Interpretation of a single-point P buffering index for adjusting critical levels of the Colwell soil P test

Soil phosphorus (P) buffer capacity is the change in the quantity of sorbed P required per unit change in solution P concentration. Because P availability to crops is mainly determined by solution P concentration, as P buffer capacity increases, so does the quantity of P required to maintain a solution P concentration that is adequate for crop demand. Bicarbonate-extractable P using the Colwell method is the most common soil P test used in Australia, and Colwell-P can be considered to estimate P quantity. Therefore, as P buffer capacity increases, the Colwell-P concentration required for maximum yield also increases. Data from several published and unpublished studies are used to derive relationships between the ‘critical’ Colwell-P value (Colwell-P at 90% maximum yield) and the single-point P buffer index (PBI) for annual medics, soybean, potato, wheat, and temperate pasture. The rate of increase in critical Colwell-P with increasing PBI increases in the order: temperate pasture medics wheat potato. Indicative critical Colwell-P values are given for the 5 crops at each of the PBI categories used to describe soil P buffer capacity as it increases from extremely low to very high.