Humic acid and fulvic acid both play crucial roles in agricultural production.
Humic acid provides a favorable soil environment and nutrient supply for plant growth by improving soil structure, enhancing soil fertility, promoting plant growth, improving water use efficiency, and improving the quality of agricultural products.
Fulvic acid excels in soil improvement, increasing fertilizer utilization, slow-release and synergistic pesticide application, stimulating plant growth, improving agricultural product quality, and enhancing crop resistance, making it a core nutrient for both soil and plants.
Although they each have their own characteristics, they complement each other in agricultural production.
The comprehensive application of these substances has brought new opportunities for agricultural development. With the continuous advancement of agricultural technology, their research and application will become more in-depth, and are expected to make greater contributions to achieving sustainable agricultural development, increasing agricultural product yield and quality, and protecting the ecological environment.
Humic Acid: A Facilitator for Soil Improvement and Plant Growth
(I) Improving Soil Structure
Humic acid promotes the formation of soil aggregates. It has good binding properties, binding soil particles together and making the soil loose and porous. Research indicates that soil aggregates are primarily formed by the interaction of fresh humic acid and calcium ions in the soil, resulting in flocculent precipitates that act as excellent binders, holding the soil together to form water-stable aggregates—stable aggregates that do not easily disperse when wet. This aggregate structure unifies water and air permeability, absorption, and retention, meeting the needs of plants for water, oxygen, and air during growth.
(II) Enhancing Soil Fertility
Humic acid can adsorb and fix nutrients in the soil, preventing nutrient loss and thus improving soil fertility. Simultaneously, humic acid can form complexes with nutrients such as nitrogen, phosphorus, and potassium in the soil, promoting nutrient uptake by plants. For example, humic acid can combine with soil calcium ions, reducing Ca-P precipitation and enhancing the phosphorus effect of fertilizers or soil. Humic acid can also combine with soil potassium ions, improving its resistance to soil adsorption and regulating the availability of soil potassium. Humic acid contains many acidic groups, which can improve soil pH and enhance the availability of soil micronutrients such as Zn, Mn, Cu, and Fe. Furthermore, humic acid, through the reaction of its phenolic hydroxyl, carboxyl, and uric acid amide groups, secretes humic acid-urea complexes, which have a good inhibitory effect on urea decomposition, improving nitrogen utilization and regulating urea properties, making it a long-lasting and slow-acting agent.
(III) Promoting Plant Growth
Humic acid has a stimulating effect on plant growth. It can increase the chlorophyll content and photosynthetic efficiency of plants, promote root development, and enhance plant resistance. Studies have found that tomato plants treated with humic acid have more lateral roots than ordinary plants, with an increase of approximately 150%-264%, and an even greater increase in length of 405%-2280%. The effects of humic acid on plants are primarily a stimulatory response similar to that of large-scale or low-dose application of exogenous auxins, closely related to biostimulants such as kininases. After applying humic acid, plant biochemical activity is further enhanced. Roots are stimulated similarly to auxins, increasing the production of plant auxins and endogenous kininases, thereby altering cell membrane permeability, accelerating plant protein synthesis, and speeding up cell growth. Root growth is significantly faster than in ordinary plants, resulting in a substantial increase in yield.
(IV) Improving Water Use Efficiency
Humic acid can reduce soil salinity and water loss, improving water use efficiency. In arid regions, the use of humic acid fertilizers can enhance plant drought resistance and protect plants from drought damage. For example, humic acid colloids are easily affected by organic acids released from roots in arid environments, decomposing into sufficiently small individual growth-regulating molecules that reach the cell membrane via the cell wall matrix, positively influencing plant gene expression and enzyme activity.
(V) Improving Agricultural Product Quality
Using humic acid fertilizers can improve the quality of agricultural products, enhancing their nutritional value and taste. Simultaneously, humic acid can delay the aging of agricultural products and extend their shelf life. Humic acid can chelate harmful heavy metals in the soil, balance soil nutrients, promote fertilizer utilization, and regulate plant physiological metabolism, thereby improving crop quality. For example, many research institutions and fertilizer production enterprises across the country have joined the ranks of researching, developing, and utilizing humic acid fertilizers, achieving significant progress in improving crop quality through their use.
