Humic acid is formed in the process of organic matters decomposition by specific microorganisms under special conditions. This product is a compound of macromolecules containing polymeric and phenolic structures, which is able to construct chelate with the nutrient elements in the soil, and consequently, enhance their uptake. The ideal amount of organic matter in the agricultural soils is between 4 - 6% whereas in Iran, organic matter of the soils, and subsequently the humic acid, is very small. Under such conditions, applying humic acid significantly compensate the foregoing deficiency.