Transformation kinetics of inorganic P forms in relation to calcareous soil properties of western Iran

This study was conducted to investigate the effect of time on chemical forms of P in 10 calcareous soils of western Iran. Phosphorus was added to soils at the rate of 200 mg kg-1 as KH2PO4. The samples were incubated for 1, 7, 15, 30, 60, 90 and 120 days at 25 degrees C and constant moisture. After incubation, P was fractionated by the sequential extraction procedure: soluble and exchangeable P (NaHCO3-P) Al + Fe-P (NaOH-P), Ca-P (HCl-P) and residual P (Res-P). The distribution of added P into different fractions consisted of two phases involving initial rapid retention followed by a slow continuous retention. In general, the majority of the P applied entered mostly in the HCl-P and Res-P fractions. After 120 days incubation, the HCl-P fraction remained the most dominant in all soils. A combination of silt and sand content of the soils together explained between 88.5 and 83.3% of the variance inNaHCO3-P and HCl-P transformation rates, respectively, 76.6 and 72.8% of which is explained by silt alone. CaCl2-P and electrical conductivity (EC) together accounted for 66.3% of the variation in the rate constant of NaOH-P. The release rate of Res-P was not significantly related to soil properties.