Fulvic Acid: A Core Nutrient for Soil and Plants
(I) Soil Improvement
Fulvic acid plays a significant role in soil improvement. It belongs to the humic substance class and can promote the formation of a more stable granular structure in the soil. According to relevant studies, it can increase the content of soil aggregates ≥0.25mm by 10-20% and the organic matter content by 10%, thereby retaining soil moisture, increasing aeration, and creating favorable conditions for crop growth.
Fulvic acid’s powerful water absorption capacity makes it a crucial factor in enhancing soil water retention. Its maximum water absorption can exceed 500%, and the weight of water absorbed from saturated atmosphere can be more than double its own weight, much greater than that of typical mineral colloids. By inhibiting crop transpiration, it slows down soil water loss, thereby increasing soil moisture content.
Regarding nutrient retention, fulvic acid, as an organic acid, increases the dissolution of minerals in the soil, providing nutrients, and also enhances nutrient availability through complexation. As an organic colloid carrying both positive and negative charges, it can adsorb cations and anions, preventing nutrient loss with water and improving fertilizer utilization, which is particularly significant in sandy soils.
Fulvic acid can also regulate soil pH. The interconversion of fulvic acid and fulvic acid salts forms a buffer system, playing a regulatory role. Simultaneously, it forms colloidal structures by complexing and chelating metal cations in the soil, utilizing its porosity (large specific surface area) to adsorb ions or molecules in the soil solution, reducing the salt concentration in the soil solution. In addition, fulvic acid contains various oxygen-containing functional groups, which can promote the growth and reproduction of beneficial bacteria and inhibit the number of harmful microorganisms. The carboxyl and phenolic hydroxyl groups also have a certain inhibitory effect on viruses.
(II) Improving Fertilizer Utilization
Fulvic acid contains functional groups such as carboxyl and phenolic hydroxyl groups, possessing strong complexing, chelating, and surface adsorption capabilities. It can reduce the loss of ammonium nitrogen, increase the distance phosphorus travels in the soil, inhibit the fixation of water-soluble phosphorus by the soil, convert ineffective phosphorus into available phosphorus, and promote root absorption of phosphorus. It can also absorb and store potassium ions, increasing the content of available potassium, with a particularly significant synergistic effect on potassium fertilizers. Experiments show that fulvic acid can increase the utilization rate of nitrogen, phosphorus, and potassium nutrients in fertilizers by more than 20%.
For nitrogen fertilizers, the active groups of fulvic acid can form complexes or chelates with nitrogen, such as fulvic acid-urea, reducing the loss of ammonium nitrogen in ammonium bicarbonate and improving nitrogen fertilizer utilization. Oxidatively degraded nitrofulvic acid can inhibit urease activity and reduce urea volatilization. Adding fulvic acid to urea slows down urea decomposition, prolongs its fertilizer effect, reduces losses, and relatively increases urea utilization by 30%, with a post-fermentation effect exceeding 15%.
For phosphate fertilizers, degraded nitrofulvic acid increases the distance phosphorus travels in the soil, inhibits soil fixation of water-soluble phosphorus, and promotes root absorption of phosphorus. Adding 10-20% fulvic acid to superphosphate, calcium carbonate, or ammonium phosphate can relatively increase fertilizer efficiency by 10-20% and increase phosphorus uptake by 28-39%.
In terms of potassium fertilizer, the acidic functional groups of fulvic acid can absorb and store potassium ions, reducing the amount of potassium lost with water in sandy soils and soils with strong leaching, preventing potassium fixation in clay soils, increasing the amount of exchangeable potassium, dissolving potassium-containing minerals, slowly increasing potassium release, increasing the content of available potassium in the soil, and increasing potassium uptake by more than 30%.
