Selecting Commercial Compost
Composting is the process of partially breaking down organic material by microorganisms such as bacteria and fungi. Organic material for compost may include materials such as grass clippings, straw, ground wood or leaves. The composting process heats the organic material to temperatures that kill weed seeds and most potential disease causing organisms. Incorporation of compost improves soil conditions and plant health.
Amounts, Specifications & Supplies:
How is compost made?
There are three methods of composting that are commonly used: windrows, aerated static piles and bins or aerated chambers. Windrows and aerated static pile are typically used for high volume composting. Bins or aerated chambers are most typical for small volume or home composting.
Windrows are piles of organic material 3 to 5 feet tall and 10 to 15 feet wide at the base. Windrows are turned periodically to allow oxygen into the center of the pile, because microorganisms which break down organic matter require oxygen to carry out the composting process. Turning is accomplished with loading equipment such as front-end loaders. The windrows are monitored to ensure that necessary temperature levels are attained for thorough composting to occur. The composting process takes several months to complete. Aerated static piles are windrows that have perforated pipe laid within the pile. The pipe forces air into the pile, which makes turning unnecessary.
Heat created by microorganisms during the composting process kills weed seeds and disease causing organisms. Temperatures should reach 140 to 160 degrees F for this process to be thorough.
Material that is properly composted should look dark in color and have the appearance of coarse soil. If you pick some up in your hand you may still see small specks of leaves, but most of the material should be decomposed and difficult to recognize. Mature compost has an earthy smell. Most commercial compost is screened to ½ inch to separate out larger particles and non-biodegradable materials such as plastic, glass, metal or rock, thus ensuring a more uniform product.
There are materials that should not be composted. Sawdust from chemically treated wood products contains toxic elements that can be harmful to plants and potentially leach into the soil. Other materials that should not be composted are diseased plants and human waste. Although proper composting reaches temperatures that kill disease-causing organisms, it is difficult to be sure that all pathogens have been destroyed.
There are four grades of compost that most sites offer. These include recycled grade, nursery grade, food refuse grade and DOT grade two. Recycled grade is produced primarily from residential yard waste. Nursery grade is produced from commercial grass clippings, composted ground wood and composted cow manure. Food refuse grade includes plant material from food processors such as sweet corn husks, pea pods, etc. Food refuse grade is recommended for vegetable gardens.
How to determine if compost is safe to use?
Information that can be used to determine the make-up of a compost product include copies of any soil or waste chemical nutrient analyses, pesticide and heavy metal analyses, and stability tests that the producer performed. (Bilderback and Powell, 1996)
The following table lists the specifications the MN Department of Transportation requires for the compost products it uses for projects.
MnDOT 3890-2 Specifications for Grade 2 compost (SKB Environmental, 1998)
|Carbon/Nitrogen Ratio||6 minimum - 25 maximum|
|Ammonia Levels, % of Total||10 % maximum|
|Percent Moisture||20 % minimum - 40 % maximum|
|pH||5.5 - 8.5|
|*Soluble Salts (mmhos/cm)||10 maximum|
*North Carolina Cooperative Extension Service recommends soluble salts levels of no more than 2.0 mmhos/cm to prevent damage to ornamental plants. (Bilderback and Powell, 1996)
pH is a measure of the acidity or alkalinity of a substance. A pH level of 7 is considered neutral. Levels higher than this (8 or greater) are alkaline. The greater the number, the higher the alkalinity. Levels lower than this (6 or lower) are acidic. The acidity of a substance increases as the pH number decreases. Ideally, good compost should fall within a pH range between 7 - 8.
Compost needs to be free from pathogens to be safe for the consumer. Check with the producer to see if compost was maintained at a minimum of 131 degrees F for 48 to 96 hours during the composting process. This ensures that the compost is free from disease-causing organisms. (Bilderback and Powell, 1996)
Three simple tests can be used to determine if compost is stable and ready to use:
- Put a handful of the material in a plastic bag and seal it for 24 hours. If it does not have an offensive smell when opened, it is probably stable.
- None of the materials in the compost should be identifiable. The compost should appear like rich, organic earth.
- If a small pile (3 cubic yards) of compost does not heat more than 20 degrees F above the temperature of the surrounding air in 24 hours, it is probably stable.
(Bilderback and Powell, 1996)
The addition of organic matter such as compost benefits the soil in several ways. Compost improves soil structure, which is usually damaged in urban sites due to construction. Compost breaks up clay soils, increasing the amount of air spaces. This allows more oxygen into the soil for plant roots and improves water drainage. In sandy soil compost increases the ability of the soil to hold moisture and nutrients. This improves drought tolerance of plants. Compost also incorporates and promotes the growth of beneficial microorganisms which are necessary for plant growth and health.
