Improve aeration and water penetration as well as water retention. Adding compost to soil builds good soil structure and texture, increasing the amount of air that can infiltrate and the amount of water it can hold. Adding compost to heavy clay soil loosens the packed soil by opening up pore spaces that, like little tunnels, carrying air and water down into the soil. Sandy soils, which tend to let water drain away too rapidly to the water table, are also improved with the addition of compost. The fine particles are united into larger ones that can hold a greater amount of water. 100 pounds of compost can hold about 195 pounds of water! By increasing the soil’s moisture-holding capacity, compost is helping the soil to become more drought talerant and also helps control erosion.
Supplying nutrients; when fresh manure is spread on a field, about 50 percent of the nitrogen is in a highly soluble form and will be washed out by rain when it is spread on a pasture. In compost, however, 95 to 97 percent of nitrogen has been converted to a much more stable form and will be slowly released, allowing plants to use it over a longer period of time. Compost doles out nutrients slowly when plants are small and at greater rates as soil temperatures warm up and the major growth period begins. (Soil microorganisms that release the nutrients from compost work harder as temperatures increase.) The benefits of adding compost will also last for more than one season. Composted manure releases about 50 percent of its nutrients in the first season and a decreasing percentage in the following years. This means that with constant additions of compost, the reserves of plant nutrients in the soil are being built up to the point where, for several seasons, little fertilizer, if any, may be needed.
Bacteria, earthworms, and pH. Compost also supports essential soil bacteria; feeds earthworms and allows them to multiply; and gradually changes soil pH levels that are either too low (acidic) or too high (alkaline).
Using compost in garden and landscape areas. Compost can be worked into garden beds by hand or with a tiller or added to the soil when planting trees, shrubs, annuals, or perennials. Compost is also an excellent mulch or topdressing around flowers, shrubs, and trees. This mulch will help your plants get through the dry summer with less need for irrigation. When using it as a mulch around trees or shrubs, start three to four inches from the trunk and spread the compost out to the dripline keeping it about three inches deep.