Mechanism of humic acid in promoting plant growth

1 Humic acid stimulates plant root growth and nutrient uptake
The stimulating effect of humic acid on the growth of plant roots is the initial driving force of its effect on plant growth. The stimulating effect of humic acid on plant root growth is mainly reflected in promoting the increase of root length and root number. The production of this effect is mainly due to the fact that after humic acid acts on plants, plants show a stimulating response similar to the addition of low dose or a large amount of exogenous auxin, which is related to the organic biostimulants such as humic acid containing cytokinases, therefore, after application, it can improve the biochemical activity of plants, produce auxin-like effects on roots, stimulate the increase of plant endophytic cytokinins and auxin, cause changes in the permeability of the cytoplasmic membrane, and promote the synthesis of plant protein and the growth of cells. This promotes plant root growth and increased yield.
Humic acid can also be absorbed by plants as a nutrient. Humic acid, especially humic acid with small molecular weight, readily reaches the cytoplasmic membrane of higher plants and is partially absorbed by plants. The promoting effect of humic acid on nutrient uptake in plant roots is mainly manifested in the absorption of nitrate. Humic acid can promote the expression of genes related to nitrate uptake and assimilation in plant roots. Humic acid can inhibit the co-directional transfer of H/NO3- by lowering the pH of the plasma membrane surface of root cells, neutralizing the alkali produced by NO3- as a nitrogen source. The stimulation of humic acid (especially small molecular weight humic acid) on the activity of H-ATPase separated on the plasma membrane determines the increase of electrochemical proton gradient, thereby promoting the uptake of NO3- by plant roots, and ultimately by promoting nitrogen uptake by plants. Therefore, humic acid can promote the uptake of nitrate nitrogen by regulating nitrate absorption, assimilation and transfer.

2. Humic acid enhances plant stress resistance
Humic acid can alleviate the effects of biotic and abiotic stresses on plant growth, improve plant morphological and physiological characteristics, have a protective mechanism for plant growth under stress stress, and play an important role in preventing oxidative stress in plant ecological security. A large number of studies have shown that under the stress of water, temperature, salt and heavy metals, humic acid stimulates the synthesis of proteins and enzymes in various organs of plants, enhances the activities of peroxidase, catalase, superoxide dismutase and other enzymes related to reactive oxygen metabolism in plants, and reduces malondialdehyde and plasma membrane permeability in plants, regulates the content of reactive oxygen species in plants, reduces the degree of peroxidation of membrane lipids, and keeps plants growing at a faster rate, thereby maintaining the permeability of cell membranes, and enhancing the resistance of plants to drought, cold, Resistance to salts and heavy metals, etc. At the same time, humic acid is also an important aspect of enhancing plant resistance to water stress. The results showed that humic acid condensate could be formed on the surface of plant roots, root epidermal cells and new root hairs, which reduced the permeability coefficient of roots and enhanced their drought resistance.
The change of humic acid on plant stress resistance is not only reflected in the change of plant morphology and physiological characteristics, but also can enhance plant stress resistance by changing the plant growth environment. Under the condition of salt stress, the application of humic acid fertilizer can reduce the conductivity of soil medium, promote the growth of seedlings, and increase their chlorophyll content. Humic acid can increase soil aggregate content, soil compactness, water-holding capacity, cation exchange capacity, and promote plant growth. In addition, under drought stress, organic acids secreted by plant roots can exert local destructive effects on humic acid colloids, causing them to release individual growth regulators, which are small enough to penetrate the cell wall matrix and enter the cell membrane, thereby affecting gene expression and enzyme activity.
Under biological stress, humic acid enhances plant stress resistance, which is mainly reflected in its inhibitory effect on plant pathogenic bacteria. Studies have confirmed that humic acid can not only reduce the survival rate of conidia of soil fungi Alternaria, Fusarium grandisarium and Fusarium oxysporum, but also inhibit the elongation of conidia hyphae, and inhibit the reproduction of conidia as its main mode of action.
3. Humic acid improves the soil environment
Humic acid can act as a binder, sticking together the clay mineral ions around it to form an agglomerate structure. The process is to first bond several clay mineral ions into micro-aggregates, and then several micro-aggregates are then bonded into large aggregates, and some large aggregates are then bonded to each other to form huge soil aggregates. Such an aggregate structure has a certain water resistance and is not easy to collapse due to excessive soil moisture, so it plays an important role in maintaining the soil aggregate structure, and the aggregate structure is one of the indispensable conditions for soil water and fertilizer retention.