Phosphate Desorption Characteristics of Some Representative Soils of Bangladesh: Effect of Exchangeable Anions, Water Molecules and Solution to Soil Ratios

Establishment of phosphate (P) retention and release capacity of soils is essential for effective nutrient management and environmental protection. In this experiment, we studied the influence of soil properties on P desorption and the relationship between phosphate sorption and desorption. Among the soil series, the Ghior soil had the highest percent clay (59.32%) and free iron oxide (15,241 mg·kg) content. Along the catena of the calcareous soils, percent clay contents increased. For sorption study, the soils were equilibrated with 0.01 M CaCl2 solution containing 0, 1, 2, 4, 8, 16, 25, 50, 100 and 150 mg·P·L solution. For desorption, three extractants namely, 4 SO  (0.005 M) as Na2SO4, 3 HCO  (0.01 M) as NaHCO3 and distilled water were used at extractant to soil ratios of 30:1, 60:1 and 100:1 (v/w). Among the sorption equations, the Langmuir equation showed better fit to the sorption data at higher P concentrations. The amount of phosphate desorbed by all the three extractants increased significantly with the increasing extractant to soil ratios. Phosphate desorption by 4 SO  and water molecules was highly correlated with pH, percent clay and free iron oxide content of the soil. Significant positive correlation (r > 0.64, P < 0.05) was observed between the amount of phosphate desorption and phosphate sorption maximum (bL). Phosphate desorption by 4 SO  and water molecules was also positively correlated with Freundlich constant, N (r > 0.67, P < 0.05) and EPC0 (r > 0.72, P < 0.05). On the other hand, a significant negative correlation (r > –0.77, P < 0.05) was observed between phosphate desorption and phosphate binding strength (KL). The results suggest that freshly sorbed phosphate ions (inner-sphere complex forming species) can be readily desobed by outer-sphere complex forming species like sulphate and bicarbonate ions. Water molecules also desorbed significant amount of freshly sorbed phosphate from the soil colloids.