Comparing measures for determination of phosphorus saturation as a method to estimate dissolved P in soil solution

In response to concerns over the translocation of P from soils to P-sensitive water bodies, there is high demand for developing simple indicators for evaluating a soil’s risk of releasing P into solution. Many studies have shown that the degree of soil phosphorus saturation (DPS), calculated as the ratio of soil P concentration to soil P sorption capacity (PSC), is good predictor of a soil’s risk of releasing P in solution. In this study we compared four different DPS indices in how well they predicted dissolved P following extraction with either a 0.01 M CaCl2 (PCaCl2) solution or deionized water (PW). The first two indices were calculated from the ratio of extractable P to extractable Al and Fe using either acid ammonium oxalate (AlOX + FeOX) or ammonium lactate solutions (AlAL + FeAL). The second two DPS indices were calculated from the ratio of either Olsen-extractable P or AL-extractable P with sorption capacity estimated from the single point P sorption index (PSI). On a subset of 11 soils, we compared the different methods for estimating PSC with fitted Langmuir sorption maximum (Smax) using data from complete sorption isotherms. Both (AlOX + FeOX) and PSI were well correlated with Smax and hence regarded as good estimates for P sorption capacity. Conversely, (AlAL + FeAL) was not significantly correlated with Smax. P saturation calculated from PSI together with PAL or POls predicted PCaCl2 and PW best, whereas P saturation calculated from ammonium oxalate predicted PCaCl2 and PW the least. We did not find notable improvements in the regression models when we added a second explanatory variable (clay content, pH or total carbon) to the models. Our results show that multiple measures of P saturation provide similar predictions of a soiĺs potential for releasing dissolved P into soil solution. This provides flexibility in how P saturation indices are calculated to identify leaching prone hotspots.