Fulvic acid is a plant growth regulator that can promote plant growth, play a crucial role in drought resistance, enhance plant stress resistance, and improve crop yield and quality.
As the component of humic acid with the smallest molecular weight and the highest activity, fulvic acid is the essence of the effective ingredients in humic acid.
According to solubility in solvents, humic acid can be divided into fulvic acid, brown humic acid, and black humic acid. As shown in the following table, fulvic acid has the best water solubility and activity!
| Humic Acid Type | Solubility | Molecular Weight | Activity |
|---|---|---|---|
| Fulvic Acid | Soluble in water | Several hundred to tens of thousands | High |
| Brown Humic Acid | Soluble in organic solvents | Tens of thousands to several hundred thousand | Low |
| Black Humic Acid | Soluble in alkali | Several hundred thousand to several million | Low |
Fulvic acid is a water-soluble grayish-black powdery substance. As a plant growth regulator, it can promote plant growth, exert a significant effect on drought resistance, enhance plant stress resistance, and increase yield while improving quality.
Its main application objects include wheat, corn, sweet potato, millet, rice, cotton, peanut, rapeseed, tobacco, mulberry, fruits, vegetables, etc. In practical agricultural applications, fulvic acid demonstrates the most stable and optimal results.
Specific characteristics: Fulvic acid has a small molecular weight, making it easily absorbed and utilized by plants; it is rich in functional groups, resulting in higher physiological activity than ordinary humic acid and strong complexation ability with metal ions; unlike humic acid, which cannot be directly dissolved in water and needs to be converted into monovalent metal salts such as potassium and sodium salts or ammonium salts (whose aqueous solutions are alkaline), fulvic acid can be directly dissolved in water, and its aqueous solution is acidic.
I. The Effects of Fulvic Acid on Soil, Fertilizers, and Plants in the Agricultural Field
| Fulvic Acid Characteristics | Physical Effects | Chemical Effects | Biological Effects |
|---|---|---|---|
| Small molecular weight | Adsorption, Permeation | Complexation, Ion exchange | Promoting root growth, Increasing yield and improving quality, Enhancing stress resistance |
| Ability to form colloids and porous structures | Improving soil structure, Preventing soil cracking and erosion, Increasing soil water-holding capacity, Darkening soil color to facilitate solar energy absorption | Regulating soil pH, Enhancing soil buffering capacity, Improving nutrient and water absorption, Increasing solubility of organic fertilizers | Stimulating growth and reproduction of beneficial soil microorganisms, Enhancing plants’ natural disease and pest resistance |
| Abundant active functional groups | – | Reducing fertilizer loss, Converting nutrients into easily absorbable forms | – |
1. Direct Effects
Promote plant growth and increase crop yield.
2. Indirect Effects
Divided into physical effects, chemical effects, and biological effects.
Physical Effects:
- Improve soil structure, prevent soil cracking and erosion.
- Increase soil water-holding capacity and enhance cold resistance.
- Darken soil color, facilitating the absorption of solar energy.
Chemical Effects:
- Regulate soil pH and enhance soil buffering capacity.
- Improve and optimize plants’ absorption of nutrients and water.
- Rich in organic matter and minerals essential for plant growth, improve the solubility of organic fertilizers, reduce fertilizer loss, and convert various nutrient elements into forms easily absorbed by plants.
- Enhance plants’ absorption of nitrogen, reduce phosphorus fixation, retain and store elements such as nitrogen, phosphorus, and potassium deep in the soil, and accelerate the process of nutrients entering plants, thereby improving the application effect of inorganic fertilizers. Therefore, fulvic acid is a “reservoir” of plant nutrient elements and physiologically active substances.
Biological Effects:
- Stimulate the growth and reproduction of beneficial microorganisms in the soil.
- Enhance plants’ natural ability to resist diseases and pests.
II. Six Major Properties and Ten Core Functions of Fulvic Acid
1. Six Major Properties
- Long-acting nature of organic fertilizers
- Quick-acting nature of inorganic fertilizers
- Growth-promoting nature of biological fertilizers
- Multi-element nature of mineral fertilizers
- Comprehensive nature of compound fertilizers
- Convenience of flushing fertilizers
2. Ten Core Functions
- Activate soil
- Improve fertilizer efficiency and retain moisture
- Alleviate soil compaction
- Enhance replanting resistance
- Strong stress resistance
- Stimulate development
- Accelerate growth
- Improve quality and reduce costs
- Promote robust root systems
- Green and environmentally friendly
3. Detailed Explanation of Fulvic Acid’s Effects
Soil Improvement:
Improve soil aggregate structure. As a humus substance, fulvic acid can affect soil properties, promoting the formation of more stable aggregate structures. It can increase the content of aggregates ≥0.25mm in soil by 10-20% and organic matter content by 10%, enabling soil to retain water, increase aeration, and benefit crop growth.
