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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 years7

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-.05
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
Cadmium 39
Nickel 420
Copper 1500
Chromium 1200
Zinc 2800
Mercury 5
lead 300
Molybdenum 18
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.

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