Die Veränderung der Phosphatintensität der Bodenlösung eines Pelosol-Pseudogleys nach Grünlandumbruch

Long-term change in phosphate intensity in a clay soil after turning grassland into arable land In a 7years field experiment on a poorly drained clay (Pelosol-Pseudogley) after turning permanent grassland into arable land, the phosphate concentration in the soil solution (P_l) decreased and P retention (P_s) increased even if as much as 265 kg P ha^?1 was added in excess of plant uptake. Neither loss in organic matter nor disaggregation of the well aggregated grassland soil by soil management seemed to be responsible for this decrease in P_l. Instead, a slight increase in pH within the 7 years and, to a lesser extent, a concomitant increase in exchangeable Ca are assumed to have caused the decrease in P_l. Laboratory experiments in which pH, exchangeable Ca and ionic strength were varied support the field results. A multiple correlation between P_l and pH plus exchangeable Ca explained 89% of the variation of P_l whereas pH and Ca alone explained only 76 and 42%, respectively. Solubility data placed the solutions around the hydroxyapatite (HA) isotherm even when P-fertilizer were added in excess of plant uptake. It seems unlikely, therefore that HA governs P_l in this soil. Instead, the results are in favour of surface P adsorption to determine P_l. The effect of exchangeable Ca and ionic strength is then explained by their effects on the thickness of the electric double layer which in turn influences P adsorption.