Worm castings, also called worm manure, or worm humus, are the end product of the breakdown of organic matter by certain earthworm species called composting worms. They are used as a form of organic fertilizer to improve soil and plant growth. The process of harvesting this material is called vermicomposting. It is practiced both on a large scale for large farms, landscaping, or commercial sale, and on a small scale for home gardens.
Worm species suitable for vermicomposting include Red Wigglers and European nightcrawlers. They may be purchased from a bait store or from commercial vermicomposting operations. Large-scale harvesting of worm castings uses either a windrow, a row of mowed hay or other grain crop, or a raised bed. Since Red Wigglers are top-dwelling worms, and the food source is placed on the top of the bed, a raised bed allows the collection of castings from below without disturbing the worms.
Vermicomposting at home is similar to the raised bed method. A large bin with air holes, and a drainage spout, and a collection tray on the bottom, is used to house the worms. Some experts recommend leaving the bottom open so that the worms are not trapped, allowing them to escape in the case of adverse conditions. Worm castings can be harvested when there is little to no visible food or bedding left in the bin.
Composting bins should ideally be made of recycled or semi-recycled plastic. Though they do not need as much drainage as plastic bins, bins made of wood will decay over time and must be replaced, and some woods contain oils that are harmful to worms. Metal bins are not recommended as they are prone to rust and can release heavy metals into the compost. Styrofoam bins may also produce a toxic environment.
Compost worms can be fed with a variety of organic material. Commercial vermicomposters use grass clippings and wood chips, cow and pig manure, sewage, food processing and grocery waste, and agricultural waste. Home vermicomposters use kitchen and garden trash including fruit and vegetable rinds and peels, coffee grounds and filters, tea bags, moldy bread, and leaves and grass. It is not recommended to use dairy products or meat because they can putrefy and attract pests.
Worm castings have 10 to 20 times as much microbial activity as the soil and food the worms ingest. They contain worm mucus, which helps the soil hold nutrients and retain moisture. They also attract other earthworms that continue to improve the soil. Vermicomposting also reduces biowaste and greenhouse gas emissions.