Castings provide nutrition right at the root. Converting humus to castings provides nutritional value to plants and organisms. As plants decay, they form phenols and formaldehyde which can inhibit growth in plants. Earthworms reduce these substances and generate auxins and cytokinins instead, which stimulate plant growth. Castings contain as much as 10 times the soluble, plant-available nutrients as the original soil.
Polysaccharides left by the earthworms help the aggregation of the soil.
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.