Enhance soil water retention. Fulvic acid is a hydrophilic colloid with strong water absorption capacity, with a maximum water absorption capacity exceeding 500%. The weight of water absorbed from saturated atmosphere can be more than twice its own weight, much higher than that of general mineral colloids. Additionally, fulvic acid inhibits crop transpiration, slowing down the rate of soil water consumption and increasing relative soil water content.
Enhance soil fertilizer retention. As an organic acid, fulvic acid not only increases the dissolution of the mineral part in soil to provide soil nutrients but also improves nutrient availability through complexation.
As an organic colloid, fulvic acid carries both positive and negative charges, enabling it to adsorb cations and anions, thereby retaining these nutrients in the soil without leaching with water and improving fertilizer utilization rate—this is particularly significant in sandy soils.
Regulate soil solution pH. The mutual transformation between fulvic acid and fulvates forms a buffer system, which regulates the pH of soil solution.
Reduce soil salinity. The colloidal structure formed by fulvic acid through complexation and chelation of metal cations in soil, along with its porous nature (large specific surface area), can adsorb ions or molecules in soil solution, reducing the salt concentration of soil solution.
Biological Effects:
Fulvic acid contains various oxygen-containing functional groups, which determine its physiological activity and thus regulate life processes. It promotes the growth and reproduction of beneficial bacteria while inhibiting the number of harmful microorganisms. The carboxyl and phenolic hydroxyl groups in fulvic acid also have a certain virus-inhibiting effect.
Improve Fertilizer Utilization Rate:
Fulvic acid contains functional groups such as carboxyl and phenolic hydroxyl groups, which have strong complexation, chelation, and surface adsorption capabilities. It can reduce the loss of ammonium nitrogen, increase the movement distance of phosphorus in soil, inhibit the fixation of water-soluble phosphorus by soil, convert invalid phosphorus into effective phosphorus, and promote root absorption of phosphorus. Additionally, fulvic acid can absorb and store potassium ions, increasing the content of available potassium—its synergistic effect on potassium fertilizers is particularly significant.
III. Fulvic Acid Can Improve the Utilization Rate of Nitrogen, Phosphorus, and Potassium in Fertilizers by More Than 20%
1. Synergistic Effect on Nitrogen Fertilizers
The active groups of fulvic acid (including carboxyl groups, hydroxyl groups, and certain groups containing P, O, N, S) are generally electron donors, which can easily form coordination compounds with many electron acceptors (polyvalent metal ions, organic groups, or ions), known as complexes or chelates.
For example, fulvic acid-urea is essentially a complex (chelate), which can reduce the loss of ammonium nitrogen in ammonium bicarbonate and improve the utilization rate of nitrogen fertilizers. Nitrohumic acid produced by oxidative degradation can inhibit urease activity and reduce urea volatilization.
Adding fulvic acid to ammonium bicarbonate can reduce the nitrogen volatilization rate from 13.1% to 2.04% within 6 days. In field trials, the fertilizer efficiency of ammonium bicarbonate lasts for more than 20 days, while that of fulvic acid ammonium can reach more than 60 days.
Adding fulvic acid, especially nitrohumic acid, to urea can form urea complexes, slowing down urea decomposition, extending fertilizer efficiency, reducing losses, and relatively improving urea utilization rate by 30% with a residual effect increase of more than 15%. Nitrogen fertilizer utilization rate tests show that after adding fulvic acid, the utilization rate increases from 30.1% to 34.1%, and nitrogen absorption increases by 10%.
The combined effect of fulvic acid and nitrogen on plant growth and development is very obvious. When nitrogen and fulvic acid are sufficient, plants can synthesize more proteins, promoting cell division and growth. Therefore, the leaf area of plants increases rapidly, providing more leaf area for photosynthesis.
It has a significant effect on promoting robust plant growth—usually, after application, the leaf color turns green quickly and the growth volume increases.
2. Synergistic Effect on Phosphorus Fertilizers
Degraded nitrohumic acid can increase the movement distance of phosphorus in soil, inhibit the fixation of water-soluble phosphorus by soil, convert invalid phosphorus into effective phosphorus, and promote root absorption of phosphorus. Using fulvic acid to protect water-soluble phosphorus fertilizers or phosphorus-based compound fertilizers can reduce phosphorus fixation, promote phosphorus absorption, and improve phosphorus fertilizer utilization rate.
Fertilizer efficiency tests show that adding 10-20% fulvic acid to superphosphate, triple superphosphate, or ammonium phosphate can relatively improve fertilizer efficiency by 10-20% and increase phosphorus absorption by 28-39%. Radioactive phosphorus tracer tests for phosphorus fertilizer utilization rate indicate that after adding fulvic acid, the seasonal utilization rate of phosphorus fertilizer increases from 15.4% to 19.3%, meaning the phosphorus fertilizer utilization rate increases by a quarter.
