Phosphate sorption experiments were conducted on two Andisols, an Oxisol, an Ultisol and a Vertisol from Hawaii to evaluate the effects of soil mineralogies and organic acids (acetic, malic and protocatechuic) on P retention. Findings in the laboratory were further verified by bioassay in the greenhouse, using lettuce as a test crop. Andisols containing a high proportion of strongly reactive alumino silicates sorbed most P, whereas the Vertisol with smectites (2:1 layered silicates) as the dominant mineral sorbed least P. Oxisol and Ultisol dominated by kaolinite and sesquioxide mineralogies were intermediate in P sorption. Similar to P, organic acids also were markedly sorbed by the soils. The structure and charge of the acids apparently controlled the sorption mechanism. It is believed that competition of the acids with P for sorption sites played a major role in releasing P into the soil solution. Among the three acids examined, malic (a hydroxy dicarboxylic acid) was most effective, protocatechuic (a dihydroxy carboxylic acid) intermediate, and acetic (a monocarboxylic acid) was least effective in increasing soluble P concentration. The magnitude of P sorbed was determined by the order in which P and acid were added: least if P was added last. Bioassay confirmed these findings and suggested that the efficiency of P fertilizers would increase significantly if they are applied along with organic acids, or more practically, along with acids-producing materials such as green manures and animal wastes.
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N. V. Hue (1991) studied this question.