Humic acid properties
Short answer: Humic acid is a weak acid. Its carboxyl and phenolic groups react with bases and metal ions, give it buffering capacity, let it form salts (humates) and bind metals such as iron and zinc. It has no single molecular formula; it is a mixture of large organic molecules.
Key chemical properties
| Property | What it means |
|---|---|
| Weak acidity | Carboxyl (–COOH) and phenolic (–OH) groups release H+; reacts with carbonates and hydroxides |
| Salt formation | With potassium or sodium it forms soluble humates; with calcium it can form insoluble precipitates |
| Buffering | Resists changes in pH in solution and soil |
| Ion exchange | Holds cations such as Ca2+, Mg2+ and K+ (CEC of humate) |
| Complexation | Binds metal ions such as Fe, Zn and Cu (iron humate) |
| Redox activity | Quinone groups can take part in oxidation–reduction reactions |
For appearance, solubility and how properties are measured, see physical properties and how they are measured.
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.
Frequently asked questions
Is humic acid acidic?
It is a weak acid. Its carboxyl and phenolic groups make it mildly acidic, and it reacts with bases to form humates.
What is the chemical formula of humic acid?
There is no single formula. Humic acid is a mixture of molecules made mainly of carbon, oxygen, hydrogen and nitrogen.
Why does humic acid bind metals?
Its carboxyl and phenolic groups form complexes with metal ions.
Does humic acid have buffering capacity?
Yes. Its acidic groups help resist changes in pH.
