Phosphate Adsorption Behaviour of the Soils of North-West India

Laboratory investigation was carried out to study the phosphate adsorption behaviour on seventeen non-calcareous soils belonging to Camborthids, Torripsamments, Haplustepts, Hapludolls of different agro-climatic zones of north-west India. The samples of each soil were equilibrated for 24 hrs at 25 ± 2°C with aqueous solution of KH₂PO₄ containing (0–500 μg P mL⁻¹) in a 1:20 soil: solution ratio. The amount of P adsorbed increased and distribution coefficient (Kd) and percentage of added phosphate adsorbed (Xad) decreased with increase in solution P concentration. Phosphate adsorption was satisfactorily described by the two surface Langmuir equation (R² = 0. 94 - 0.99, P = 0.01) and Freundlich equation (R² = 0.98 - 0.99, P = 0.01). The values of b2 (adsorption maxima of lower energy sites) were 1.41 to11.11 folds higher than b1 (adsorption maxima of higher energy sites); whereas the values of k1 (bonding energy constants of higher energy sites) were 0.96 to 41.52 folds higher than those 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 of region-I (b1) was positively and significantly correlated with organic carbon (r = 0.64^**), clay (r = 0.74^**), and CEC (r = 0.56^*) and adsorption maxima of region-II (b2) was positively and significantly correlated with clay (r = 0.81^**), CEC (r = 0.59^*), exchangeable Al (r = 0.53^*) and exchangeable Fe (r = 0.58^*) content. Individually, none of the soil properties were significantly correlated with bonding energy constants (k1, k2) of Langmuir and K and n of Freundlich equations except CEC. The CEC had significant negative correlation (r = −0.49^*) with n. The stepwise multiple regression equations revealed that soil properties (pH, EC, OC, CEC, clay, Al, Fe, and available P) jointly contributed for 77.3% variation in b₁, 91.8% in b₂, 27.7% % in k₁, 27.2% in k₂, 66.5% in K, and 76.4% in n in these soils.