Predicting the degree of phosphorus saturation using the ammonium acetate-EDTA soil test

As a result of the important role played by phosphorus (P) in surface water eutrophication, the susceptibility of soils to release P requires evaluation. The degree of phosphorus saturation, assessed by oxalate extraction (DPS(ox)), has been used as an indicator. However, most laboratories do not include DPS(ox) in routine soil tests because of cost and time. This study evaluates the suitability of the ammonium acetate extraction in the presence of EDTA (AAEDTA), the standard soil test P (STP) in Wallonia (Southern Belgium), to predict DPS(ox); we also compared it with the Mehlich 3 extraction. Ninety-three topsoil samples were collected in agricultural soils throughout Wallonia. Good correlations were found between the AAEDTA and the Mehlich 3 methods for P, Fe and Al (r = 0.85, 0.77 and 0.86, respectively). An exponential relationship was found between P(AAEDTA) and DPS(ox). Results of principal component analysis and regression demonstrated that STP can be used to predict DPS(ox) (r = 0.93) after logarithmic transformation. Soil test Al was also a good indicator of the P sorption capacity (PSC(ox)) of soils (r = 0.86). Including the clay fraction in regression equations only slightly improved the prediction of PSC(ox) (r = 0.90), while other readily available data (such as pH or organic carbon) did not significantly improve either DPS(ox) or PSC(ox) predictions.