Minerals control phosphorus solubility in long-term-cultivated calcareous soils

Little information is available on phosphorus (P) solubility in long-term cultivated calcareous soils. Improved characterisation of P-containing minerals and soil P species in calcareous soils leads to better management of crop production, water quality and soil quality. In this study, we investigated the solubility relationships of P for 20 surface-soil samples from Hamedan, western Iran, with a wide range of physical and chemical properties. Two equilibration times (0.5 and 168h) were used to evaluate the effect of equilibration times on P activities. We observed up to 67% decrease in mean P concentration when equilibration time was increased from 0.5 to 168h. Solubility diagrams support the stability of hydroxyapatite with 0.5h equilibration and hydroxyapatite and -tricalcium phosphate with 168h equilibration. Geochemical modelling predicted that dicalcium phosphate, dicalcium phosphate dihydrate and magnesium-P minerals would be unstable and thus would gradually dissolve and supply P in solution in these calcareous soils. The information obtained can be used to predict the behaviour of P and its availability for agricultural crops in calcareous soils.