Compost is a product made from the controlled microbial decomposition of organic wastes. Performed under aerobic conditions (the presence of oxygen), high temperatures are generated for a period of time sufficient to destroy weed seeds and plant and animal pathogens. The characteristics of composts will depend on both the degree of decomposition and the raw ingredients that are used. However quality compost should have a reduced carbon to nitrogen ratio (less than 30:1), a balanced pH (between 6.0 and 8.0) and be free of weed seeds and foreign debris. Compost should not have excessive salts or phytotoxic residues from improper handling.
Composts have value as sources of plant nutrients and organic matter. Although compost can provide significant amounts of nutrients, they are marketed and used as soil additives. For most applications compost should be mixed with soil or other materials (e.g. peat, sand or perlite) to make a growing mix. When using compost to amend soil the soil should be first tested to identify nutrient levels, pH, and organic matter content.
Most soils will benefit from addition of organic matter and thus the addition of compost either as mulch or when incorporated into the soil can significantly improve plant growth.
Increased soil organic matter content will improve the soils ability to absorb and hold water thus reducing the need for irrigation. Soil organic matter also improves the cation exchange capacity and thus plays an important role in holding and releasing nutrients. As the compost breaks down in the soil nutrients are released slowly, thus unlike most commercial fertilizers the nutrients are not washed away by rain.
Common uses for compost in the home landscape include; amending soils for vegetable gardens and planting beds, topdressing lawns, and mulching.
Amending Soils
In general most compost is effective when used at about 20 to 30 % by volume in the soil. Thus adding one to three inches of compos ant tilling the compost into the top six inches is an effective way to improve soils used for vegetables and planting beds. Using compost at higher rates may produce any increased plant response and could in fact reduce plant growth.
Topdressing Lawns
Many lawn areas suffer from excessive compaction and inadequate organic matter. By topdressing with a thin layer of compost on an annual basis the quality of the soil can be greatly improved. When topdressing spread a layer of ¼ to ½ inch of compost over the grass and rake the compost out to fill in bare spots. Topdressing is best performed in the fall when cool air temperatures have slowed the growth of the grass. At this time root growth is active and nutrients will be absorbed into the roots. Grass seed can be spread with the compost and thus areas where the grass is thin can be reseeded at the time topdressing is performed.
Mulching
Composts make good mulches when spread over the surface at a depth of two to three inches. Unlike wood mulches and various colored mulches manufactured from wood, compost will release nutrients over time. Thus by using compost for mulching the quality of the soil will be improved.
