Mulch is a protective layer of material, either organic (like wood chips) or inorganic (like gravel), applied to the soil surface. Erosion is the process by which soil particles are displaced and transported, most commonly by wind and water. Mulch is highly effective for erosion control, acting as a physical barrier against the forces that move topsoil. This protective function prevents the loss of the nutrient-rich upper soil layer across a landscape.
How Mulch Stabilizes Soil and Slows Runoff
The primary mechanism by which mulch controls water erosion is intercepting the kinetic energy of raindrops before they strike the soil surface. When rainfall hits bare ground, the impact dislodges soil particles in a process known as splash erosion. A layer of mulch absorbs this energy, preventing the soil from being broken apart.
By cushioning the impact of rain, the mulch layer maintains the soil’s natural structure and prevents the formation of a surface crust. Bare soil repeatedly hit by rain becomes compacted and sealed, significantly reducing water infiltration. This protective cover promotes better water infiltration, allowing moisture to seep into the earth rather than quickly flowing over the surface.
Mulch minimizes sheet erosion by slowing the velocity of surface water flow. As water moves across a mulched area, the physical resistance reduces its speed. This reduction in flow velocity allows more time for water to infiltrate the soil, reducing the overall volume of runoff. Less runoff and slower movement mean fewer soil particles are carried away. This action can reduce sediment loss and runoff by over 90 percent in some cases.
The material also helps mitigate wind erosion, particularly on exposed slopes. Mulch slows the wind speed directly at the soil surface, reducing the risk of soil displacement. Furthermore, as organic mulches decompose, they contribute to the soil’s structure, making it more robust and resistant to movement.
Choosing Mulch Materials for High-Erosion Areas
Selecting the correct mulch material is important for maximizing its effectiveness, especially on sloped or vulnerable ground. Organic mulches like shredded bark or wood chips are often preferred because their fibers naturally interlock. This interlocking creates a continuous mat over the soil that resists being washed or blown away during storms.
In contrast, mulches made of large, uniform chunks or coarse bark tend to roll and move more easily, making them less suitable for steep slopes. Shredded wood fiber, often applied hydraulically as hydromulch, is highly effective because it adheres well to the soil. It can also be mixed with tackifiers for extra binding strength, which is particularly useful in critical areas where erosion potential is high.
Straw and hay are very effective, particularly as a temporary cover for newly seeded areas on light to moderate slopes. Straw is lightweight and inexpensive, providing excellent coverage, but it requires stabilization with netting or a tackifier to prevent it from blowing away. The application rate aims for sufficient coverage without smothering new seeds.
Inorganic options, such as crushed stone or gravel, offer a long-term solution for extreme slopes or areas with heavy water flow. While they provide strong erosion protection and do not decompose, they lack the soil-improving benefits of organic mulches. These materials are effective in unvegetated areas where soil improvement is not the primary goal.
Proper Application on Slopes and Vulnerable Ground
The technique used to apply mulch is crucial for successful erosion control, especially on any gradient. For general coverage, the mulch should be applied in a consistent layer typically between 2 to 4 inches deep. This range provides a thick enough barrier for protection without becoming unstable and sliding down a slope.
On steep grades, the mulch must be physically secured to the soil to prevent displacement by gravity or water runoff. This is often accomplished by covering the loose material with biodegradable netting or erosion control blankets (made from materials like jute or coir). Anchoring the netting with staples ensures the mulch stays in contact with the soil until vegetation can be established.
An alternative method for securing loose organic mulch involves tackifiers, which are binding agents that create a temporary crust on the surface. These can be sprayed over the mulch after spreading or mixed in during a hydraulic application process. For very steep terrain (slopes greater than a 3:1 ratio), specialized erosion control blankets or hydraulically applied wood fiber mulch are necessary for adequate stabilization.
To manage water flow on longer or steeper slopes, small check dams can be incorporated into the mulching plan. These barriers are installed perpendicular to the flow of water, helping to break up the slope length, slow runoff, and trap sediment. This combined approach of proper material selection, correct depth, and physical anchoring ensures the mulch layer provides maximum protection against soil erosion.