Humic acid is rich in carboxyl, phenol, quinone, carbonyl, methoxy and other active functional groups, which have a positive effect on plant growth, crop yield, soil microbial community structure, etc. This paper focuses on the relevant mechanisms of humic acid in regulating plant growth from five aspects: humic acid promotes nutrient uptake by plants, contains hormone-like substances or regulates plant endogenous hormones, interacts with plant roots, improves the nodulation and ammonia fixation ability of legumes, and improves the structure and activity of soil microbial community, in order to provide a theoretical basis for the application of humic acid in agriculture.
Humic acid is a type of organic macromolecular hydrocolloid formed by the decomposition and transformation of microorganisms and a series of geochemical processes, which has a strong ability to absorb water, store water, and improve soil moisture content
Humic acid has unique biological activity and functional properties, which in turn has a wide range of applications in many fields such as improving soil fertility, promoting plant growth, and controlling heavy metal pollution. As a natural bioregulator or biostimulant widely used in sustainable agriculture, humic acid can improve soil ecology, promote plant growth, promote nodulation and nitrogen fixation in legumes, improve plant resistance, and increase crop yield.
Humic acid can stimulate the growth of plant roots, especially in terms of increasing length and the number of roots, which has a more pronounced effect. It can affect the root state and improve growth indicators such as seedling height and stem thickness.
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Humic acid can reduce the impact of abiotic stress on plant growth, and play a protective role in the growth of plants under stress by improving plant morphology and physiological characteristics. Under drought conditions, humic acid improves leaf hydration and oxidation capacity, promoting root and aboveground growth
Under salt stress, humic acid increased the proline content in kidney beans, reduced membrane permeability and reactive oxygen species production.
The effects of different levels of humic acid on the biological nitrogen fixation of cowpea were tested, and it was found that foliar spraying of humic acid significantly promoted the root growth of cowpea, and increased the number of roots, nodules, dry weight and nitrogen content of cowpea.
Humic acid can change soil physicochemical properties, specific bacterial communities, and soil enzyme activity. Previous studies have explored the effects of humic acid on the physical and chemical properties, microbial diversity and enzyme activity of continuous cropping peanut soil, and found that humic acid increased soil nutrient content, including soil total nitrogen and total phosphorus, total potassium, available nitrogen, available phosphorus, available potassium and organic matter content. At the same time, the activities of enzymes such as urease, sucrase and phosphatase in soil were significantly increased.
The application of humic acid has a great effect on beneficial bacteria in the soil. Previous studies have further confirmed that humic acid can specifically promote the growth of beneficial bacteria and significantly alter the bacterial community.