Abstract
Soil humic acids were extracted from an Entisol, an Inceptisol and an Andisol in this study. We examined the effects of different concentrations of humic acids on growth and nutrient uptake of rice (Oryzasativa) seedlings grown hydroponically. Rice growth was inhibited and the symptoms of chlorosis appeared at the addition of 75 to 100 mg L^<-1> of humic acids. The concentration of K in rice seedlings was almost constant, but the concentrations of Mg and Ca increased with increasing concentration of humic acids. In culture solution, Ca^<2+> and Mg^<2+>, which were concentrated near the negatively charged surface of the humic acids, would move to the root surface by means of ion exchange with H^+ released from the root surface and then absorbed by the roots, resulting in the increase of Ca and Mg concentrations in the seedlings. Fe^<3+> and Cu^<2+> did not undergo ion exchange with H^+ because they were coordinated with the carboxyl groups and formed the complexes with humic acids. Fe and Cu concentrations in the seedlings decreased, leading to chlorosis, because Fe^<3+> and Cu^<2+> were not absorbed as rapidly by the roots. The addition of large amount of Fe^<3+> for 20 days mitigated the chlorosis of rice seedlings hydroponically cultivated with 100 mg L^<-1> of humic acids. The addition of 1.2 mg L^<-1> Fe^<3+> solution prevented the chlorosis of rice seedlings. The results suggest that the addition of Fe equivalent to more than 5% of the carboxyl groups of humic acids can stimulate the growth of rice plant by preventing chlorosis development.
