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EFFECT OF HUMIC ACID ON ALUMINUM TOXICITY IN CORN PLANTS

We conducted greenhouse experiments to study the effect of humic acid on corn seedlings grown in sand, receiving 0 to 50 mg kg−1 Al and 0 to 350 mg kg−1 humic acid (HA). Fertilization was supplied in the form of a Hoagland-Arnon solution adjusted to pH 4.8. In the absence of HA, increasing amounts of Al in the growth media sharply decreased dry matter production. Total dry weight was reduced from 6.8 g/four plants at 0 Al to 4.0 g/four plants at 50 mg kg−1 Al. Plants treated with 50 units Al developed symptoms of Al toxicity, which appeared as marginal chlorosis and necrosis. The addition of HA to pots containing Al increased dry matter production by 32.5 to 42.5% over those with 0 unit HA, and the plants appeared healthy and green. The beneficial effect of HA was also reflected in the Al and P contents of the corn leaves. Leaf-Al concentrations increased linearly with increased Al treatments at 0 units HA, but decreased from a high of 86.6 μg g−1 at 50 units Al and 0 unit HA to 60.5 and 57.4 μg g−1 with 100 and 350 units HA, respectively. The increase in leaf-Al content as a result of Al stress coincided with a decrease in leaf tissue P contents. However, addition of HA suppressed the reduction in concentrations of leaf P, which accompanied Al stress. At treatments with 50 units Al, plants receiving HA contained more P than those at 0 unit HA. Indications suggested that high amounts of Al in solution reacted with P to form insoluble Al phosphates. However, the presence of HA may have chelated most of the Al, rendering it inactive for reaction with P.