Sorption of phosphorus in calcareous paddy soils of Iran: effects of soil/solution ratio and pH

The sorption of phosphorus (P) was measured in calcareous paddy soils of Iran at room temperature as a function of pH and soil/solution ratio using batch experiments. Seven soils of different properties and five soil/solution ratios, 1:10, 1:25, 1:50, 1:100, 1:150, and eight pHs, 2, 3, 4, 5, 6, 7, 8, 9 were employed for this study. The pH value of these seven soil samples was adjusted to cover a range of 2–9 by addition of 1 M HCl or 1 M NaOH. The sorption of P was affected by the soil solution pH; at low pH the sorption of P was high. The soil/solution ratio was found to have significant effect on the sorption of P. A larger soil/solution ratio, i.e., more soil for a constant mass of solution, resulted in a smaller maximum P sorption value. This result also held true for Freundlich distribution coefficient at most studied pHs. There were significant correlations between P sorption and Freundlich distribution coefficient and pH and soil/solution ratio. The equations obtained can be used to predict the solubility of P and Freundlich distribution coefficient as a function of pH and soil/solution ratio in calcareous paddy soils.