Recommendations for using yard waste compost to control soil erosion on steep slopes are presented in this NebGuide.
Bruce Dvorak, Extension Environmental Engineer
- Compost Recommendations
- Slope Recommendations
- Compost Application Recommendations
- Compost Filter Berm Recommendations
- Seeding Recommendations
- Summary Recommendations
Construction contractors need to control soil erosion on construction sites, especially on steep roadway cuts or fill slopes on building sites. Erosion control is especially important, and more difficult, on steep slopes approaching 3:1 or greater. Environmental blankets, silt fences and hydro-seeding are common erosion control practices. Research and demonstration studies have shown that yard waste compost placed as a 2-inch blanket over bare subsoil or incorporated with a tiller into the subsoil, and seeded is an effective long-term erosion control practice.
These recommendations are based on the results of a demonstration study performed in Nebraska on steep slopes (3:1) conducted in 2004 using LinGro® yard waste compost (NebGuide G1625). Included here is guidance regarding compost specifications, application on steep slopes, compost filter berms and grass establishment.
These recommendations were developed based on a demonstration study using LinGro® yard waste compost, which is processed from municipal grass cuttings and leaves by the city of Lincoln, Neb. The finished compost is well-aged and screened through 0.5 inch openings for use as a soil amendment. LinGro® yard waste compost is commercially available. Similarly processed and screened compost may work as well.
Slopes of 3:1 and shallower are typical for roadway, embankment, stream bank and construction fill slopes. Compost blankets can be used on slopes up to 3:1 by spreading compost uniformly across the slope. Pneumatic blowers or other mechanical spreaders can be used. Test and calibrate spreaders for the proper depth of application before covering large areas.
Compost blankets used on slopes greater than 3:1, such as 2:1, may require additional anchoring to keep the blanket in place during heavy rainfall and wind. Compost can be anchored on steep slopes by spreading geo-fabric or plastic netting over the compost blanket. The netting should be anchored using metal stakes at approximately 6 feet intervals along and perpendicular to the slope. A substitute for the netting may be a liquid tackifier similar to that used for hydro-seeding.
An unincorporated 2-inch compost blanket is recommended for erosion control on slope treatments based on its effectiveness at reducing soil loss in the demonstration study (NebGuide G1625) and other similar studies. A 2-inch unincorporated compost blanket was found to be adequate in studies in Iowa. Studies in Oregon and Washington, where rainfall is greater than in Nebraska, recommend a 3-inch blanket?. Prior to applying a compost blanket, the subsoil should be scarified or tilled with a mechanical tiller. This will allow plant roots to more easily enter the soil and will help hold the compost in place.
Compost can be placed using a pneumatic blower truck. This method provides a uniform application if a skilled operator is used, plus it can reach some hard-to-access areas. For larger areas, alternative mechanical application methods may be less expensive. An experienced skid steer loader operator may back up with a full bucket of compost and slowly vibrate the bucket up and down to distribute a relatively even layer of compost. Practice and testing of this technique is recommended before using it over a large area.
The recommended thickness for incorporated compost is 2 inches of compost mixed thoroughly into 6-8 inches of soil and incorporate with a mechanical tiller or disc. If heavy rain occurs shortly after tillage, slopes with incorporated compost may have erosion rates greater than non-incorporated practices because of the soil disturbance caused by tillage. However, incorporated compost may lead to deeper root development, especially in otherwise poor soils, which in turn can aid in grass growth and reduce erosion in the long-term. Incorporation of compost may be important in areas of high wind or heavy thunderstorms to prevent blowing and washing of the compost on the surface.
Research has shown that a compost filter berm removes sediment from runoff similar to a silt fence. However, filter berms are subject to failure if storm events are too severe, or the wrong gradation of compost filter material is used. In the demonstration study, the fine, 0.5-inch screened compost did not work well as a compost berm. Therefore, the berm should be built using coarser compost following specifications, such as those by Filtrexx. A berm following these specifications performed adequately for the demonstration study.
|Figure 1. Filtrexx schematic diagram of filter berms and filter sock.|
Compost berms should be installed parallel to the base of the slope, on the contour. Low spots along the berm may cause runoff water to concentrate and the berm to fail. Berms should be a minimum of 1-foot high by 2-feet wide. In extreme conditions (i.e., 2:1 slopes or greater), a second berm may be constructed at the top of the slope to prevent excess runoff on the slope from above. Berm size based on slope and slope length is shown in Table I. If the berm is to be left as a permanent filter or as a long-term part of a landscape, it should be seeded to permanent vegetation. Compost filter berms should not to be used in areas of concentrated runoff, swales or runoff channels. If direct water flow is possible, use a filter sock, such as Filtrexx FilterSocks, in similar diameters to the berm and stake according to specifications (Figure 1)
It is recommended that compost blankets be seeded with a locally specified grass mix. Seeds can be sown on top of a compost blanket or on top of incorporated compost. Seeds should be lightly worked into the soil surface. A tall fescue seed mixture was used for the demonstration study in Lincoln, Neb., and performed well for the study. A vegetation growth evaluation as part of the demonstration study showed that compost is capable of supporting vegetation without additional irrigation or fertilizer provided adequate rainfall occurs. However, testing soil and/or compost to determine available levels of nitrogen, phosphorous and potassium should be done to determine if adequate nutrients are present to support grass growth and provide healthy soil in the long term.
|Table II. Summary of Compost Use Recommendations|
|Compost||Well cured yard waste compost (grass and leaves), weed free, screened to 0.5-inch (e.g. LinGro® yard waste compost)|
|Slopes||Slope < 3:1 → 2-inch compost blanket
Slope > 3:1, (i.e., 2:1) → Additional slope stability may be necessary
|Compost Blanket Treatments||2-inch blanket, uniformly applied across slope, having an adequate nutrient content to support grass growth|
|Incorporated Compost Treatments||May improve grass establishment in poor soil. Mechanically incorporate 2 inches of compost in 6 – 8 inches of soil|
|Compost Filter Berm Treatments||Compost size and filter berm dimensions, should follow specifications similar to Filtrexx specifications (Filtrexx, International, 2003). See Table I.|
|Vegetation||Should be used in combination with compost applications (incorporated and non-incorporated); use locally approved grass mix|
|Target Treatments||Permanent and temporary projects|
|Maintenance||Weed suppression (i.e., herbicide) as needed;
Irrigation → May not be necessary if adequate rainfall occurs
Starter fertilizer may not be necessary
N, P and K samples prior to compost addition
The projects that would benefit the most from compost use are those needing permanent solutions to erosion control and vegetation establishment, or those that are temporary projects to be completed at a future date. The temporary projects include those where the finish site work (i.e., grading) is completed, but the project will not progress beyond that point for three to five years in the future. For temporary projects, compost may limit the need to re-grade after several years of soil erosion. Table II summarizes the recommendations for using compost for erosion control on steep slopes.
Filtrexx, International (2003), Specifications for Using Filtrexx Filter Berms, Filtrexx International, LLC, 35481 Grafton Eastern Road, Grafton, OH 44044. http://www.filtrexx.com/
|To simplify technical terminology, trade names sometimes may be used. No endorsement of products is intended nor criticism implied of products not mentioned.|
Visit the University of Nebraska–Lincoln Extension Publications Web site for more publications.
Index: Waste Management
Issued July 2006