Laboratory validation of soil phosphorus storage capacity predictions for use in risk assessment

Soil P concentrations in agricultural soils have increased over the years, increasing concerns about eutrophication of surface waters. Sandy soils are particularly prone to P leaching due to limited P retention capacity. This study tested the validity of a measure of soil P storage capacity (SPSC) for sandy soils amended with dairy and poultry manure by evaluation of SPSC response to P gain or loss under controlled laboratory conditions. Forty soil samples representing A, E, and Bt horizons were collected from two dairy and two poultry operations within the Suwannee River Basin.. Soils were packed into 1.5-cm-diameter columns and 0.05 M KCl solution was passed through the column using unsaturated flow at pore water velocities of approximately I cm d(-1). Then the soils were leached with known quantities of P. Several P addition and leaching cycles followed and the whole experiment lasted approximately 30 mo. Phosphorus in the leachate was measured after each P addition. The P saturation ratio (PSR) was calculated from oxalate P, Al, and Fe extracts analyzed before and after the study. The SPSC of the soils was calculated based on a threshold PSR of 0.15 for the oxalate solutions. Changes in SPSC due to repeated P additions corresponded to predicted values calculated from P loading amounts, taking into consideration the P concentration before additional P loading. Results support the validity of SPSC as a means of estimating P loading rates that pose low environmental risk for specific sandy soils.