Temporal Changes in Available Phosphorus in Some Calcareous Soils

Application of phosphorus (P) fertilizers as solid or liquid fertilizers to soils leads to re-actions of orthophosphate ions with various soil components which remove P from the so-lution phase and convert it to less soluble phosphates. This study was aimed at determin-ing soil properties that affect the rate of phosphate retention in ۲۸ contrasting calcareous soils from different geographical regions (۱۴ soils from Western Azarbaijan in Iran (WAI) and ۱۴ soils from Western Australia in Australia (WAA). The results showed that the mean apparent recoveries of applied available P (Olsen P-based recovery) after ۱۶۰ d of incubation at field capacity was found to be ۷% for the soils of WAI and ۲۵% for the soils of WAA. A study of correlation coefficients showed that, for the soils of WAA, there was a negative relationship between the recovery of applied P and the clay content (r = -۰.۵۹, P≤ ۰.۰۵), whereas there was a positive relationship between the recovery of P and the active CaCO۳ (ACCE) content (r = ۰.۶۴, P ≤ ۰.۰۱). For the soils of WAI, there was a negative correlation between the Olsen P-based recovery and the clay content (r = -۰.۶۱, P ≤ ۰.۰۱). The decline in extractable P with time was best fitted to a second order kinetic equation. The kinetic rate constant (k) ranged from ۰.۰۹۲ to ۰.۵۵ mg kg-۱d-۱ for the soils of WAI and from ۰.۳۱ to ۰.۹۲ mg kg-۱d-۱ for the soils of WAA. The kinetic rate constant (k) increased with increasing the ratio of the clay content to ACCE for all soils of WAI. A re-gression analysis study showed that the ratio of clay/ACCE was the major factor govern-ing P retention.