What is compost?
In simplest terms, compost improves the soil so that the plants growing in that soil will be healthier and more productive. Whether it is spread in a layer on the surface of the soil or is worked down into the soil by hand or with a tiller, it serves primarily as a soil amendment. Compost is not a fertilizer, although it adds some nutrients to the soil. It is not a pH moderator like limestone, though it can alter the pH of the soil. Basically, it is humus, a component that "conditions" soil, improving its quality, texture, and structure so that it provides a more effective environment for plant roots that grow in it. While it directly improves the soil, it is, of course, indirectly improving plant quality of life. Let's take a closer look at some of the more specific benefits offered by compost.
Here's what compost can do for the soil:
improve soil texture
improve soil drainage and water storage
increase microbial activity
moderate soil temperatures
moderate pH
improve soil fertility, reducing fertilizing needs
Here's what compost in or on the soil does for plants
reduce weed problems as a mulch and killer of weed seeds
reduce watering needs
reduce insect problems
reduce disease problems