Making and Using Compost

Christopher J. Starbuck
Department of Horticulture

Compost is partially decomposed organic matter. It is dark, easily crumbled, and has an earthy aroma. It is created by biological processes in which soil-inhabiting organisms break down plant tissue. When decomposition is complete, compost has turned to a dark brown, powdery material called humus. The processes occurring in a compost pile are similar to those that break down organic matter in soil. However, decomposition occurs much more rapidly in the compost pile because the environment can be made ideal for the microbes to do their work (see Figure 1).

Why make compost?

Gardeners often have difficulty disposing of leaves, grass clippings and other garden refuse, particularly in urban areas. Missouri law bans these materials from landfills, so it has become even more important to find environmentally sound ways to dispose of them. These byproducts of the garden and landscape can be turned into useful compost with no more effort than it takes to bag and haul them away. Home composters avoid hauling or utility costs associated with centralized composting facilities and end up with a valuable soil conditioner or mulch for the landscape and garden.

The value of compost

Good compost consists primarily of decomposed or partially decomposed plant and animal residues, but may also contain a small amount of soil. Compost improves both the physical condition and the fertility of the soil when added to the landscape or garden. It is especially useful for improving soils low in organic matter.

Organic matter in compost improves heavy clay soils by binding soil particles together into "crumbs," making the soil easier to work. Binding soil particles also helps improve aeration, root penetration and water infiltration and reduces crusting of the soil surface. In sandy soils, additional organic matter also helps with nutrient and water retention. Compost also increases the activity of soil microorganisms that release nutrients and other growth-promoting materials into the soil.

Although compost contains nutrients, its greatest benefit is in improving soil characteristics. You should consider it as a valuable soil amendment rather than a fertilizer, because additional fertilization may be necessary to obtain acceptable growth and yields.

Compost also is a valuable mulching material for garden and landscape plants. It may be used as a topdressing for lawns and, when it contains a small amount of soil, as part of a growing medium for houseplants or for starting seedlings.