When you hike through the woods, drive past a cornfield, or mow your lawns, you tend to focus on the plants. Especially the tassel-topped stalks, the blooming flowers, and the green grass borders. But have you paid attention to the soil? Soil is one of the most underappreciated natural resources present on Earth. Hence the causes of soil erosion are many.
Healthy soil is the foundation for agriculture and plays a crucial role in protecting the water we drink, the air we breathe, the food we eat, and even our climate. Soil erosion occurs when the topmost layer of the land or soil wears away due to several factors. These factors include topography, climate, soil structure and composition, and vegetation. There are two leading causes of soil erosion, namely wind, and water.
Causes of Soil Erosion – Loss of Top Soil
When the topmost layer of the soil becomes loose, it is vulnerable to the natural forces of water and wind to blow the soil particle away. This loss of topsoil particles is soil erosion and almost irreversible loss. The process of Soil erosion occurs naturally and affects all types of landforms. In agriculture, soil erosion means the wearing away of the field’s topsoil by the natural physical forces of wind and water or through details associated with farming activities such as tillage.
Causes of Soil Erosion – Three Step Process
The soil erosion process involves three distinct steps: soil detachment, movement, and deposition. This happens by water, wind, or tillage. Topsoil comprises a high content of organic matter, fertility, and soil life. Soil erosion can reduce cropland productivity and pollute adjacent wetlands, watercourses, and lakes.
The process of soil erosion is a slow process that continues relatively unnoticed. It can also be a fast process that can occur at an alarming rate, causing severe loss of topsoil. Low organic matter, soil compaction, loss of soil structure, poor internal drainage, salinization, and soil acidity are other harsh soil degradation conditions that can accelerate soil erosion.
In the geological process of soil erosion, earthen materials, such as soil, sediments, and rocks, are worn away and transported over time by natural forces such as water or wind. This process becomes faster due to poor management or other human impacts on land. For example, the Colorado River’s natural process of river erosion created the Grand Canyon when it cut wide and deep through the rock over millions of years.
Similarly, the glacial erosion carved out the iconic landscape of Yosemite National Park. The soil erosion rate depends on several factors, including the soil’s makeup, vegetation, and wind and rain intensity. Since our activities can influence the speed of soil erosion, we have the power and the responsibility to solve one of the planet’s most significant environmental challenges.
The difference between weathering and erosion is that materials are worn away but not transported in the process of weathering. And erosion is the opposite of deposition, when natural forces leave earthen materials behind. In Ontario, Canada, the widespread occurrence of water erosion coupled with the severity of off-site and on-site impacts have made water erosion the focus of soil conservation efforts in Ontario, Canada.
Causes of Soil Erosion – Rainfall and Runoff
The greater the duration and intensity of a rainstorm, the higher the potential for soil erosion. The impact of raindrops falling on the soil’s surface can break down soil aggregates and disperse the aggregate material. The raindrop splash and runoff water easily remove lighter aggregate materials such as very fine sand, silt, clay, and organic matter; more incredible raindrop energy or runoff amounts are required to move more extensive gravel and sand particles.
Soil movement by rainfall or the raindrop splash is usually most noticeable and most incredible during the short-duration but high-intensity thunderstorms. The soil erosion caused by long-lasting and less-intense storms is not as spectacular or prominent as the one produced during thunderstorms. However, soil loss can be significant, especially when compounded over time.
Surface water runoff happens when excess water on a slope cannot be absorbed into the soil or trapped on the surface. The runoff increases when there is reduced infiltration due to soil compaction, crusting, or freezing. Runoff from agricultural land is most significant during spring when the soils are typically saturated, snow is melting, and vegetative cover is minimal.
