Humic acid is a principal component of humic substances, which are the major organic constituents of soil (humus). Humic sub-stances are formed by the microbial degradation of dead plant matter, such as lignin. They are very resistant to further biodegrada-tion. A typical humic sub-stance is a mixture of many mole-cules, some of which are based on a motif of aro-matic nuclei with phenolic and carboxylic substituents, linked to-gether. The functional groups that contribute most to sur-face charge and reactivity of humic substances are phenolic and carboxylic groups. Humic and fulvic acids are commonly used as a soil supplement in agriculture. Organic soil amendments (humus) are beneficial to plant growth due to the release of its constituent nutrient elements into inorganic forms and through its effect on the soil fertility through their water-holding capacity.
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