Phosphorus fractions in grassland soils

The partitioning of P among major chemical forms was studied in 110 non-fertilized, semi-natural grassland soils of southeastern Sweden, comprising a wide range of soil acidity and other chemical properties. The P fractions bound to Ca and Fe were closely related to soil acidity, expressed as pH-KCl, especially when calculated as % of the total pool of mineral P. The relationship was inversely linear for log10P-Fe, whereas it was strongly positively curvilinear for log10P-Ca, with a sudden decrease of this fraction below pH-KCl 4.5. Phosphate bound to Al was inversely related to pH-KCl, but the variability accounted for was rather low. Easily exchangeable phosphate, interpreted as the most labile form of P-Al, was more closely related to soil acidity, with very low values at pHg-KCl > 5.5. Soil concentration of organic P was mainly a function of its contents of organic matter. Data were treated by PCA and stepwise regression analysis.