"Earthworm castings are the best imaginable potting soil for greenhouses or houseplants, as well as gardening and farming. It will not burn even the most delicate plants and all nutrients are water-soluble, making it an immediate plant food. Earthworm castings, in addition to their use as a potting soil, can be used as a planting soil for trees, vegetables, shrubs, and flowers..."

-- R.E. Gaddie and D.E. Douglas, Earthworms For Ecology and Profit, Vol. I "Scientific Earthworm Farming," 1975, p. 175.