Citrate and oxalate influence on phosphate, aluminum, and iron in tropical soils

A laboratory study was conducted to examine the effects of increasing concentrations of oxalate and citrate anions on soil phosphate (P), aluminum (Al), and iron (Fe) solubility and P sorption/desorption from some volcanic ash soils. Duplicate 1.0 g soil samples were shaken with 10-mL portions of 0.01 M NaCl solution containing varying concentrations (0-160 mM) of sodium oxalate or citrate. Subsequently, the samples were equilibrated with increasing concentrations of NaH2PO4 . H2O to measure P sorption and with 0.01 M NaCl for P desorption. Results showed that P release from soils increased as concentrations of organic anions increased. At lower concentrations, P release was less than that determined in the NaCl solution. P sorption maxima decreased with increasing loading rates of organic anions; with the oxalate treatment, this decrease amounted to 91% in the soil with the highest organic matter content, suggesting that oxalate was more effective than phosphate in competing for binding sites on the exchange complex. Relationships between organic anions concentrations and release of Al and Fe from the soils were highly significant.