Advances in humic acid for promoting plant growth and its mechanism

Objectives Humic acid plays an important role in Chinese agricultural production. Many research results have showed that humic acid could prompt plant growth. In this study, the researches on humic acid for prompting plant growth were systematically summarized, and the mechanism was focused on stimulating roots growth, regulating nutrition transformation, and affecting microbial activity. According to review of the research status of humic acid prompting plant growth, we aimed to provide the references for the further researches and application of humic acid at last.Major advances 1) Humic acid produces effects on plant like biostimulants. In detail, humic acid can improve H+-ATPase activity of roots, simulate root elongation and the increase of lateral root points, which is conducive to the increase of root activity, as well as the expansion of the contact area between roots and soil nutrition. 2) Humic acid regulates the metabolism of plant and improves the environment of plant growth under stress. Based on this, the abiotic and biotic stress is eliminated and the plant is prevented from the injury, which is conducive to plant growth. 3) Humic acid reacts with nitrogen, phosphorus and potassium, and new compounds are synthetized. In addition, humic acid has competition effects with phosphate, and adsorption effects with potassium ions. All the effects are beneficial to immobilize and activate the nutrients in soil and fertilizer. Thus, the availability and the sustainable release of the nutrients in soil and fertilizer are enhanced, and plant growth is prompted. 4) Humic acid also plays an important role in regulating soil enzyme activities related to soil or fertilizer nutrition transformation, and microbial community structure and biomass. Owing to the regulation, the nutrition is activated in soil, and the goal of nutrition preservation is achieved. Therefore, it is ensured that plant growth can get permanent nutrient supply. 5) The promotion of humic acid on plant growth is affected by the structural characteristics, addition amounts of humic acid, and crop species.Suggestions and expectations The limited technology and the discrepancy of researches impede deeper and systematic research in the mechanism of humic acid prompting plant growth. Therefore, the future study will focus on the basic characteristics of humic acid, the dominating factor analysis of humic acid action, the principal pathway of humic acid prompting plant growth in soil-plant systems, and the effect of humic acid on soil functional microorganism.