Adsorption and desorption of phosphate in some semi-arid tropical indian vertisols

The P adsorption and desorption characteristics of four Vertisols belonging to three agriculturally important soil series were studied. The amounts of P adsorbed by the soils at 0.2 μg/ml equilibrium solution P concn was low and ranged from 34.3 to 79.5 μg/g soil. The phosphate adsorption was very well described by Langmuir and Freundlich isotherms. The P adsorbed by a Vertisol (BR-1) fertilized with different rates of P in the previous season (0, 10, 20 and 40 kg P/ha) was similar (34.3-41.3) indicating little effect of fertilization on P adsorption. The correlation studies indicated that the DTPA-extractable Fe was the most important factor accounting for P adsorption in these soils and clay and CaCO3 content were relatively less important. The capacity of the two extractants and EUF (electro-ultrafiltration) to desorb the adsorbed P followed the order: EUF (400V, 80°C)> sodium bicarbonate > EUF (200V, 20°C) > calcium chloride. The average amounts of P desorbed from the four Vertisols using these methods were 74, 63, 50 and 3% resp. of the adsorbed P. In the Begamganj soil, the amount of P desorbed by EUF (400V, 80°C) exceeded 100%, indicating that all of the adsorbed P was desorbable including some native P. The results show that the Vertisols studied have low phosphate adsorption capacity and that the P they adsorbed is easily desorbabl