Improved microbial activity Compost contains literally billions of microorganisms. The more you encourage microbial activity in the soil, the better plant roots can pick up nutrients. Whether your soil is a heavy clay or a sandy mix, adding compost will benefit its structure. Clay soil has particles so fine that they stick tightly together and do not easily allow in air and water.
Compost binds to the clay particles and helps open up the soil.
Sandy soil has particles that are very coarse with lots of air space that allows water to drain through it very quickly. Compost fills these gaps and helps the soil retain water. Improved soil chemistry Soil conditions can range from extremely alkaline or acidic to nutrient excesses or deficiencies. Compost helps soil chemistry by softening these extremes. Many micronutrients are tied up in the soil and are totally unavailable to plants.