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 pH-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.