Organic fertilizer, as a natural soil amendment, has a multidimensional and far-reaching effect on soil improvement, from the physical structure to chemical properties, and even biological activity, all of which can have positive and lasting effects. The core mechanism is to activate the material cycle and energy flow in the soil ecosystem by inputting organic matter and its derivatives, thereby comprehensively improving soil quality.

1、 Optimization of soil physical structure: After applying organic fertilizer to the soil, humus is generated through microbial mineralization and humification. This type of substance acts as a binder, promoting the bonding of soil particles into water stable aggregates through hydrogen bonding and van der Waals forces.

The formation of aggregates reconstructs the three-phase structure of soil: micropores (<30 μ m) enhance capillary water holding capacity, leading to an increase in soil water storage during drought periods; Macropores (>30 μ m) improve aeration and reduce the risk of root hypoxia during waterlogging. In addition, the hydrophobic lipid wax components of humus form a protective film on the surface of the granules, inhibiting water evaporation and delaying capillary water movement, achieving dual regulation of drought resistance and flood prevention.

Organic fertilizers can enhance the buffering capacity of soil, and the high thermal capacity (2-3 J/g · K) and low thermal conductivity of humus can stabilize soil temperature fluctuations. For example, reducing the depth of frozen soil layers in winter and lowering the peak surface temperature in summer can create suitable habitats for roots. Humus can adsorb and fix harmful substances in soil, such as heavy metal ions, and reduce their bioavailability through chelation, precipitation, and other methods, thereby reducing the absorption of heavy metals by crops and lowering the safety risks of agricultural products. At the same time, the carbon dioxide produced during the decomposition process of organic fertilizers can increase the concentration of carbon dioxide in the air around the soil, which is beneficial for crop photosynthesis, improves photosynthetic efficiency, and indirectly promotes crop growth.

2、 Reconstruction of Soil Chemical Properties

1. Nutrient slow-release and effectiveness regulation: Organic fertilizers contain nitrogen, phosphorus, potassium, as well as trace elements such as B, Zn, and Mo. However, unlike fertilizers that release ions quickly, their nutrients exist in the form of organic macromolecules such as proteins and nucleic acids, and need to be gradually mineralized through microbial enzymatic hydrolysis. For example, proteins are degraded into amino acids by proteases and then mineralized into ammonium nitrogen, which synchronizes the nutrient release curve with crop demand and reduces the risk of leaching. The chelation of humus is crucial for the availability of phosphorus. In acidic soils, humic acid forms chelates with aluminum/iron ions, reducing the formation of closed state phosphorus; In alkaline soil, it inhibits the fixation of calcium to phosphorus.

2. Acid base buffering and heavy metal passivation humic substances contain functional groups such as carboxyl (- COOH) and phenolic hydroxyl (- OH), and their dissociation constants endow them with strong acid-base buffering capabilities. In crop rotation experiments, it was found that organic fertilizer increased soil cation exchange capacity (CEC) and buffering capacity, effectively alleviating the pH drop caused by nitrogen fertilizer.

Organic fertilizers have a double-edged sword effect on heavy metal pollution: on the one hand, low molecular weight organic acids such as oxalic acid and citric acid produced by decomposition can activate metal ions such as Cd and Pb;

On the other hand, humic acid forms insoluble complexes through hydrophobic interactions and coordination bonds, reducing its bioavailability.

Long term application of organic fertilizers can also achieve sustained improvement in soil fertility. Although the application of chemical fertilizers can quickly provide nutrients for crops, long-term single application of chemical fertilizers can lead to problems such as decreased soil organic matter, structural damage, and reduced microbial activity, gradually causing soil fertility to decline. The application of organic fertilizers can continuously supplement soil organic matter, maintain and improve the physical, chemical, and biological properties of the soil, and form a virtuous cycle. With the improvement of soil fertility, the stress resistance of crops will also be enhanced. When facing unfavorable factors such as drought, floods, pests and diseases, they can demonstrate stronger adaptability, thereby stabilizing and improving crop yield and quality.

3、 Enhancement of soil biological activity

1. Microbial community optimization: There are a large number and diverse types of microorganisms in soil, such as bacteria, fungi, actinomycetes, etc. They are the core components of soil ecosystems. Organic fertilizers provide abundant carbon, nitrogen, and energy sources for these microorganisms, significantly promoting their reproduction and activity. Microorganisms produce a large number of enzymes during the decomposition of organic fertilizers, such as urease, phosphatase, sucrase, etc. These enzymes can accelerate the decomposition and transformation of organic nutrients in the soil, converting organic nutrients into inorganic nutrients that crops can directly absorb. At the same time, some microorganisms also have functions such as nitrogen fixation, phosphorus solubilization, and potassium solubilization. For example, rhizobia can symbiotically fix nitrogen with leguminous crops, converting nitrogen from the air into usable nitrogen; Some bacteria and fungi can decompose insoluble phosphorus and potassium compounds in the soil, releasing available phosphorus and potassium, and increasing the content of available nutrients in the soil.

2. Construction of Micro Ecological Defense System

Beneficial microorganisms in soil can also inhibit the reproduction of harmful pathogens, reducing the occurrence of crop soil borne diseases through competition for nutrients, secretion of antibacterial substances, and other means. Beneficial microorganisms such as Trichoderma and Bacillus inhibit the proliferation of pathogens through nutritional competition and antibiotic inhibition. Research has found that the incidence of soil borne diseases in farmland treated with organic fertilizer for three consecutive years has decreased. The number of soil animals such as earthworms increases synchronously, and their pores form a biological pore network, which enhances the oxygen diffusion coefficient. 4、 Improvement of Agricultural System Efficiency 1 The dual improvement of crop yield and quality promotes the optimization of soil structure, promotes deep root rooting, and enhances water use efficiency. Experiments in the semi-arid region of northwest China have shown that deep rotary tillage combined with organic fertilizer can improve the water use efficiency of corn and increase grain yield. Humic acid enhances the activity of phosphoenolpyruvate carboxylase and promotes sugar accumulation; At the same time, chelation of heavy metals reduces crop absorption, increases vitamin C content in fruit from apple producing areas in Shaanxi, and reduces lead/cadmium residues to below 30% of the EU standard limit. 2. Systematically reducing the negative externalities of the environment by replacing chemical fertilizers with organic fertilizers to reduce nitrogen and phosphorus runoff losses and alleviate eutrophication of water bodies. Its carbon sequestration potential is particularly outstanding, with every 1% increase in soil organic matter equivalent to fixing 25 tons of CO ₂/ha. In the melon experiment in the Hetao area, the combination of controlled-release fertilizer and organic fertilizer achieved soil organic matter balance while maintaining yield, transforming fertilization behavior into a carbon sink process. In short, the improvement of soil by organic fertilizers is a systematic process that enhances soil fertility and quality comprehensively through optimizing physical structure, regulating chemical properties, activating biological activity, and other aspects, laying a solid soil foundation for the sustainable development of agriculture. This improvement effect is not achieved overnight, but requires long-term persistence to show the best results, and its impact is far-reaching, creating a good soil environment for subsequent crop growth.