There is some concern that addition of organic matter will deplete soil nitrogen levels. While this may be somewhat true for uncomposted material, the plant matter in compost is already broken down by soil microorganisms so compost will not deplete the soil of nitrogen.
Most sites sell compost by the cubic yard. Prices range from $8.00 - $16.00 per yard, depending upon the grade purchased.
Recommended application rates for compost from the Composting Counsel, 1994. (SKB Environmental, 1998)
|Type of Application||Use||Recommended Usage Rates (approx.)|
|Homeowners||Common landscape & garden uses||1 inch application or 20% by volume|
|Sports turf||Construction mixes for golf courses||5 - 20% by volume depending on application|
|Sports turf||Topdressing mixes||5 - 20% by volume for golf courses; up to 100% for fields after aeration|
|Landscapers||New turf establishment||1 - 2 inches tilled to a depth of 5 inches|
|Landscapers||Turf renovation and topdressing||1/8 to 1/2 inch topdressed after aeration|
|Landscapers||Planting bed preparation||1 - 2 inches tilled into raised beds|
|Landscapers||Mulching||2 - 3 inches evenly applied|
|Landscapers||Backfill for tree planting||30% by volume|
|Nurseries||Field application as a soil amendment||1 - 2 inches incorporated 5 inches deep|
|Nurseries||Band application for shade trees||2 inches applied in two foot bands|
|Nurseries||Liner beds incorporated||1 - 2 inches incorporated pre-planting|
|Nurseries||Liner beds mulched||1 - 2 inches mulched post planting|
|Nurseries||Container mixes||5 - 40% by volume depending on plant type|
|Topsoil Blenders||Soil amendment for many blends||10 - 50% by volume for blends|
|Roadside||New seed establishment
Upgrading of soil
|1 inch disked to a depth of 4 inches|
|Roadside||Erosion control||1 - 2 inches as a coarse mulch|
|Roadside||Mulch for tree plantings||2 - 3 inches applied evenly|
|Roadside||Planting beds at rest stops or interchanges||1 - 2 inches tilled into raised beds|
|Landfills||Daily cover||used to replace soil|
|Landfills||Vegetation establishment during closure||1 inch disked into soil|
|Silviculture||Seedling establishment||1 - 2 inches disked where possible|
|Silviculture||Mulch||1 - 2 inches evenly applied|
|Agriculture||General field soil amendment||1/4 - 1 inch incorporated|
|Agriculture||Specialty crop production||1/4 - 2 inches incorporated or as a mulch|
Tools and Equipment:
- Shovels - 'Grain scoop' style and standard digging type
- Silage or 'Pitch' fork
- Rototiller - to work compost into soil
A soil test should be done to determine soil pH and whether supplemental nitrogen, phosphorus and potassium are required in the soil. These can be worked into the soil at the same time the compost is incorporated. Phosphorus and potassium infiltrate the soil very slowly and need to be worked into the soil where they can be accessed by plant roots.
Where to purchase commercial compost:
The following is a list of companies in the Twin Cities area that sell commercial compost:
Buberl Recycling and Compost Inc.
5750 Memorial Avenue North
Composting Concepts Inc.
15843 South 45 Street
435 Reed Street South
Waterville, MN 56096
Minnesota Mulch & Soil
P.O. Box 270101
St. Paul, MN 55127-0101
SMC Compost Services
805 Yankee Doodle Road
630 Malcolm Avenue
9600 S. Glendenning Road
Cottage Grove, MN
17750 Pilot Knob Road
Barker, James C. 1996. Organic Composting for Horticultural Use. North Carolina Cooperative Extension Service EBAE 171 - 93.
Bilderback, T.E. and M.A. Powell. 1996. Using Compost in Landscape Beds and Nursery Substrates. North Carolina Cooperative Extension Service AG - 473 - 14.
Johnson, Eric S. 1997. How to Compost.
Johnson, Eric S. 1997. What NOT to Compost.
Personal interview with Tom Halbach, Professor of Soil Science, University of Minnesota, June 1, 1999.
Rosen, Carl J., Nancy Schumacher, Robert Mugaas, and Thomas R. Halbach. 1990. Composting and Mulching: A guide to managing organic yard wastes. University of Minnesota Extension Service AG - FO - 3296.
SKB Environmental. 1998. www.skbinc.com
This report was developed by Nancy A. Henry-Socha, Student, Department of Horticultural Science, University of Minnesota.