The decomposition of various minerals by solutions of humic acids has been demonstrated by many investigators. The character of the action depends on the nature of the humic substances, as well as on the resistance of the minerals. Humic substances, like organic compounds of an individual nature, promote the conversion of a number of elements into forms available to plants . The effect of humic acids on the conversion of iron into available forms protecting plants from chlorosis even in the presence of a high P205 content, was demonstrated by DeKock in 1955. In the supply of trace elements and rare elements to plants, an important role is played by compounds in which they are linked with humic substances in the form of chelates.
Numerous reports are available on the presence of auxin type reactions by humic substances.
It is also welt established that humic substances increase the germination capacity of seed and the vitamin content of plants. Many investigators have observed a positive effect of humic substances on the growth of various groups of microorganisms. They attributed this effect to the presence of iron in the humic acids or to their colloidal nature, or they regarded humic substances as organic catalysts. In experiments on the infiltration of NH4NO3 into sunflower leaves, the presence of humic acids was found to increase the percentage content and total amount of nitrogen. Khristeva believes that humic acids entering the plant at early stages of development are a supplementary source of polyphenols, which function as respiratory catalysts.