Dilute Acid Extraction is a useful Indicator of the Supply of Slowly Available Phosphorus in Vertisols

Long-term studies suggest readily available P, measured using a bicarbonate extractant, can be replenished by slowly available inorganic P pools. The aim of this study was to estimate the supply of slowly available P to the soil solution. Readily and slowly available P pools were measured using two agronomic soil P tests (the bicarbonate (Colwell) and dilute acid (Bureau of Sugar Experiment Stations; BSES) extractants, respectively) in vertisol soils of the northern grains region (NGR) of eastern Australia. The amount of P supplied to the soil solution was estimated by the iron oxide sink method accumulated over 96 h (FeOt-P). The readily available P pool (i.e., Colwell-P) was strongly correlated with FeOt-P, and this pool was likely to be replenished by the slowly available P pool (i.e., BSES-P) when BSES-P concentrations were >61 mg P kg⁻¹. The slowly available P pool did not supply the readily available P pool when the Ca (mg kg⁻¹)/P (mg kg⁻¹) ratio in the BSES extract was >74:1, suggesting the presence of Ca phosphates of low solubility affected potential buffering. Using a multiple linear regression, approximately 75% of the variability in the ability of the slowly available P pool to supply soil solution P could be explained by the quantity of BSES-P and the ratio of Ca/P in the BSES extract.