Study of the Phosphorus Sorption Behaviour in the Cultivated and Virgin Soils of Khoy Region

To study the behaviour of phosphorus (P) sorption in the agricultural soils of Khoy region and the effect of long-term cultural management with the application of poultry manure on the P sorption parameters, bath experiments were carried out with 16 soil samples (8 cultivated and 8 virgin soils) and 9 initial P concentrations from 0 to 30 mg L-1 in 0.01M CaCl2 as a background solution. After equilibrium, the remaining amount of P in solution was measured and the experimental sorption data were fitted to the Langmuir (R2=0.93-0.99) and Freundlich (R2=0.87-0.99) models. The results showed that P sorption was increased with enhancing the initial P concentration, eventually reaching the steady-state plateau. Based on the coefficient of determination (R2) and the standard error of estimate (SE), both isotherms models, Langmuir and Freundlich, showed a relatively good fit to the experimental data. The maximum mono layer sorption of Langmuir (qmax) varied from 233 to 486 and from 340 to540 mg kg-1, and the energy parameter of Langmuir (KL) ranged from 0.12 to 0.50 and from 0.22 to 0.71 for the cultivated and virgin soils, respectively. Freundlich sorption capacity (KF) and intensity (n) parameters showed the same trends and KF varied from 36.4 to 123 and 59.3 to 145.2; also n varied from 1.18 to 1.50 and 1.47 to 1.71 in the cultivated and virgin soils, respectively. Consequently, all sorption parameters and the buffering indices showed a decreasing trend in the cultivated soils, as compared to the corresponding virgin soils and the cultural and fertilization management; especially, the application of the poultry manure in this region reduced phosphorus sorption by soil and then increased phosphorus availability to plants. Hence, less fertilizer would be needed to maintain a favourable P concentration in the soil solution for the optimum plant growth.