Compost is the end product of composting. Composting is a degradation process brought about by bacteria and fungus organisms. Large amounts of organic kitchen, garden, lawn, and/or farm refuse can be reduced in a relatively short time to a pile of black, crumbly humus which makes an ideal soil conditioner.
Compost added regularly to soil will certainly benefit the soil. The soil's structure will improve, since humus contains substances which cause aggregation (sticking together) of soil particles. In a clay soil this means that the microscopic individual particles will be clumped together and more air spaces will be opened up between clumps. Without these air spaces the clay particles stick tightly to each other, forming a nearly impenetrable barrier to water and gases. This is why clay is so sticky when it is wet and hard when dry.
In sandy soils, the large sand particles are clumped with humus too, the humus adding its nutrient- and moisture-holding capacity. Normally, water and nitrogen fertilizers leach quickly from sandy soil, making it necessary to add them frequently.
A less widely recognized benefit from compost is that it contains humic and other organic acids which help to degrade compounds naturally present in the soil into the simpler form that plants use. These elements, or ions, can then be held by the humus particles, which contain many ion exchange sites on their surfaces. The ions are released into soil water, and plant roots are able to take them up.
Because there are so many ion exchange sites on humus particles, humus increases the buffering capacity of the soil. This condition helps to prevent rapid leaching of lime and nutrients as well as reducing the effects of over-liming and over-fertilizing. For example, when a soil's pH is increased too much by adding too many wood ashes, the most economical way to correct the condition is generally to add compost, which will absorb (take up on the surface) the extra ions that produce the high pH. (compost itself is somewhat acid because of the acidic products made by microorganisms.) In other words, compost buffers the effects of other soil additives.
Compost and other organic matter turns the soil dark brown or blackish and increases heat-absorbing capabilities to a small extent. Compost reduces soil erosion because it allows water to percolate into lower soil layers, rather than puddle on top and then run off. This quality also reduces crusting of soil. Compost provides food for earthworms, soil insects, and microorganisms, many of which will, over the years, help balance the populations of less desirable soil fauna. Mycorrhizal fungi, which have been proven to benefit plants through their association with plant roots, are also prolific in high humus soil. Finally, the products from the breakdown of plant and animal refuse contain many fertilizing elements in and of themselves, including trace elements not available from commonly used synthetic fertilizers.