Turf

Using Compost as a Soil Amendment (Post-Construction Soil BMP)

Compost is the product resulting from the controlled biological decomposition of organic materials that has been sanitized through the generation of heat and stabilized to the point that it is beneficial to plant growth. It is an organic matter resource that has the unique ability to improve the chemical, physical, and biological characteristics of soil.

Key Considerations

Healthy, undisturbed soils provide important stormwater management functions including efficient water infiltration and storage, adsorption of excess nutrients, filtration of sediments, biological decomposition of pollutants, and moderation of peak stream flows and temperatures. In addition, healthy soils support vigorous plant growth that intercepts rainfall, returning much of it to the sky through evaporation and transpiration. Common development practices include removal of topsoil during grading and clearing, compaction of remaining soil, and planting into unimproved soil or shallow depths of poor quality imported topsoil. These conditions typically produce unhealthy plants that require excessive water, fertilizers and pesticides, further contaminating runoff.

To maintain the natural soil qualities, impacts to undisturbed soils should be avoided and minimized during the construction process. When impacts are unavoidable and soils have been compacted or otherwise disturbed, compost can be used as an amendment to regain some of the characteristics of undisturbed soils.

Figure of Comparison of Soil Infiltration after Compaction (from John Barten, Three Rivers Park District)

Figure 1 shows the effect that compaction of soils has on infiltration of water into sandy and clay soils. Uncompacted sandy soils will infiltrate up to 12 inches of water per hour. When compacted, the infiltration rate decreases to 1 inch or less per hour or a 90% reduction in the infiltration of water. Uncompacted clay soils are able to infiltrate up to 9 in per hour. However, when compacted, the infiltration rate drops to less than a ½ inch per hour or a 95% reduction in the infiltration of water. This illustrates how compacted soils contribute a significantly greater volume of runoff to the storm water system. Later discussion shows how compost can help to off-set the effect of compaction.

Establishing soil quality and depth regains greater stormwater function in the post development landscape, provides increased treatment of pollutants and sediments that result from development and habitation, and minimizes the need for some landscaping chemicals, thus reducing pollution through prevention. Establishing a minimum soil quality and depth is not the same as preservation of naturally occurring soil and vegetation. However, establishing a minimum soil quality and depth will provide improved onsite management of stormwater flow and water quality.

Benefits:

Compost can be used as a soil amendment to:

• Increase infiltration
• Reduce runoff
• Improve soil porosity
• Increase soil moisture holding capacity (reduce water demand of lawns and landscaping)
• Reduce erosion
• Absorb certain pollutants (increase cation exchange capacity)
• Reduce fertilizer needs
• Reduce pesticide and herbicide needs

Both yard – leaf compost and the manure compost could be used for turf applications. However, if using manure compost, the fertilizer application may need to be adjusted downward so as to not over fertilize the turf and inadvertently create nutrient runoff.

Compost maturity is another important factor. Using compost that has been properly aged as a post-construction soil amendment promotes healthy root and plant growth and will prevent damage to turf and plantings. When immature compost is applied to soils it continues to decompose and the process of decomposition robs nitrogen from the plants and stunts plant growth, possibly even killing the plant.

To facilitate the creation of consistent compost products throughout the United States, the U.S. Composting Council (USCC) created the Seal of Testing Assurance Program (STA). This voluntary program requires participating compost facilities to perform a uniform set of tests on their compost products. Composters who are STA participants are required to furnish test information to compost buyers. This gives the purchaser of the compost the agronomic information needed (such as pH, particle size and test results from a number of other parameters) to successfully use the compost.

Turf Establishment or Incorporation in Soil as an Amendment

When purchasing compost to be used for turf establishment or incorporation into soil as a post-construction soil amendment, look for the specifications listed in Table 1.

Application Guidelines

The goal in amending compacted soils with compost is to reach or exceed the stormwater management benefits of naturally occurring soil and vegetation. Compost amended soils will improve on-site stormwater management and reduce long term operation and maintenance costs for off-site water treatment best management practices. Developing a Soil Management Plan is an important first step in minimizing and mitigating impacts to native soils and maximizing onsite stormwater management benefits.

In areas where remaining topsoil or subsoil will be amended in place, it is important that, at a minimum, certain soil quality and depth improvements are achieved, as follows:

Soil Quality: For soils in planting areas, a minimum dry weight organic matter content of 10% is recommended. For soils in turf areas, a minimum dry weight organic matter content of 5% is recommended. Soil pH should range from 6.0 to 8.0 or match the pH of the original topsoil (WDOE, 2005).

Depth: Within the construction limits, a minimum, uncompacted depth of 12 inches is recommended (Kunz and Jurries, 2001, WDOE, 2005). In high traffic areas, a minimum uncompacted depth of 18 inches is recommended. Table 1 summarizes how to achieve these depths in planting areas and turf areas.

