The combined addition of citric and aromatic organic acids to an acid soil prolongs phosphorus availability

The role of organic acids (both aliphatic and aromatic) in increasing soil P availability is well established. However, the effect of the combined addition of aliphatic and aromatic organic acids on soil P availability is not well studied, which is particularly important when assessing the efficacy of heterogeneous natural organic acid sources, such as compost and biochar, on soil P availability based on their total abundance of organic acid compounds. In this study, some naturally occurring aliphatic (oxalic and citric) and aromatic (benzoic, 4-hydroxybenzoic, and protocatechuic) organic acids were incubated with an acidic soil for up to 110 days and their effect on soil P availability (Olsen extractable P) and phosphorus buffering capacity (PBC) were studied. The aliphatic and aromatic organic acids were added as individual and combined treatments. The experimental results suggested that aliphatic organic acids were more effective in increasing Olsen extractable P and decreasing soil PBC when compared to aromatic organic acids. The effect of citric acid on increasing soil P availability was prolonged when applied in combination with aromatic organic acids. The contrasting chemical nature and complementary effect between aliphatic and aromatic organic acids were suggested as the reason for this phenomenon.