EFFECT OF TEMPERATURE ON PHOSPHATE SORPTION AND DESORPTION IN TWO ACID SOILS

We investigated the temperature effect on P sorption and desorption in two acid soils at five temperatures in the range of 5 to 45°C. Phosphate was sorbed from 0.01 M Cacl2 solution containing four P concentrations between 10 and 100 ppm P equilibrated for 6 days. Phosphate was desorbed by shaking with 0.01 M CaCl2 solution for 6 hours. Phosphate sorption increased, but desorption decreased with increasing temperature, when sorption and desorption were initially at the same temperature. When desorption followed sorption at 25°C, the amounts of P desorbed increased with increasing temperature. It appears that the sorption process was an overriding factor in the desorption processes, when the sorption temperature was higher than the desorption temperature. The calculated values of the apparent heat of reaction (ΔH) for P sorption and desorption in the two soils ranged from 1.6 to 3.3 kcal/mole. The results obtained in this study show that the temperature at which P is sorbed should be given primary consideration when using P sorption and desorption isotherms for evaluating fertilizer P requirements of crops grown at different temperature regimes.