Influence of phosphorus source and soil moisture on the solubility of phosphorus

Abstract

A laboratory study was conducted on two calcareous soils which had beeu previously cropped with lucerne. Equilibrium soil solutions were obtained from soil samples which were taken after the last harvest of lucerne. Monocalcium phosphate (r/npCa + pH,PO,) and lime (pH -I/zpCa) potentials and dicalcium phosphate potentials (pCaHPO,) were calculated from pH, Ca and P measurements in these solutions.A plot of the lime and phosphate potentials on a solubility diagram showed that all the experimental values were near or above the solubility curve for dicalcium phosphate dihydrate. There was little difference between all phosphorus sources on the Machete soil, and between the treatments on the Wisconlith soil. Lime potentials of the two soils were similar, Phosphate potentials of the Machete soil were lower than those of the Nisconlith soil. The moisture regimes tested did not affect the lime and phosphate potentials of either soil.Dicalcium phosphate potential of hydroxyapatite was significantly higher than for all the other sources on the Machete soil, and also that of monoammonium and monocalcium phosphate on the Nisconlith soil. The lowest pCaHPO, values occurred with the Nisconlith soil Moisture did not influence pCaHPO, on either soil.