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

Measurement of the available soil P pool is required for making P fertiliza tion recommendations. Extractants capable of estimating this P pool are nee ded. The objective of this study was to explore the utility of the acid amm onium 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 investigat ed 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 weathe red soils and three slightly weathered soils amended with varying amounts o f 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 ra ised the pH of the oxalate solution and reduced the concentration of the ox alate anion to differing degrees, thus making this method unreliable for qu antifying available P pools in calcareous soils. In highly weathered soils, the relationship between the quantity of available P and P-ox was describe d by a linear equation, Y = 294.7 + 1.05 X (r(2) = 0.81), where Y is estima ted 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 experimen t.