INFLUENCE OF LOW-MOLECULAR-WEIGHT ORGANIC-ACIDS ON THE SOLUBILIZATIONOF PHOSPHATES

A range of low-molecular-weight organic acids were identified in rhizo sphere soil, leaf litter, and poultry manure compost. Laboratory and g reenhouse experiments were carried out to examine the effects of seven low-molecular-weight organic acids on phosphate adsorption by soils, and the solubilization and plant uptake of P from soil pre-incubated w ith monocalcium phosphate and North Carolina phosphate rock. Acetic, f ormic, lactic (monocarboxylic), malic, tartaric, oxalic (dicarboxylic) , and citric (tricarboxylic) acids were used in the study. The additio n of organic acids decreased the adsorption of P by soils in the order tricarboxylic acid > dicarboxylic acid > monocarboxylic acid. The dec reases in P adsorption with organic acid addition increased with an in crease in the stability constant of the organic acid for Al (log K(Al) ). Organic acids extracted greater amounts of P from soils incubated w ith both monocalcium phosphate and phosphate rock than water did. Alth ough more phosphate was extracted by the organic acids from monocalciu m phosphate - than from phosphate rock - treated soils in absolute ter ms, when the results were expressed as a percentage of dissolved phosp hate there was little difference between the two fertilizers. The amou nt of P extracted by the organic acids from both fertilizers increased with an increase in log K(Al) values. The addition of oxalic and citr ic acids increased the dry matter yield of ryegrass and the uptake of P in soils treated with both fertilizers. The agronomic effectiveness of both fertilizers increased in the presence of organic acids and the increase was greater with the phosphate rock than with the monocalciu m phosphate. The results indicated that organic acids increase the ava ilability of P in soils mainly through both decreased adsorption of P and increased solubilization of P compounds.