EFFECTS OF HUMIC AND FULVIC-ACIDS ON THE RHIZOTOXICITY OF LANTHANUM AND ALUMINUM TO CORN

Effects of varying lanthanum (La) or aluminium (Al) concentrations (0- 30 mu M) On corn (Zea mays L.) root elongation were examined in the pr esence and absence of (i) humic acid (HA) at 35 mg carbon (C)/L, or (z z) fulvic acid (FA) at 15 mg C/L, using dilute nutrient solutions. The organic acids were extracted from a mixture of decomposed grass (Sorg hum halepense) and lucerne (Medicago sativa) hay. In the absence of ad ded HA or FA, the addition of La at greater than or equal to 5 mu M an d Al at 30 mu M was toxic to the root growth of corn. The rhizotoxic e ffects of La at 5 and 10 mu M were negated by KA. The ability of FA to overcome La rhizotoxicity was much less, significantly ameliorating t he toxic effects of mu M La but not those of 10 or 30 mu M La. HA and FA did not precipitate La from solution. Both organic acids ameliorate d Al toxicity by complexing Al and reducing monomeric Al in solution. It is concluded that concentrations of HA and FA, commonly present in soil solutions, are capable of forming non-rhizotoxic complexes with L a, hence plant tolerance to La in the soil solution may be appreciably higher than would be indicated by results of solution culture experim ents in which these ligands are not present.