What is Compost: Compost soil is very rich soil and used for many purposes. A few of the places that it is used are in gardens, landscaping, horticulture, and agriculture. The compost soil itself is beneficial for the land in many ways, including as a soil conditioner, a fertilizer to add vital humus or humic acids, and as a natural pesticide for soil. In ecosystems, compost soil is useful for erosion control, land and stream reclamation, wetland construction, and as landfill cover.