PHOSPHATE ADSORPTION AND DESORPTION BY CALCAREOUS SOILS OF SYRIA

To further our understanding of P dynamics in calcareous soils, the P-adsorption isotherms of 12 contrasting surface soils were studied. Methodology evaluation indicated that equilibrium of phosphate adsorption from 0.02M KCl solution containing a range of concentrations of phosphate (KH2PO4) was achieved in 6 hours. Both the Freundlich and Langmuir equations were tested for their ability to describe the adsorption curves, and the former was found superior. Phosphate-adsorption power of soils was significantly related to the clay content of the soil, but to no other measured soil property. It is noteworthy that there was no relationship between P adsorption and CaCO3 content, and this supports recent evidence that iron oxides may play a more active role. Four of the 12 soils used in the above study were selected for the P adsorption - desorption investigation. Estimation of fertilizer requirements of soils using P-adsorption isotherms ranged from 44.7 to 58.7 mug P/g soil. Soil 4, which had already received 300 kg P2O5/ha, required the least application, having adsorbed P from P fertilizer added. Fertilizer had little effect on P adsorption. In all four soils there was desorption of P varying from 0.09 to 24.8% of the adsorbed P with 0.02M KCl extractant consistently giving higher recovery than 0.01M CaCl2. The amount of P desorbed increased with the amount of P originally sorbed, but the relation- ship is not described by any simple function. The results indicate that applied P was, at least, not all irreversibly fixed and that some was released even after 20 hours soil drying at 25-degrees-C.