Soil Erosion, Soil Degradation, and Soil Pollution: Causes, Manifestations, and Impacts

Soil erosion, soil degradation, and soil pollution are issues that directly affect soil fertility, agricultural productivity, and ecosystem balance. A proper understanding of these phenomena helps producers identify risks, protect soil resources, and develop appropriate farming practices.

1. Soil Erosion

What is soil erosion?

Soil erosion is the process by which soil particles, particularly the nutrient- and organic matter-rich topsoil layer, are detached from their original location, transported, and deposited elsewhere, primarily under the influence of water, wind, or other physical agents.

In agricultural production, water erosion is the most common and significant form, especially on sloping land, bare soil, or land with insufficient vegetation cover.

Water erosion generally occurs in the following sequence:

Rainfall → breakdown of soil structure → detachment of soil particles → transport by surface runoff → deposition

Erosion directly removes the topsoil layer, where much of the organic matter, nutrients, microorganisms, and active root systems are concentrated. Therefore, prolonged erosion can reduce soil fertility and productivity.

1.2. Types of Soil Erosion

Sheet erosion: The relatively uniform removal of the topsoil layer by rainfall and surface runoff. This form of erosion can be difficult to detect in its early stages but may result in substantial soil loss over time.

Rill and gully erosion: When surface runoff becomes concentrated, water forms small channels or rills on the soil surface. If the process continues, these channels can expand into larger gullies, resulting in severe soil loss.

Wind erosion: Occurs when wind removes small, dry, and lightweight soil particles from the surface. It is commonly found in dry areas, bare land, and areas lacking vegetation cover.

2. Soil Degradation

What is soil degradation?

Soil degradation is the process of declining soil fertility, which consequently reduces the productive capacity of the soil. Soil degradation can be classified into three main groups:

  • Physical degradation: Typical manifestations include soil hardening, compaction, structural deterioration, reduced porosity, and reduced water infiltration. These conditions restrict root development and increase surface runoff.
  • Chemical degradation: Typical manifestations include acidification, salinization, nutrient depletion, reduced cation exchange capacity (CEC), phosphorus fixation, and Al/Fe toxicity. These processes reduce the soil's ability to supply nutrients.
  • Biological degradation: Typical manifestations include reduced organic matter content, reduced microbial biomass, and decreased microbial diversity. These conditions reduce mineralization, nutrient cycling, and the soil's natural capacity for recovery.

Degradation Processes on Sloping Land

Decline in Organic Matter, Humus, and Nutrients

On sloping land, erosion and leaching cause losses of organic matter and nutrients. Erosion removes nutrient- and humus-rich topsoil, while leaching carries soluble nutrients deeper into the soil profile or beyond the root zone. These processes become more severe under conditions of heavy rainfall, steep slopes, and insufficient ground cover.

The loss of topsoil and organic matter reduces the soil's ability to retain water and nutrients, maintain soil structure, and support microbial activity, thereby reducing soil fertility and nutrient availability to crops.

Reduction in Cation Exchange Capacity and Base Saturation

Prolonged organic matter loss, leaching, and erosion can reduce the amount of clay colloids and organic matter, resulting in lower cation exchange capacity (CEC) and reducing the soil's ability to retain and supply nutrient cations such as Ca²⁺, Mg²⁺, K⁺, and NH₄⁺.

At the same time, leaching removes base cations such as Ca²⁺, Mg²⁺, K⁺, and Na⁺, leading to lower base saturation (BS) and an increased risk of soil acidification.

When CEC and base saturation decline, the soil's nutrient-holding capacity decreases, the risk of nutrient loss increases, and overall fertility declines. This may also increase Al³⁺ toxicity and restrict phosphorus uptake by crops.

Increased Soil Acidity

Leaching of Ca²⁺, Mg²⁺, K⁺, and Na⁺ reduces base saturation while increasing H⁺ and Al³⁺, causing the soil to become progressively more acidic. Long-term use of ammonium-based nitrogen fertilizers may also contribute to soil acidification.

As the soil becomes more acidic, Al and Fe present in the solid phase may become more soluble and chemically active, particularly under low-pH conditions. High concentrations of soluble Al³⁺ can be toxic to roots, restricting root development and reducing water and nutrient uptake. Acidic soil conditions may also alter the activity and composition of soil microbial communities.

Increased Mobile Iron and Aluminum and Greater Phosphorus Fixation

When soil pH decreases, the solubility and activity of Al- and Fe-containing compounds may increase. Active forms of Al and Fe can adsorb and bind phosphate, reducing phosphate concentrations in the soil solution and decreasing phosphorus availability to crops.

The loss of organic matter may also contribute to increased phosphorus fixation because organic matter can compete for phosphate adsorption sites and form complexes with some Fe and Al. However, the extent of phosphorus fixation depends significantly on soil pH, clay mineral type and content, Fe–Al oxide content, organic matter, and other soil chemical properties.

Deterioration of Soil Structure

Soil structural degradation is the process of breaking down or reducing the stability of soil aggregates, thereby changing the distribution and continuity of pore spaces within the soil.

On sloping land, factors such as erosion, loss of organic matter, excessive tillage, inappropriate cultivation practices, and the use of agricultural machinery can contribute to structural degradation. As organic matter declines, the ability of soil particles to bind together into stable aggregates also decreases, making the soil more susceptible to dispersion, surface crusting, and erosion.

In basalt-derived soils rich in iron and aluminum oxides, changes in organic matter content, wetting–drying conditions, and cultivation practices can alter soil structure and physical properties. However, it should not be broadly assumed that degraded basalt soils contain “almost no calcium humate or magnesium humate,” or that the remaining organic matter is primarily bound to sesquioxides, unless supported by specific analytical data.

