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different sites. The highest concentrations were found in composts produced at sites in the most urban areas. Gener. ally it has been considered safe to use garden produce grown in soils with total lead levels less than 500 mgfkg (parts per million). The lead levels in the yard waste composts are con- siderably less than this suggested fir. ,. Other trace metals such as cadmium, nickel, copper, chromium, and zinc are also present in compost in small quantities. The Minnesota Pollution Control Agency has established maximum con- centrationsof these elements for composts that are applied to soil. The allowable levels in milligrams per kilogram on a dry weight basis are: Cadmium 10 Nickel 100 Copper S00 Chromium 1000 Zinc 1000 Typical concentrations of these elements found in municipal yard waste composts are many times less than the allowable levels. Use of Compost as a Soil Amendment Compost is used as an organic amendment to improve physi- cal, chemical, and biological properties of soils. Adding compost will increase the moisture holding capacity of sandy soil, thereby reducing drought damage to plants. When added to heavy clay soils, compost will improve drainage and aeration, thereby reducing waterlogging dam- age to plants. Compost will increase the abilityof the soil to •^ hold and release essential nutrients. The activity of earth- worms and soil microorganisms beneficial to plant growth will be promoted with compost additions. Other benefits of addingcmnpost include improved seed emergenceand wa- ter infiltration due to a reduction in soil crusting. Over time, yearly additions of compost will create desirable soil structure, makiig the soil much easier to work. For im- proving 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. How- ever, 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 aulrient content of municipal leaf compost. Differences are probably due to several factors including age of the com- post, 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 fer- tilizer, 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 up (5). For the majority of municipal yard waste com- posts, the UN ratio,, Inss than 20 toI(Table 3:unts of Thus, whine composts may not supply significant especially in the short run, nitrogen tie-up should not be a major concern with most yard waste composts. Approxi-mately 1 cup of ammonium nitrate a 15 lb actual nitrogen) _ v per 3 bushels (100lbs compos0 is required to provide the ad- ditional nitrogen needed by most garden plants. 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 compostshould not pose any problems when diluted by mixing into the soil and in fact is beneficial to plants ;rowingon acid soils. Because of the al. kaline pH, yard waste composts do not appearwell suited for use on acid loving plants such as azaleas and blueberries. Use of Compost in Potting Soils Leaf compost can ue used as a component of potting mixes. Generally, no more than one quarter to one third by volume of the potting mix should be compost: over time some of the compost is likely to decompose and the volume of the pot- ting soil will be reduced. In addition, high levels of compost in a mix may cause waterlogging and poor aeration for roots. Although proper composting destroys most weed seeds and disease organisms, some may still survive due to incomplete mixing. To obtain a completely pasteurized leaf compost, it is necessary to heat the material in an oven until the tempera- ture of the center reaches 160-F and is maintained for 30 minutes. Use of Compost as a Mulch Mulches are used in gardens to suppress weeds, reduce soil erosion, modify the soil temperature (cooler in the summer, warmer in the winter), and conserve soil moisture. Yard waste compost makes an ideal mulch for annual and peren- nial garuens. All that needs to be done is to apply a 3.6 inch layer of compost around the base of the plant. Periodically throughout the summer, you may need to add more compost aver the old layers to maintain the heriefits of the mulch. The soil environment beneath the mulch is favorable for promoting earthworms which in turn are valuable It aerat- ing the soil. Organic matter is gradually added to the soil as the mulch decomposes. For annual gardens, the mulch can be worked into the soil at the end of the season to further im- prove soil physical properties. For perennials, it may be ben- eficial to remove the mulch in the spring to allow the soil to thaw out faster. Asdiscussed above, a well decomposed Yard waste compost will nat tie up nitrogen. Therefore, additional nitrogen beyond that recommended for plant growth is un- necessary. If uncomposted or partially composted leaves are used, one tablespoon of a high nitrogen fertilizer should be applied to each bushel of mulch. Alternatives to Composting Grass Clippings The question of whether to collector not to collect grass clip- pings has been around for a long time. However, it is now agreed that as long as the grass is not excessively long and cl ippings de not thickly cover the lawn surface after mowing, there is normally no need to collect the clippings. Aside from reducing the work involved in lawn maintenance, leaving the grass clippings benefits the turf by returning nutrients and organic matter to the soil. if evenly distributed, clippi alien left on the lawn can be equivalent to one fertilizer application per year. In order to keep your lawn looking healthy and to control the amount of clippings generated, several maintenance prac- tices should be followed. It is important that mowing height be properly adjusted. The height of the cut will depend on