Causes of Soil Erosion – Soil Erodibility
Soil erodibility is a measure or an estimate of the ability of the soil to resist erosion. Soil erodibility is based on the physical characteristics of each soil. The texture is the principal characteristic affecting erodibility, but the structure, permeability, and organic matter, also contribute to it. Soils with faster infiltration rates, improved soil structure, and higher organic matter levels have a greater erosion resistance. Sandy loam, sand, and loam-textured soils tend to be less erodible than silt, fine sand, and certain clay-textured soils.
Tillage and other cropping practices that reduce soil organic matter levels cause poor soil structure and compaction and increase soil erodibility. For example, compacted subsurface soil layers can decrease infiltration and runoff. The formation of a soil crust tends to seal the surface and reduces infiltration. On some sites, a soil crust might decrease the amount of soil loss from raindrop impact and splash. However, a corresponding increase in runoff water can contribute to more severe erosion problems.
Past erosion also affects a soil’s erodibility. Due to the poorer soil structure and lower organic matter, many exposed subsurface soils on eroded sites tend to be more erodible than original soils. The lower nutrient levels associated with subsoils contribute to lower crop yields and generally poorer crop cover, providing less crop protection for the soil.
Causes of Soil Erosion – Slope Gradient and Length
The steeper and more prolonged the field slope, the higher the risk for erosion. Soil erosion by water increases as the length of the slope increases due to the more significant accumulation of runoff. Consolidation of small fields into larger ones results in longer slope lengths with increased erosion potential due to increased velocity of water, which permits a greater degree of scouring (carrying capacity for sediment).
Causes of Soil Erosion – Cropping and Vegetation
The potential for soil erosion increases if the soil has no or minimal vegetative cover of plants and crop residues. Plant and residue cover protects the soil from raindrop impact and splash, slows down the movement of runoff water, and allows excess surface water to infiltrate.
The erosion-reducing effectiveness of plant and crop residues depends on the type, extent, and quantity of cover. Vegetation and residue combinations that completely cover the soil and intercept all falling raindrops at and close to the surface are the most efficient in controlling soil erosion (e.g., forests, permanent grasses). Partially incorporated residues and residual roots are also important as these provide channels that allow surface water to move into the soil.
The effectiveness of any protective cover also depends on how much protection is available during various periods during the year. It is relative to the amount of erosive rainfall that falls during these periods. Crops providing a complete protective cover for a significant time of the year (e.g., alfalfa or winter cover crops) can reduce erosion.
This is much more than can crops that leave the soil bare for a more extended period (e.g., row crops), particularly during periods of highly erosive rainfall such as spring and summer. Crop management systems favor contour farming, and strip-cropping techniques can further reduce the amount of erosion. To minimize erosion on annual row-crop land, leave a residue cover more significant than 30% after harvest and over the winter, or inter-seed a cover crop.
Causes of Soil Erosion – Tillage Practices
The potential for soil erosion by water is severely affected by tillage operations. It depends on the direction, depth, and timing of plowing, the type of tillage equipment, and the number of passes. Generally, the lesser the vegetation or residue covers disturbance near the surface, the more effective will be the tillage practice in reducing water erosion. Minimum till or no-till practices are very much effective in reducing soil erosion by water.
Tillage and other practices performed down and upfield slopes create pathways for surface water runoff and can accelerate soil erosion. Contour farming techniques and Cross-slope cultivation discourage the concentration of surface water runoff and limit soil movement.
Soil erosion remains a crucial challenge for agriculture. Many farmers have made significant progress in tackling soil erosion issues on their farms. Due to continued advances in soil management and crop production technology that have maintained or increased yields despite soil erosion. However, others are unaware of the increasing problem on farmland. When the property is damaged, and productive soil areas are lost, then only there is the awareness.
The increase in extreme weather conditions predicted with climate change will magnify the existing wind and water erosion situations. Furthermore, it will create new areas of concern. The next time you are out walking or driving, take a moment to appreciate the soil. Plan a little celebration for World Soil Day that falls on December 5 and give thanks to the soil that’s keeping it all together for life on Earth.