When soil structure deteriorates, both large and small pore spaces are affected, influencing porosity, water infiltration, aeration, water retention, and root development.

Increased Soil Compaction

Soil compaction may result from frequent operation of machinery and vehicles on the soil, especially when soil moisture is high, combined with a decline in organic matter and structural stability.

When soil becomes compacted, bulk density increases and total porosity decreases, particularly the proportion of large pores. This reduces water infiltration and gas exchange while increasing the mechanical resistance encountered by growing roots.

Compaction can also reduce root development, restrict the activity of aerobic microorganisms, and decrease the efficiency of water and nutrient use by crops.

It should be noted that erosion and leaching are not directly equivalent to soil compaction. However, they can contribute to the loss of organic matter and structural degradation, making the soil more susceptible to compaction under inappropriate cultivation practices.

Reduced Water Infiltration and Water-Holding Capacity

When soil structure deteriorates, soil porosity and pore distribution are altered, particularly when the soil becomes compacted or develops a surface crust. As a result, soil water infiltration may decrease. Rainwater or irrigation water that cannot infiltrate the soil increases surface runoff. On sloping land, this raises the risk of erosion and topsoil loss.

At the same time, declining organic matter and changes in soil structure may reduce soil water-holding capacity, causing plant-available water to decrease or shortening the period during which water remains available between rainfall or irrigation events. However, the extent of these changes depends on soil type, clay content, organic matter, soil structure, and hydrological conditions.

This can create a reinforcing feedback loop:

Loss of organic matter and structural degradation → reduced water infiltration → increased surface runoff → increased erosion → further loss of soil and organic matter

On sloping land, without appropriate soil protection and water management practices, this feedback loop can progressively worsen land degradation while also increasing the risk of concentrated runoff, landslides, and flash floods under suitable terrain and heavy rainfall conditions.

3. Soil Pollution

What is soil pollution?

Soil pollution is the presence or accumulation of pollutants in soil at concentrations or levels that may adversely affect soil quality, organisms, crops, water resources, or human health.

Soil pollutants may include heavy metals, pesticide residues, hydrocarbons, toxic organic substances, radioactive materials, and other chemical compounds. Pollution assessment should consider soil characteristics, pollutant type, concentration, mobility, and exposure levels, while also referring to applicable soil environmental standards and regulations.

Causes of Soil Pollution

Pollution from Chemical Fertilizers

Excessive fertilizer application or fertilization that does not correspond to crop requirements can increase nitrate and other soluble nutrient concentrations in the soil, thereby increasing the risk of nutrient leaching and eutrophication of water bodies.

Certain fertilizers or poor-quality raw materials may also introduce impurities such as Cd and other metals into the soil, creating a risk of long-term accumulation.

Pollution from Pesticides

Improper use of pesticides in terms of product type, dosage, timing, or application technique can increase pesticide residues in the soil. Some active ingredients may persist and adversely affect soil microorganisms, non-target organisms, and soil ecosystems.

Pollution from Urban and Industrial Wastewater

Wastewater and other wastes that have not been adequately treated may introduce heavy metals, organic compounds, oils, chemicals, and other pollutants into the soil. These substances may accumulate in the soil, migrate into groundwater, or enter the food chain.

Pollution from Mining Activities

Mining, mineral processing, and related activities can generate waste rock, dust, acid mine drainage, and heavy metals, altering soil properties and increasing the risk of contamination of surrounding soil, water resources, and ecosystems.

Other Sources of Soil Pollution

Soil may also become polluted by landfills, transportation activities, industrial production, livestock waste, chemical spills, and other emission sources.

Saltwater intrusion should not be directly classified as soil pollution; it is more appropriately considered a form of soil degradation caused by salt accumulation. However, wastewater or human activities may contribute to the intensification of this process.

Relationship Between Soil Erosion, Degradation, and Pollution

Soil erosion, degradation, and pollution are closely interconnected and may influence one another.

Erosion removes topsoil rich in organic matter, clay, and nutrients, while soil degradation reduces soil structure, water-holding capacity, and nutrient-retention capacity. As these properties deteriorate, the soil's ability to retain and buffer pollutants may also change, increasing the risk that pollutants will be washed away or dispersed into surface water and groundwater.

Conversely, certain pollutants such as heavy metals and persistent pesticide residues may be toxic to soil organisms, reducing biological activity and contributing to the degradation of soil functions.

Therefore, soil conservation should combine pollution-source control, erosion prevention, maintenance of organic matter, protection of soil structure, and the appropriate use of fertilizers and pesticides. These measures are essential for maintaining long-term soil quality, crop productivity, and environmental safety.

Conclusion

Soil erosion, soil degradation, and soil pollution are three closely interconnected processes that collectively reduce soil quality and functionality. Erosion removes topsoil, organic matter, and nutrients; degradation reduces the physical, chemical, and biological properties of soil; while pollution increases the concentration of substances that may adversely affect soil, crops, ecosystems, and human health.

These processes may interact and form a cycle of degradation: the loss of soil and organic matter reduces the soil's ability to retain water, nutrients, and pollutants; degraded soil becomes more susceptible to erosion and loses its capacity to recover; meanwhile, pollutants may further reduce biological activity and the soil's natural functions.

Therefore, soil protection should not focus solely on erosion control, but should involve integrated management aimed at maintaining organic matter, protecting soil structure, balancing nutrients, using fertilizers and pesticides appropriately, controlling waste sources, and maintaining vegetation cover.

The ultimate objective is to maintain healthy soil that can retain water and nutrients, support crops and ecosystems, and ensure both agricultural productivity and the long-term sustainable use of land.