Fulvic acid combined with phosphorus participates in photosynthesis, respiration, energy storage and transfer, cell division, cell enlargement, and other processes in plants.
3. Synergistic Effect on Potassium Fertilizers
The acidic functional groups of fulvic acid can absorb and store potassium ions, reducing potassium loss with water in sandy soils and soils with strong leaching. Fulvic acid can prevent potassium fixation in clay soils and increase the content of exchangeable potassium. It also has a dissolving effect on potassium-containing minerals, slowly increasing potassium release and improving the content of available potassium in soil.
Additionally, fulvic acid can stimulate and regulate crop physiological metabolic processes through its biological activity, increasing potassium absorption by more than 30%.
The combined use of fulvic acid and potassium can promote photosynthesis, significantly improve plants’ absorption and utilization of nitrogen (which is quickly converted into protein), and promote efficient water use by plants.
4. Promote Trace Element Absorption and Effectively Solve Nutrient Deficiency Symptoms
Fulvic acid chelates medium and trace elements to form fulvic acid chelates with strong mobility and easy absorption by crops, which are transported to nutrient-deficient parts of crops, effectively solving nutrient deficiency symptoms.
In addition to macronutrients such as nitrogen, phosphorus, and potassium, crop growth and development also require trace elements such as iron, boron, manganese, zinc, molybdenum, and copper. These elements are components of various enzymes in crops and play important roles in promoting normal crop growth and development, enhancing disease resistance, increasing yield, and improving quality.
Most trace elements in soil are in an invalid state that is difficult for plants to absorb, and applying trace element fertilizers to soil is also easily fixed by soil.
Studies have shown that fulvic acid can undergo chelation reactions with trace elements such as iron and zinc to form fulvic acid trace element chelates with good solubility and easy absorption by plants, such as fulvic acid-Zn, fulvic acid-Mn, and fulvic acid-Fe. These chelates are conducive to root or foliar absorption and can promote the transport of trace elements from roots to above-ground parts.
Tests indicate that the amount of fulvic acid iron entering plants from roots is 32% more than that of ferrous sulfate, and its mobility in leaves is twice that of ferrous sulfate, increasing chlorophyll content by 15-45% and effectively solving leaf yellowing caused by iron deficiency.
5. Effects of Fulvic Acid Combined with Individual Medium and Trace Elements
Rice is a silicon-loving crop. Applying fulvic acid silicon fertilizer to rice increases grain weight per panicle, promotes early maturity, and enhances disease resistance. It can also improve crop resistance to diseases and pests, reduce pest damage, enhance lodging resistance, increase air permeability in crops, improve stress resistance, reduce phosphorus stability in soil, improve agricultural product quality, and facilitate storage and transportation.
Combining fulvic acid with calcium fertilizer can reduce fruit absorption, increase fruit firmness, improve fruit storage resistance, reduce decay, and increase vitamin C content.
Combining fulvic acid with magnesium fertilizer can promote plant photosynthesis and protein synthesis, improving crop yield and agricultural product quality.
Combining fulvic acid with sulfur can increase protein content, improve the quality of cereal crops, increase vitamin A content and oil content of oil crops, and simultaneously improve the quality of fruits, vegetables, sugar beets, and other varieties, while enhancing crop cold and drought resistance.
Zinc combined with fulvic acid is absorbed by plants in the form of Zn²⁺. In nitrogen metabolism, it can effectively change the ratio of organic nitrogen to inorganic nitrogen in plants, greatly improving drought and low-temperature resistance and promoting healthy growth of branches and leaves. It also participates in chlorophyll formation and prevents chlorophyll degradation and carbohydrate formation.
6. Slow-Release and Synergistic Mechanism of Pesticides
Humic acid has surfactant functions, which can reduce the surface tension of water and exert emulsifying and dispersing effects on pesticides. It can also undergo hydrogen bond association or ion exchange reactions with many pesticides to varying degrees.
As a colloidal substance with high viscosity and large surface area, fulvic acid can produce strong physical adsorption on pesticides. Additionally, fulvic acid itself has bacteriostatic and disease-resistant effects, so compounding it with fungicides is equivalent to compounding two pesticides.
It can promote early coloring and ripening of fruits, similar to the ripening effect of ethylene. It also promotes cell division, elongation, and differentiation, similar to the effect of more than two plant hormones.
7. Improve Agricultural Product Quality
Fulvic acid enhances the synthesis and transport of sugars, starches, proteins, fats, and various vitamins through enzymes. It can stimulate the activity of polysaccharidases, converting polysaccharides into soluble monosaccharides, thereby increasing fruit sweetness. It also improves the ratios of total sugar to nicotine and potassium to chlorine in tobacco leaves, enhancing tobacco quality, and increases the total sugar content and vitamin C content in fruits such as watermelons and cantaloupes.
