Mobilization Characteristics of Phosphate Adsorbed Onto the Sorption Sites of Soils By Oxalic Acid.

Phosphorus (P) is an essential nutrient for plant growth, and is one of the least available mineral nutrients, particularly in highly weathered, tropical soil. Phosphorus accumulation in fertilized soils becomes serious problem for agriculture and the environment. In this investigation, we conducted a laboratory scale investigation to find the most desirable displacement methods of the adsorbed phosphate onto the soil particle surfaces. To do this, the soil columns packed with soil samples containing approximately 20% clay particles were saturated with phosphate solution as KH₂PO₄, and then oxalic acid solutions with the five levels of concentrations ranging from 10^-5 to 10^-1 cmol L^-1 were applied to observe the mobilization of solid-phase phosphate through the soil columns. Apart from this, a batch experiment was also conducted to observe the influence of dilution factors on the displacement of phosphate by oxalic acid. The soil to solution ratios as dilution factor were 1:1, 1:2.5, 1:5, 1:10, and 1:20. The results from column experiment showed that the increase in concentration of oxalic acid gradually increased the mobilization of phosphate ions recovered as effluent. The breaking concentration of oxalic acid was lowered to 10^-4 cmol L^-1 and 5 x 10^-5 cmol L^-1 for dilution factors of 1:10 and 1:20, respectively. The curve fit obtained from the graph can be described by exponential growth when the dilution factors were 1:1, 1:2.5, and 1:5 while the sigmoidal shape for 1:10 and 1:20, showing the mineralization of P were significantly dependent on the dilution factor.