Nutrient Precaution

When leaching of nutrients could be harmful to a receiving water, is it important to take the compost source into consideration. Because compost made from biosolids or animal manure tends to be higher in nutrients, there is the possibility of nutrient leaching. In general, adequately composted tree and grass material presents less of a problem than animal waste or mixed municipal compost. These types of compost are less appropriate for certain uses in areas in close proximity to water bodies. Note that the use of potential nutrient leaching compost as a filter material in such things as compost socks or filter bags should be avoided whenever excess nutrient (see previous section on Materials Specification) content of water flowing through the filter and into a receiving water would cause a problem. Specification of compost without extractable phosphorus is recommended in cases when nutrients are a receiving water concern.

Other Benefits and Emerging Uses of Compost

In addition to improving the stormwater management functions of compacted soils, compost has several other beneficial uses. The first part of this Fact Sheet addressed soil compost for uses as a post-construction BMP. Because there are so many benefits for compost, its use in construction runoff control is also discussed in the following paragraphs. Many of the uses of compost certainly overlap and can serve both construction and post-construction purposes.

Erosion Control and Storm Water Management Uses of Compost

Compost Blanket Application

If you are considering using compost as a “blanket” to reduce or prevent erosion (See Table 3), the soil blanket should be a composted, weed free organic matter source derived from: agricultural, food, or industrial residuals; yard trimmings; or source-separated or mixed solid waste. Particle size shall be as described below in the product parameters table. The compost shall possess no objectionable odors, will be reasonably free (< 1% by dry weight) of foreign matter and will meet the product parameters outlined below.

Well-composted product will provide the best planting medium for grass, wildflower, legume seeding or ornamental planting. Very coarse composts may need to be avoided if the slope is to be landscaped or seeded, as it will make planting and crop establishment more difficult. Composts containing fibrous particles that range in size produce a more stable mat.

A. Construction Requirements:

Compost mulch shall be uniformly applied to a depth described below. Areas receiving greater precipitation (see Table 4), possessing a higher erosivity index, or which will remain unvegetated, will require greater application rates.

The compost should be spread uniformly on up to 1:2 slopes, then track (compact) the compost layer using a bulldozer or other appropriate equipment, if possible. Alternatively, apply compost using a pneumatic (blower) or slinger type spreader unit. Project compost directly at soil surface, thereby preventing water from moving between the soil-compost interface. Apply compost layer approximately 3 feet beyond the top of the slope or overlap it into existing vegetation. On highly unstable soils, use compost in conjunction with appropriate structural, stabilization and diversion measures. Follow by seeding or ornamental planting if desired.

Compost Filter Berm Application or Sediment Control

A. Description:

This work consists of constructing a raised berm of compost on a soil surface to contain soil erosion, control the movement of sediment off site, and to filter storm water.

B. Materials:

Filter berm media should be a composted, weed free organic matter source derived from: agricultural, food, or industrial residuals; yard trimmings; source-separated or mixed solid waste. Particle size may vary widely. The compost shall possess no objectionable odors, will be reasonably free (< 1% by dry weight) of man-made foreign matter and will meet the product parameters outlined below.

Where seeding of the berm is planned, use only well composted product that contains no substances toxic to plants. Avoid coarse composts if the berm is to be seeded, as it will make establishment more difficult.

The Landscape Architect/Designer shall specify the berm dimensions depending upon specific site (e.g., soil characteristics, existing vegetation) and climatic conditions, as well as particular project related requirements. The severity of slope grade, as well as slope length, will also influence compost application.

C. Construction Requirements

Parallel to the base of the slope or other affected areas, construct a berm of compost to the size specifications outlined in Table 6.

In extreme conditions and where specified by the Landscape Architect/Designer, a second berm shall be constructed at the top of the slope or silt fencing shall be installed in conjunction with the compost berm. Where the berm deteriorates, it shall be reconstructed. Do not use filter berms in any runoff channels (concentrated flows).

Other Uses

In addition to improving the stormwater management functions of compacted soils, compost has several other beneficial uses

Soil Reclamation

Compost can be used to reclaim highly disturbed and low quality soils on sites of old factories, landfills, and brownfields. Application rates in such situations often range from 25 to 175 tons per acre, much higher than typical compost application rates. Benefits include improved soil quality and enhanced plant establishment (Alexander, 1999).

Wetland Construction

Due to its similar physical and chemical properties to certain wetland soils, compost is being used to mimic hydrology, soil properties and plant community composition wetland functions.

Pollution Remediation

Compost has been shown to be effective in degrading or immobilizing several types of contaminants, including hydrocarbons, solvents, and heavy metals (Alexander, 1999).

Pollution Prevention

Compost has been included as a component of biofilters and bioswales to treat contaminated air and water with great success (Alexander, 1999). Compost treated areas have also be shown to be effective at reducing erosion and stormwater runoff (Glanville, et. al., 2003). Because contaminants adhere to soil particles, this limits the amount of sediment and contaminants reaching water bodies.

Additional Information

Clean Washington Center. Minnesota Pollution ControlAgency.