In fact, the bacterial population of a cast is much greater than the bacterial population of either ingested soil, or the earthworm's gut. An important component of this dark mass is humus. Humus is a complicated material formed during the breakdown of organic matter. One of its components, humic acid, provides many binding sites for plant nutrients, such as calcium, iron, potassium, sulfur and phosphorus. These nutrients are stored in the humic acid molecule in a form readily available to plants, and are released when the plants require them .
Castings contain slow release nutrients which are readily available to plants.
Castings contain the plant nutrients which are secreted by the earthworms. They dissolve slowly rather than allowing intermediate nutrient leaching. The product has excellent soil structure, porosity, aeration and water retention capabilities. The product can insulate plant roots from extreme temperatures, reduce erosion and control weeds. The activity of the earthworm gut is like a miniature composting tube that mixes, conditions, and inoculates the residues.