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Extension > Garden > Yard and Garden > Soils > Composting and mulching guide > Use of compost as a soil amendment

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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