A simple phosphorus buffering index for Australian soils

Soil phosphorus (P) buffering capacity (PBC) is an important soil property that influences the amount of P fertiliser available for plant uptake. However, current methods of determining PBC are time-consuming and uneconomic in most commercial soil testing programs. The current study examined simpler methods of measuring the PBC of a wide range of Australian soils. Phosphorus sorption and extractable P data from 290 soils (initial data set) were collated to define the range of PBC values of Australian agricultural soils. Independently, detailed chemical and physical analyses were undertaken on a second set of 90 agricultural soils (principal data set), which were selected to represent the range of soil properties measured on the initial data set. Relationships between PBCO&S (Ozanne and Shaw 1968) values (P sorbed between solution P concentrations of 0.25 and 0.35 mg P/L) and 11 different single-point P sorption indices and selected soil properties were examined for the principal data set. Whilst relationships between PBCO&S values and selected soil properties such as oxalate-extractable iron and aluminium, and clay content, were generally poor, strong relationships existed between all of the single-point P sorption indices and PBCO&S. Results suggest that PBCO&S values were most closely related to the P buffering indices (PBI+ColP and PBI+OlsP)when a single addition of 1000 mg P/kg was added to soil and either the Colwell or 4.59 x Olsen extractable P were added to the amount of P sorbed: PBI+ColP = (P-s + Colwell P)/c(0.41) PBI+OlsP = (P-s + 4.59 x Olsen P)/c(0.41) where P-s is the amount of P sorbed (mg P/kg) from a single addition of 1000 mg P/kg, and c is the resulting solution P concentration (mg P/L). This index provides a simple and accurate method for estimating PBC, a fundamental soil property that influences the P fertiliser requirements of different soil types.