Mechanisms by which citric acid increases phosphate availability

Aims: Cluster roots release carboxylic acids, especially citric acid, into the rhizosphere. We investigated how the citrate ion and the acidity interact to release phosphate. Methods: We prepared solutions with differing citrate concentrations and differing pH and mixed these solutions with a soil. We investigated the effects of: pH, duration of mixing, the presence of chloroform, solution:soil ratio, and the effects of background electrolyte. We compared the results obtained with citrate with those obtained with arsenate. Results: In the absence of chloroform, decomposition of citrate began after one hour. In the presence of chloroform it was delayed until after 24 hours. The higher the pH, the faster the decomposition. In the presence of chloroform, the ratio between phosphate and citrate in solution was steady between one and 24 hours. We chose a duration of six hours for further observations. Desorption of phosphate by citrate was most marked near pH 4. The peak in desorption was sharp in 0.01 M CaCl₂ because, at high pH, much of the citrate was complexed with calcium. These effects of pH were modelled by assuming that divalent phosphate ions were displaced by divalent citrate ions. When compared with arsenate, citrate ions were better at dissolving surface iron atoms and thus releasing phosphate, but much worse in competing with phosphate ions in solution for sorption sites. Conclusions: The acid component of citric acid has direct effects. It moves the pH into a region in which the HPO4⁻ ion dominates and therefore facilitates uptake; it acts as a preservative thus slowing down citrate decomposition, and it directly increases desorption. However its most important effect occurs through interaction with citrate with greatest desorption near pH 4.