Sorption of organic acids in acid soils and its implications in the rhizosphere

Organic acids have been implicated in many soil-forming and rhizosphere processes, but their fate in soil is poorly understood. We examined the sorption of four simple short-chain organic acids (citric, oxalic, malic and acetic) in five acid soils and on synthetic iron hydroxide (ferrihydrite). The results for both soils and ferrihydrite indicated that the sorption depended on concentration in the following order of strength: phosphate much greater than oxalate > citrate > malate much greater than acetate. The sorption reactions in soil were shown to be little influenced by pH, whereas for ferrihydrite, sorption of all ligands increased strongly with decreasing pH. The sorption of organic anions onto ferrihydrite was influenced to a lesser extent by the presence of metal cations in solution. From the results we calculated that when organic acids enter solution they rapidly become sorbed onto the soil's exchange complex (> 80% within 10 min), and we believe that this sorption will greatly diminish their effectiveness to mobilize nutrients from the rhizosphere.