Sandy soil has large particles with large air pores, causing these soils to drain rapidly without adequate water retention. In sandy soil, compost helps to form larger aggregates from unconnected particles so that water and nutrients are retained. Clay soils have small particles which have small air pores. They drain poorly, retaining so much water that the soil cannot hold adequate air for vegetation. In clay or silt, compost breaks up the compacted particles to form aggregates so that water and air can move through the soil.
Root systems can also forge their way through the soil more easily.
Soil health is very important to an overall healthy landscape or garden, and compost is a critical component of achieving soil health. Others are aeration of the soil and covering all bare soil with mulch. This web site focuses on compost, but we will give a capsule of the logic behind these two critical steps. Aeration allows oxygen to reach microorganisms that break down the organic matter. Methods of aeration range from simply punching holes in the ground with a pitchfork or pole to rototilling.