The influence of the amount and the origin of calcium carbonates on the isotopically exchangeable phosphate in calcareous soils

The kinetics of the isotopic exchange of phosphate ions in soils with and without phosphate added in the laboratory were examined in relation to the amount and origin of the CaCO3 they contained. The isotopic exchange index, ‘Pr/Pe’, and the recovery of added phosphate were inversely proportional to carbonate content in soils containing carbonates of similar geological origin; soils from the Lower Lias showed the biggest change in Pr/Pe with carbonate content.In soils from the Cretaceous Chalk, the first-order rate of isotopic exchange of the ‘slow’ phosphate fraction was constant. It increased to a larger but constant value in the soils incubated for 6 months after adding phosphate in the laboratory. This rate constant is therefore specific to the calcium phosphates in a group of soils derived from the same calcareous parent material and with similar phosphate manuring.