Effect of soil properties on the kinetics of phosphate desorption in acid soils

Kinetics of P desorption was studied in seven acid soils, varying in texture, organic carbon (OC) and extractable Al and Fe (1 N NH4OAc, pH 7.0) content. Desorption of P was determined 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 was estimated by distilled water at water-to-soil ratio of 40:1, on an end-over-end shaker for 5 to 180 minutes. None of the soil properties significantly affected the amount of P desorbed. However, a significant exponential regression was obtained between clay/OC ratio and amount of P desorbed during three hours reaction period. The rate of P desorption was satisfactorily described by the modified Elovich equation [Pd= (1/a) In (ab) + (1/a) Int] and the equation of Sharply et al. [Pd=KP0t^αw^β)]. The constants a and α were negatively related to extractable Al, whereas the constants b and K were positively related to OC and extractable Al and negatively with pH. The constants a and b decreased exponentially with clay/OC ratio, while constants α and K tended to decrease with increasing clay-, Al- and Fe/OC ratios only in soils of similar agro-climatic regions. These ratios could not be used to predict the P desorption in soils of distinct agro-climatic regions which differed in content and composition of humus and clay mineralogy