The acidic humic acid has a carboxyl group and a phenolic hydroxyl group in the molecular structure to make it weakly acidic. Therefore, humic acid can be quantitatively reacted with carbonates, acetates, and the like. A buffer solution consisting of humic acid and its salts can regulate the pH of the soil and allow the crop to grow at a suitable pH.

Ion exchange Some functional groups on the humic acid molecule, such as H+ on the carboxyl-COOH, can be replaced by metal ions such as Na+, K+, NH4+ to form weak acid salts, so they have a high ion exchange capacity. The ion exchange capacity of humic acid is related to pH. When the pH is increased from 4.5 to 8.1, the ion exchange capacity of humic acid increases from 1.7 mmol/g to 5.9 mmol/g.

Complexation performance Since humic acid contains a large number of functional groups, it can form complexes or chelates with some metal ions (such as Al3+, Fe2+, Ca2+, Cu2+, Cr3+, etc.). The functional groups involved in metal complexation or chelation in the structure of humic acid are generally carboxyl groups and phenolic hydroxyl groups, possibly carbonyl and amine groups.

Physiological Activity The physiological activity of humic acid refers to the ability of humic acid to promote the physiological activities of living organisms.
The physiological activity of humic acid appears to stimulate plant growth and metabolism, improve fruit quality and enhance plant stress resistance, but it is related to the concentration of humic acid and the molecular weight of humic acid.