarely been investigated for predicting the available P pool in highly weathered soils, where Fe and Al oxides are usually abundant. We estimated P(ox) in five highly weathered soils and three slightly weathered soils amended with varying amounts of P. The size of the available P pool in each soil was also estimated by a modified Hedley sequential extraction and verified in a 14-crop exhaustive greenhouse experiment. Results indicated that P(ox) in all soils appeared to be largely plant-available. P(ox) was found unsuitable for quantifying the available P pool in the calcareous, slightly weathered soils. Carbonates raised the pH of the oxalate solution and reduced the concentration of the oxalate anion to differing degrees, thus making this method unreliable for quantifying available P pools in calcareous soils. In highly weathered soils, the relationship between the quantity of available P and P(ox) was described by a linear equation, Y = 294.7 + 1.05 X (r(2) = 0.81), where Y is estimated available P in mg P kg(-1), and X is P(ox) in mg P kg(-1). The P(ox) was a good predictor of the available P pool in the highly weathered soils that were measured by the Hedley procedure and a multicrop exhaustion experiment.