Phosphate adsorption and desorption by calcareous soils of Syria

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Abstract
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 μg 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 extradant 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°C.