Humic acids can be classified in various ways. Based on the most common criteria—solubility in solvents and color—they are primarily categorized into three types: fulvic acid, brown humic acid, and black humic acid. Among these, fulvic acid has the lowest molecular weight and highest activity, while black humic acid has the highest molecular weight and the poorest water solubility.
Classification by solubility and color
1. Fulvic acid: Soluble in water, acids, alkalis, and ethanol; has a molecular weight of approximately 300–400 Daltons. It exhibits the best water solubility and highest activity, and is often referred to as the “essence” of humic acids.
2. Brown humic acid: Soluble in acetone or ethanol; its molecular weight and activity fall between those of fulvic acid and black humic acid.
3. Black humic acid: Soluble only in alkaline solutions; has a molecular weight ranging from 10⁴ to 10⁶ Daltons. While it has the poorest water solubility, its soil-improving effects are more long-lasting.
Other common classification methods
1. By source: Divided into natural humic acids and artificial humic acids; natural humic acids can be further subdivided based on their occurrence, such as soil humic acid, coal-derived humic acid, and aquatic humic acid.
2. By raw material properties: Divided into mineral-derived humic acids and biochemical humic acids; mineral-derived types generally offer better activity and stability.
3. By formation method: Divided into primary (native) humic acids and regenerated humic acids.
The benefits of humic acids for plants can be summarized as follows: improving soil, promoting root growth, increasing fertilizer utilization efficiency, enhancing stress resistance, and improving fruit quality. They serve as versatile soil conditioners and fertilizer enhancers.
1. Improving soil structure: Humic acids help transform compacted soil into an aggregate structure, improving aeration and water retention, which allows roots to penetrate deeper. They also regulate soil pH—lowering the pH in saline-alkali soils—while immobilizing heavy metals and reducing their toxicity.
2. Promotes root development: Humic acid stimulates hormone synthesis within the plant and accelerates root hair cell division. This results in faster, more abundant, and stronger root growth, naturally enhancing the plant’s ability to absorb water and nutrients. Data indicates that the rate of root formation can increase by over 40%.
3. Improves fertilizer utilization efficiency: It releases nitrogen, phosphorus, and potassium that have become fixed in the soil; for instance, phosphorus utilization efficiency can more than double, and potassium loss is reduced. It is said that while 100 catties of chemical fertilizer might previously have been required per mu, adding humic acid allows the same results to be achieved with just 80 catties, thereby saving money and reducing soil salinization.
4. Enhances stress resistance: When faced with adverse conditions such as drought, low temperatures, or disease, humic acid boosts the activity of protective enzymes within the crop, strengthening its resistance to drought, cold, and disease. For example, soaking seeds in humic acid can significantly mitigate cold-induced damage to corn seedlings.
5. Improves quality and increases yield: Humic acid promotes photosynthesis and increases chlorophyll content, enabling fruit to accumulate more sugars and nutrients. This results in better flavor and appearance while reducing the incidence of deformed fruit. Data shows that cash crop yields can increase by more than 15%.
