As name implies, earthworm casting are natural fertilizers excreted by earthworms. Earthworm castings are biologically active natural fertilizers containing different types of bacteria with their metabolites like proteins and enzymes. The casting also contain small pieces of plant materials and animal manure that serves as active compost. In addition, the castings are rich in water-soluble micro and macro plant nutrients including nitrates, phosphorus, magnesium, potassium and calcium, manganese, copper, zinc, cobalt, borax, iron, carbon and nitrogen. When you apply earthworm castings, these nutrients are immediately available for the plants to absorb.