Phosphate Sorption on Some Arid, Semi-arid and Dry Sub-humid Soils of North-West India

Phosphate sorption was studied on 28 surface (0–0. 15 m) soil samples of alkaline-calcareous soils belonging to Camborthids, Torripsamments, Ustochrepts, Hapludolls of arid, semi-arid and dry sub-humid regions of north-west India. The samples of each soil were equilibrated for 24 lirs at 25 ± 2 C with aqueous solution of KH₂P0₄ containing 0 to 500 μg P mL⁻¹) in a 1: 20 soil: solution ratio. The amount of P sorbed increased and distribution coefficient (Kd) and percentage of added phosphate adsorbed (Xad) decreased with increasing solution P concentration. Phosphate sorption was satisfactorily described by the two surface Langmuir equation (R²= 0. 95-0. 99, P = 0. 01) and Freundlich equation (R² = 0. 94-0. 99, P = 0. 01). The values of b₂ (adsorption maxima of lower energy sites) were 1. 41 toll. 11 folds as compared to b (adsorption maxima of higher energy sites); whereas the values ofk (bonding energy constants of higher energy sites) were 0. 95 to 41. 52 folds of k₂ (bonding energy constants of lower energy sites). The contribution of high and low energy sites to total adsorption maxima ranged from 8. 25 to 41. 47 and 58. 53 to 91. 75%, respectively. Inverse relationship was observed between adsorption maxima and bonding energy constants. The adsorption maxima for both the regions increased with organic carbon, clay, CaCO, and extractable A1 and Fe. The organic carbon and CEC had significant negative correlations with bonding energy constant k₂ of Langmuir and n of Freundlich equations, respectively. However, when the effect of individual property (e. g. clay or CaCO,) was compared for soils not varying in other soil properties, it was found to be highly marked.