What causes soil compaction, and what methods can be used to prevent and control it?

　　Soil compaction refers to the hardening and clumping of the soil surface, resulting from a lack of organic matter and poor soil structure. Under external factors such as irrigation or rainfall, the soil structure is disrupted and soil particles become dispersed; upon drying, cohesive forces cause the surface to harden into compacted clods. So, what causes soil compaction, and how can it be prevented and managed?

　　I. Seven Major Factors Causing Soil Compaction

　　1. The farmland soil is excessively clayey, with a shallow plow layer. Clay soils contain a high proportion of clay particles, resulting in fewer capillary pores and poorer aeration, water permeability, and heat‑retention capacity. After rainfall or irrigation, these pores are easily blocked, leading to the formation of a crust on the soil surface.

　　2. Organic fertilizer is severely lacking, and the amount of straw returned to the field has decreased. Insufficient replenishment of organic matter in the soil results in low soil organic matter content and deteriorated soil structure, which impairs microbial activity and, consequently, the formation of soil aggregate structures. This leads to excessive soil acidity or alkalinity, ultimately causing soil compaction.

　　3. Excessive use of plastic products. Plastic mulch films and plastic bags, among others, are not properly removed and cannot fully decompose in the soil, forming harmful clumps that are difficult to break down when applied, leading to soil compaction.

　　4. Long-term, one-sided application of chemical fertilizers. There is a severe shortage of organic manure; nitrogen fertilizers are heavily favored over phosphorus and potassium, leading to a decline in soil organic matter and an inability to replenish humus in a timely manner. This not only causes soil compaction but may also result in cracking. The reason chemical fertilizers contribute to soil compaction is that they contain only cations or anions—elements that plants require—and plants selectively absorb the beneficial ions, thereby causing soil acidification or salinization.

　　(1) Excessive application of nitrogen fertilizer: For every unit of nitrogen supplied to microorganisms, 25 units of carbon are consumed. This carbon is derived from soil organic matter; when organic matter levels are low, microbial activity is impaired, which in turn hinders the formation of soil aggregate structures and leads to soil compaction.

　　(2) Excessive application of phosphate fertilizers: Phosphate ions in the fertilizer combine with cations such as calcium and magnesium in the soil to form insoluble phosphates, which not only waste the fertilizer but also disrupt the soil aggregate structure, leading to soil compaction.

　　(3) Excessive application of potash fertilizer: Potassium ions in potash fertilizers have a particularly strong exchange capacity, capable of displacing polyvalent cations that contribute to the formation of soil aggregate structures. Meanwhile, monovalent potassium ions lack bridging functionality; as a result, the bridging bonds within the soil aggregates are disrupted, leading to the breakdown of aggregate structure and ultimately causing soil compaction.

　　5. Agricultural practices such as compaction and deep tillage disrupt the structure of the topsoil. Deep mechanical tillage damages the soil’s aggregate structure. Moreover, only a portion of the fertilizer applied annually is taken up by the current season’s crops; the remainder becomes fixed in the soil, leading to the accumulation of soluble salts and resulting in soil compaction.

　　6. Accumulation of harmful substances. In some areas, groundwater and industrial wastewater contain high levels of toxic substances; prolonged use for irrigation can lead to excessive accumulation of these toxins, resulting in compaction of the surface soil.

　　7. Wind and sand, as well as heavy rainfall, lead to soil erosion. After exposure to wind and sand or intense rain, fine soil particles from the surface layer are washed away, disrupting the soil structure and resulting in soil compaction.

　　II. The Harm Caused by Soil Compaction

　　Root vitality declines. Soil compaction reduces aeration, thereby decreasing the oxygen available to roots. This directly results in diminished aerobic respiration and, in some cases, shifts to anaerobic respiration, leading to reduced energy supply from the roots—conditions that are highly detrimental to plant growth.

　　Soil compaction also restricts root penetration; as the soil hardens, roots are unable to grow downward and access the nutrients applied to the soil, resulting in fertilizer waste and residual buildup. Moreover, its water- and nutrient‑retention capacities decline, ultimately leading to reduced yields.

　　This leads to nutrient deficiency. Such deficiencies are not always due to a lack of the specific element in the soil; rather, they often result from reduced root uptake caused by factors like soil compaction, unsuitable soil pH, or uneven water supply.

　　III. Methods for Alleviating Soil Compaction

　　1. Increase the application rate of organic fertilizers. For example, use crop straw‑based organic fertilizers and high‑quality commercial organic fertilizers, and apply microbial inoculants with a high microbial content.

　　2. Promote soil-testing and formula‑based fertilization, enhance farmers’ understanding of the roles of different fertilizers, and tailor fertilizer application rates to crop yields and soil fertility conditions. This approach not only curbs the indiscriminate use of chemical fertilizers but also reduces unnecessary inputs, thereby boosting economic returns.

　　3. Break with outdated farming practices, further promote straw return to the field, no-till mulching, and fallow‑field pilot programs, and strive to minimize unnecessary soil erosion in order to preserve soil structure.