A simple phosphorus buffering index for Australian soils

L. L. Burkitt, P. W. Moody, C. J. P. Gourley and M. C. Hannah

Abstract

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 PBC_O&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 PBC_O&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 PBC_O&S. Results suggest that PBC_O&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 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 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.

phosphorus sorption capacity, single-point phosphorus sorption index, phosphorus retention index, soil properties, Colwell phosphorus, Olsen phosphorus.

Keywords: phosphorus sorption capacity, single-point phosphorus sorption index, phosphorus retention index, soil properties, Colwell phosphorus, Olsen phosphorus.

Australian Journal of Soil Research 40(3) 497 - 513

Full text doi:10.1071/SR01050

© CSIRO 2002