A water solution of citric, malic, oxalacetic, succinic, fumaric acids at the concentration found in the root exudates of maize seedlings affected humic substances above 10 000 ds (HMS) by shifting their molecular size towards values below 10 000 ds (LMS). The percent of LMS fraction obtained was significantly correlated with the release of Ca, Mg, Fe, and Zn. Addition of HCl was ineffective on the molecular size, while the metal release occurred to a lesser extent. K salts of the organic acids were ineffective on both molecular size and metal content of humic substances. When the organic acids added to the humic solution were neutralized by 0.1M KOH, the original distribution of molecular size and the metal content were restored to a large extent. The above behaviour suggests that the degree of association and the metal content of humic substances in the rhizosphere might be modulated by the organic acids released by plant roots accordingly with the resultant pH. Fe, Mg and Zn proved to be more involved in the association of humic substances than Ca.