Determination of phosphorus buffering capacity and soil phosphorus requirement in four toposequence from arid and semi-arid regions using sorption isotherms (A case study: Isfahan and Shahrekord)

Phosphate reactions with soil components are important from the viewpoint of plant nutrition and P fertilizer use efficiency. Phosphorus buffering capacity in calcareous soils at upper-, mid-, and lower-slope positions of two arid and two semiarid landscapes were investigated. Sorption data were fitted to the Langmuir, Freundlich, Temkin and Van Hay models to determine P buffering indices. The best fit is found to be with the Freundlich isotherm for all landscapes in terms of the coefficient of determination (R2=0.865-0.994) and the standard error of estimate. Maximum P sorption (derived from Langmuir and Freundlich) were found at lower-slope position of four landscapes. Among the physico-chemical properties of soils, di-thionate extractable Al (Ald) and Fe (Fed) were positively correlated with maximum P sorption showing that P availability in these soils was affected by Fe and Al oxides. Buffering capacity indices such as maximum buffering capacity, standard buffering capacity and K at lower-slope position were higher than other position. All P buffering indices in landscapes showed a positive correlation with Ald. High correlation coefficients between P buffering indices let us to use any buffering indices for prediction of P availability in soils. For maximum production, appropriate P management is needed at lower- slope position (agricultural lands) on the basis of higher P buffering indices and standard P requirement.