Effect of soil properties on the kinetics of phosphate desorption in some arid, semi-arid and dry sub-humid soils of North-West India

Kinetics of P desorption was studied in ten alkaline- calcareous soils belonging to arid, semi-arid and dry sub-humid regions of North-west India and varying in texture, CaCO₃, organic carbon and extractable Al and Fe (1N NH₄OAc pH 7.0). Desorption of P was investigated in soil samples treated with 200 μg P g ⁻¹ soil and incubated at 25±2°C and 60% water holding capacity for 8 weeks. The amount of P desorbed by the distilled water at water-to-soil ratio of 40:1, on an end-over-end shaker for 5 to 180 minutes, was determined. Individually, none of the soil properties significantly affected the amount of P desorbed. However, a significant exponential regression was obtained between clay/organic carbon (OC) ratio and amount of P desorbed during three hours reaction period. The rate of soil P desorption was satisfactorily described by the modified Elovich equation [Pd = (1/a) ln (ab) + (1/a) lnt] and the equation of Sharpley et al. (Pd = K P₀ t^αW^β). The constants (a and b) decreased with increasing CaCO₃ and CaCO₃/OC ratio and α increased with CaCO₃ content, whereas ‘a’ increased and constants (α and K) decreased with clay/OC ratio. The results evidenced that clay, and CaCO₃/OC ratios could not be universally used to predict the P desorption and restricted only to the soils of similar pedological origin.