IV. Fulvic Acid Enhances the Synthesis and Transport of Sugars, Starches, Proteins, Fats, and Various Vitamins Through Enzymes
1. Regulate Enzymatic Reactions and Enhance Plant Life Activities
Enzymes are biological catalysts for plant life activities, and the effectiveness of enzymes is reflected by their activity. Fulvic acid can stimulate the activity of polysaccharidases, hydrolyzing pectin in young cell walls, softening cell walls, and facilitating cell elongation and division (both longitudinally and transversely). Therefore, it has a significant effect on promoting the growth of young cells in new tissues, especially on root growth points.
Fulvic acid contains a large number of carboxyl groups, which can inhibit the activity of auxin oxidase, reducing auxin destruction and increasing its content, thereby promoting root and stem growth. This results in crops taking root faster, developing more lateral roots, increasing root volume and elongation, and enhancing their ability to absorb water and nutrients.
Fulvic acid can promote the activity of invertase, starch phosphorylase, and enzymes related to protein and fat synthesis, increasing the synthesis and accumulation of substances such as sugars, starches, proteins, fats, nucleic acids, and vitamins. It also promotes the activity of transferases, accelerating the transport of various metabolites from stems, leaves, or roots to fruits and grains, which directly affects the improvement of crop yield and quality.
2. Increase Chlorophyll Content and Promote Photosynthesis
Fulvic acid can promote the absorption and transport of medium and trace elements by plants, significantly increasing leaf chlorophyll content. It can also inhibit the activity of proteolytic enzymes, slowing down chlorophyll decomposition. Additionally, fulvic acid improves the activity of protective enzymes, reducing the damage of reactive oxygen species to chlorophyll. All these help maintain and increase chlorophyll content, promote photosynthesis, and enhance the accumulation of photosynthetic products.
3. Enhance Respiration
Fulvic acid can enhance the activity of respiratory enzymes, especially terminal oxidase, thereby strengthening respiration, continuously releasing energy and producing many intermediate products to support plant life activities. Enhanced respiration promotes root absorption and material synthesis.
4. Promote the Absorption and Transport of Mineral Elements
Many trace mineral elements such as Fe, Cu, Zn, Mn, B, and Mo are components of enzymes or coenzymes involved in plant metabolic activities, or have important effects on the activity of various enzymes and plant stress resistance, and some are components of cell structural substances.
Fulvic acid can form soluble complexes (chelates) with mineral elements in soil, which improves crops’ absorption of many trace elements.
5. Enhance Crop Stress Resistance Mechanisms
One of the indicators of plant stress resistance is the content of ABA (abscisic acid: a plant hormone that inhibits growth) in plants.
Under any stress conditions, the ABA content in plants will increase. Abscisic acid is the “first messenger” that activates the expression of stress resistance genes in plants, effectively activating the plant’s stress resistance immune system.
V. Physiological Mechanisms of Fulvic Acid Improving Plant Stress Resistance
Fulvic acid can increase the ABA content in plants.
1. Drought Resistance Mechanism
Fulvic acid increases the ABA content in plants. As a growth inhibitor, ABA can reduce stomatal aperture, decrease water transpiration, promote the accumulation of osmotic adjustment substances such as proline, and help plants retain more water. At the same time, it promotes root development and improves root activity, enabling crops to absorb more water and nutrients under drought conditions. These two effects complement each other, similar to “opening up sources and reducing expenditures.”
It also increases the activity of substances that scavenge free radicals, such as SOD (superoxide dismutase) and CAT (catalase), reduces the degree of membrane lipid damage, delays plant senescence, and allows crop life activities to proceed relatively normally with less impact from drought.
2. Cold Resistance Mechanism
Fulvic acid can increase the ABA content in plants. ABA can increase the proline content. As an important osmotic adjustment substance, proline can stabilize the structure of biological macromolecules, lower the freezing point, and provide freezing protection, thereby improving plant cold resistance.
3. Disease Resistance Mechanism
Fulvic acid directly increases soil organic matter content, providing an excellent environment for beneficial microorganisms. Beneficial populations gradually develop into dominant populations, inhibiting the growth of harmful pathogens. In addition, plants themselves grow robustly due to favorable soil conditions, enhancing their disease resistance. Thus, the occurrence of diseases, especially soil-borne diseases, is greatly reduced.
Fulvic acid has a significant inhibitory effect on fungi and can prevent many diseases caused by fungi.
Fulvic acid contains phenolic and benzene carboxylic acid structures such as carboxyl and hydroxyl groups, which are the same as the active ingredients of some phenoxycarboxylic acid and phenolic pesticides, giving it a certain bacteriostatic and antiviral effect.
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