PHASE-EQUILIBRIA AND KINETICS OF ORTHO-PHOSPHATE IN SOME IRAQI SOILS

We collected soil samples from four different agricultural projects. Potassium orthophosphate was added to the samples at rates of 0, 25, 100, 200, 400, 800, and 1600 ppm P. The samples were incubated for 0.5, 24, 168, 1000, and 3000 h at constant temperature and moisture. Ec, pH, and concentrations of water-soluble orthophosphate, calcium, and magnesium were determined in the soil water extracts after the incubation. The results showed that a sharp decrease occurred in the concentration of water-soluble P, within 0.5 h after P addition. Using calcium phosphate equilibria phases indicated that .beta.-tricalcium phosphate was the dominant form after 3000 h of addition. Precipitated forms were dicalcium phosphate dihydrate, octacalcium phosphate and .beta.-tricalcium phosphate. Using Mg-phosphate equilibria phases revealed that bobiernite was the dominant form after a short time and a high level of P addition. Calcium phosphate controlled the solubility of phosphate more than Mg-phosphate.