It serves as a soil conditioner, enriching the soils and improving their structure. Compost can be used to dilute clay. When thoroughly mixed with clay, compost particles bind with the smaller clay particles to loosen the soil. Surface water and plant roots can then move quickly into the more porous soil. When compost is mixed with sandy soils, these soils hold more water and nutrients.
Compost attracts and supports earthworms and other desirable soil microorganisms.
As earthworms burrow, they aerate soils, transport minerals essential for plant growth and help convert organic matter to humus. Many microorganisms release essential plant nutrients as they break down organic materials. Compost is also an effective mulch when spread over the soil surface around annual and perennial flowers and under trees and shrubs. As little as inch of compost broadcast over the soil surface helps conserve moisture, reduces erosion, buffers the soil surface from temperature extremes, adds nutrients to soils and suppresses weeds. As the composting process continues, heat generated by these composting organisms rises from the top of the compost pile.