(III) Slow-Release Synergistic Pesticides
Fulvic acid functions as a surfactant, reducing the surface tension of water and emulsifying and dispersing pesticides. As a viscous colloidal substance with a large surface area, it may have a strong physical adsorption effect on pesticides. Furthermore, fulvic acid itself has antibacterial and disease-resistant effects; its combination with fungicides is equivalent to the combination of two pesticides.
(IV) Stimulating Plant Growth
Fulvic acid is similar to endogenous plant hormones, promoting plant growth in multiple ways. In terms of seed germination, it has an effect similar to gibberellin, promoting seed germination, uniform emergence, and seedling growth. In root growth, it has effects similar to auxin, promoting root growth and activity. In leaf growth, it increases leaf size, thickness, and greenness, delaying the senescence of lower leaves, similar to the effect of cytokinin. In transpiration, it shrinks stomata and reduces transpiration, similar to the effect of abscisic acid. In fruit ripening, it promotes earlier fruit coloring and ripening, similar to the ripening effect of ethylene. Simultaneously, it also promotes cell division, cell elongation, and differentiation, similar to the effects of two or more plant hormones.
(V) Improving the Quality of Agricultural Products
Fulvic acid enhances the synthesis and transport of sugars, starches, proteins, fats, and various vitamins by enzymes. It can stimulate the activity of polysaccharide enzymes, converting polysaccharides into soluble monosaccharides, thereby increasing fruit sweetness. For example, it can increase the total sugar content and vitamin C content in melons such as watermelons and cantaloupes, and also increase the ratio of total sugar to nicotine and potassium to chlorine in tobacco leaves, improving tobacco quality.
Fulvic acid regulates enzymatic reactions and enhances plant life activities. It can stimulate the activity of polysaccharide enzymes, hydrolyze pectin in young cell walls, soften cell walls, and promote the growth of tender cells in new tissues, with a significant effect on the root meristem. Containing a large number of carboxyl groups, it can inhibit the activity of auxin oxidase, reducing auxin degradation and increasing its content, which is beneficial to root and stem growth and increases the crop’s ability to absorb water and nutrients. It can also promote the activity of invertase, starch phosphorylase, and some enzymes related to protein and fat synthesis, increasing the synthesis and accumulation of various substances, and promoting the activity of transferases, accelerating the transport of metabolic products, and improving crop yield and quality.
Fulvic acid can promote the absorption and transport of micronutrients in plants, significantly increasing the chlorophyll content in leaves, and inhibiting the activity of proteolytic enzymes, slowing down chlorophyll decomposition. It can also increase the activity of protective enzymes, reduce the damage of chlorophyll to reactive oxygen species, maintain and increase chlorophyll content, promote photosynthesis, and increase the accumulation of photosynthetic products.
Fulvic acid enhances the activity of respiratory enzymes, particularly terminal oxidases, thereby increasing respiration, releasing energy and producing intermediate products, and promoting root absorption and synthesis.
(VI) Enhancing Crop Stress Resistance
Under any adverse conditions, the ABA (abscisic acid) content in plants increases. Abscisic acid is the “first messenger” that initiates the expression of stress-resistance genes in plants, effectively activating the plant’s stress-resistance immune system. Fulvic acid can increase the ABA content in plants, thus enhancing crop stress resistance. Fulvic acid can regulate physiological and ecological changes in plants. Under adverse stress conditions such as temperature, water, salinity, and heavy metals, it enhances the activity of enzymes related to reactive oxygen species metabolism, reduces plant plasma membrane permeability, regulates the content of reactive oxygen species in plants, reduces the degree of membrane lipid peroxidation, and enables plants to maintain a faster growth rate. Furthermore, fulvic acid can also enhance plant resistance to adverse conditions by altering the plant’s growth environment, such as reducing the electrical conductivity of the soil medium, increasing soil aggregate content and compaction, and improving soil water retention and cation exchange capacity.
