The trace element deficiency of plant occurs by the chelate ability of soil humic substances.

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When humic substances are included in nutrient medium of the crops cultivation, divalent cation (Ca, Mg) concentrates it to the carboxyl group of the humic substances, and Ca (Mg) absorption by the plant root is promoted. Essential element of iron and carboxyl groups formed 3 coordination stable chelate, and the iron was not absorbed by plant root and the plant caused iron deficiency. Carboxyl group of humic acid which form iron chelate was different from the carboxyl group which could concentrate the divalent cations.