University of Aberdeen, Aberdeen, U.K.
*Present address: All India Co-ordinated Soil Test Corp Response Correlation Project (ICAR), Saidabad, Hyderabad 500659, (A.P.)
The hydrothermal effect on the inorganic phosphate phase equilibria was evaluated in six contrasting soil groups at two soil moisture regimes (0.5 and 1 moisture equivalent), two levels of P (500 and 1000 ppm) and five temperature levels (30 to 70°C). Soil samples placed in polyethylene bags, heat sealed and kept in metal vacuum desiccator was found to be most suitable way of conducting incubation experiments without moisture loss. The incubation was carried out for 10 days, by which time an apparent equilibrium was reached. The nature of transformations was estimated by following the soil phosphate fractionation procedure.
A physico chemical approach has been proposed to estimate the enthalpy changes in the transformation reactions. The transformations of Al-P and Fe-P into Ca-P were found to be endothermic. The enthalpy changes for the reaction involving Al-P varied from 1.28 to 5.52 k cal/mol and where Fe-P was involved the figures ranged from 0.22 to 3.76 k cal/mol.
Phosphate fixation, soil phosphate equilibria, enthalpy
