PHOSPHORUS SORPTION IN RELATION TO SOIL PROPERTIES FOR THE MAJOR SOIL TYPES OF SOUTH-WESTERN AUSTRALIA

The P sorption characteristics of 97 soils that are representative of the agricultural areas of Western Australia were described using Langmuir and Freundlich equations. The Langmuir P maximum (x(m)) ranged from 11 to 2132-mu-g g-1 soil and the Freundlich k coefficient ranged from 1 to 1681. Clay content, DCB Fe and Al, oxalate Fe and AL, and pyrophosphate Al were positively related to x(m) and k. By using stepwise regression analysis, the combination of DCB and oxalate-soluble Al predicted more than 75% of the variation in the P sorption coefficients. Reactive Al compounds may thus be responsible for much of the P sorption by these soils. Soil pH in 1 M NaF (pH 8.2), which is normally used for the detection of allophanic material, was strongly related to the P sorption coefficients and might therefore be used as a quick test for predicting the P sorption capacity of soils.