BU-03296     Revised 2000

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Composting and Mulching: A Guide to Managing Organic Yard Wastes

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Municipal Composting of Yard Wastes

If building your own compost heap is impractical, there are municipal composting sites available in many counties. Depending on the location, leaves only or leaves and grass clippings may be dropped off at the composting sites. Some city compost programs also have curbside pick up in the fall. Completed compost is sometimes available free of charge from these sites. For information on the nearest composting site, contact your local county extension educator, county solid waste officer, or city recycling coordinator.

There has been concern about using municipal waste compost because of contamination with lead and other trace metals. Contamination may be due to direct exposure of leaves and grass to automobile exhaust or to inclusion of street sweepings (which might contain high levels of lead from automobile exhaust) in the compost pile. A study at the University of Minnesota⁷ shows chemicals present in yard waste composts from 11 different sites in the seven-county metropolitan area. The mean and ranges of elemental concentrations in the compost piles over two years are presented in Table 3.

Table 3. Chemical Characteristics of Municipal Yard Waste Composts Mean of 11 compost sites over 2 years 7
Chemical Characteristic
	Concentration (dry weight basis)
	Mean	Range
Carbon %	19.3	4.4-41.4
Nitrogen %	1.3	0.3-4.2
Carbon/Nitrogen*	15.4	11-25
Phosphorus %	0.19	0.05-0.5
Potassium %	0.39	0.04-2.71
Calcium %	3.02	0.70-8.04
Magnesium %	0.54	0.09-1.34
Iron %	0.25	0.06-0.31
Aluminum %	0.27	0.06-0.31
Manganese mg/kg**	420	223-1261
Sodium mg/kg	154	36-921
Zinc mg/kg	88	39-585
Copper mg/kg	11	3-143
Boron mg/kg	41	7-141
Lead mg/kg	49	1-380
Cadmium mg/kg	0.4	<0.1-1.4
Chromium mg/kg	6.3	1.2-52.5
Nickel mg/kg	7.3	1.7 - 33.3
pH	7.6	4.5-8.3
Samples for metal analysis were dry ashed and resuspended in 2N HCl. Metals were determined using an inductively coupled plasma spectrometer.
* Ratio of carbon to nitrogen (See text for further explanation).
< means "less than."
**mg/kg=milligrams per kilogram, which is the same as parts per million

The study shows a wide range in lead values from the different sites. The highest concentrations were found in composts produced at sites in the oldest urban areas, with high automobile traffic and a history of use of lead-based paints. Generally it has been considered safe to use garden produce grown in soils with total lead levels less than 300 mg/kg (parts per million). The lead levels in most of the yard waste composts were considerably less than this suggested limit. Other trace metals such as cadmium, nickel, copper, chromium, and zinc are also present in compost in small quantities. Based on US-EPA standards, the Minnesota Pollution Control Agency has established criteria for an exceptional quality compost that takes into account the maximum allowable concentrations of these elements for unrestricted use of composts. The allowable levels in milligrams per kilogram on a dry weight basis are:

Arsenic	41	Zinc	2800
Cadmium	39	Mercury	5
Nickel	420	Lead	300
Copper	1500	Molybdenum	18
Chromium	1200	Selenium	36

Typically, municipal yard waste composts contain far less than the allowable levels of these elements. If you are concerned about the composition of the municipal compost, ask the operators if a recent chemical analysis is available.

Use of Compost as a Soil Amendment

Compost is used as an organic amendment to improve physical, chemical, and biological properties of soils. Adding compost will increase the moisture-holding capacity of sandy soils, thereby reducing drought damage to plants. When added to heavy clay soils, compost will improve drainage and aeration, thereby reducing waterlogging damage to plants. Compost increases the ability of the soil to hold and release essential nutrients and promotes the activity of earthworms and soil microorganisms beneficial to plant growth. Other benefits of adding compost include improved seed emergence and water infiltration due to a reduction in soil crusting.

Over time, yearly additions of compost will create desirable soil structure, making the soil much easier to work. To improve soil physical properties, add and incorporate 1-2 inches of well-decomposed compost in the top 6-8 inches of soil. Use the lower rate for sandy soils and the higher rate for clay soils. To a limited extent, compost is a source of nutrients. However, nutrient release from compost is slow and the nutrient content is often too low to supply all the nutrients necessary for plant growth. As noted in Table 3, there is a wide variation in nutrient content of municipal leaf compost. Differences may be due to several factors, including age of the compost, amount of water added, plant species, and the amount of soil that becomes mixed into the pile during turning.

It is usually necessary to supplement compost with some fertilizer, particularly nitrogen. If the C/N ratio of the compost is less than 20 to 1, nitrogen will tend to be released rather than tied up5. For the majority of municipal yard waste composts, the C/N ratio is less than 20 to 1 (Table 3). Thus, while composts may not supply significant amounts of nitrogen, especially in the short run, nitrogen tie-up should not be a major concern with most yard waste composts. Approximately 1 cup of ammonium nitrate (0.15 lb. actual nitrogen) per 3 bushels (100 lbs. compost) is required to provide the additional nitrogen needed by most garden plants. Compost that is immature or not well decomposed should be used primarily as a mulch. Incorporation of immature compost into the soil may result in nitrogen deficiency and poor plant growth. Have your soil tested every few years to determine whether supplemental phosphorus and potassium are required.

The pH of most yard waste composts is usually between 7.0 and 8.0. This slightly alkaline pH of compost should not pose any problems when diluted by mixing into the soil and, in fact, is beneficial to plants growing on acid soils. Because of the alkaline pH, yard waste composts may not be suited for use on acid-loving plants such as azaleas and blueberries.

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