Physical Properties of Humic Acid and How to Measure Them
This page covers the physical properties of humic acid products (appearance, solubility, colloid behaviour, moisture) and how they are measured. For acidity, ion exchange and metal binding, see chemical properties of humic acid.
Short answer: Humic acid is a black to dark brown amorphous solid. In its acid form it does not dissolve in water or acid but dissolves in alkali; its potassium and sodium salts dissolve in water. In solution it behaves as a colloid and can settle out with acid or with calcium and magnesium. It absorbs moisture from air.
Physical properties at a glance
| Property | Humic acid (acid form) | Potassium / sodium humate |
|---|---|---|
| Appearance | Black or dark brown powder or granules | Black powder, shiny flakes or crystals, granules |
| Solubility in water | Insoluble | Soluble (grade dependent) |
| Solubility in alkali | Soluble | Soluble |
| In acidic or hard water | Precipitates | Can flocculate at low pH or with high calcium/magnesium |
| Solution colour | Dark brown to black | Dark brown to black |
| Moisture | Absorbs moisture; store dry | Absorbs moisture; store dry and sealed |
How the main properties are measured
| Property | Common test | Why it matters |
|---|---|---|
| Humic acid content | Extraction and gravimetric or titration method; state the basis (dry basis) | Compare products only on the same method and basis |
| Water solubility | Dissolve a weighed sample, filter, weigh the insoluble residue | Needed for drip and foliar use |
| Moisture | Loss on drying | Affects content figures and storage |
| pH | pH of a 1% solution | Affects compatibility in blends |
| Particle size | Sieve analysis | Spreading, dissolving and dust |
| K2O | For potassium humate (method) | Potassium value of the product |
Storage and handling
- Keep in sealed bags in a dry place; humic products take up moisture and can cake.
- Avoid dust when handling powders; follow the product’s Safety Data Sheet.
- Test solubility in your own water before large-scale use in liquid systems.
Frequently asked questions
What colour is humic acid?
Black to dark brown, both as a solid and in solution.
Is humic acid soluble in water?
Humic acid in its acid form is not; it dissolves in alkaline solutions. Potassium and sodium humate are water-soluble.
Why does my humic solution form sediment?
Low pH, hard water (calcium and magnesium) or insoluble matter in the product can make it settle.
How should humic acid be stored?
Sealed, dry and away from moisture, because it absorbs water from the air.
