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.