Earthworm Castings as Plant Growth Media
Researchers worldwide have demonstrated that earthworm castings (vermicompost) have excellent aeration, porosity, structure, drainage, and moisture-holding capacity. This paper describes the effects of vermicompost on plant growth. The following is based upon the paper “The Potential of Earthworm Composts as Plant Growth Media,” by Clive Edwards and Ian Burrows in Earthworms in Waste and Environmental Management (edited by Clive Edwards and Edward Neuhauser).
In 1980, Edwards and Burrows and their associates launched an extensive research program in England to examine earthworms, their castings, and their effect on plant growth. Because the physical structure of the plant growth media produced from organic wastes depends on the original material, an assortment of animal, vegetable and industrial wastes were tested as feedstock, including swine, chicken, duck, turkey, cattle, potato, brewery, paper, and mushroom wastes. Plants included in the tests were (a) several varieties of ornamental shrubs; (b) vegetables such as aubergine, cabbage, capsicum, cucumber, lettuce, radish, and tomato; and (c) bedding plants such as alyssum, antirrhinum, aster, campanula, calceolaria, cineraria, coleus, French marigold, plumose asparagus, polyanthus, salvia, and sweet pea.
The resulting nutrient content of vermicompost from animal waste feedstocks was compared to that of a commercial plant growth medium (Levington compost) which had inorganic nutrients added. The following nutrients were analyzed: nitrogen, phosphorus, potassium, calcium, magnesium, and manganese. The nutrient content was much higher in the vermicomposts for most elements except magnesium (a magnesium sulphate can be used to rectify this deficiency). The researchers noted that many of the nutrients in waste materials (including nitrogen, potassium, phosphorus, calcium and magnesium), when processed by earthworms, are changed into forms more readily taken up by plants.
Although plants prefer a growth medium on the acid side of neutral (6.0 pH), organic wastes and earthworm castings are usually more alkaline (>7.0 pH). Therefore, Edwards and Burrows recommend that castings be mixed with an acid medium such as peat.
The wide range of plants tested were successfully grown in both undiluted wastes or several mixes including 3:1 or 1:1 ratios of vermicompost to peat, Kettering loam, or pine bark. Plant growth was reported to be better with vermicompost than when recommended commercial growing media was used, and seeds germinated faster for most plant species grown in vermicompost.
Seedling emergence of tomatoes, cabbage, and radish was much better in vermicompost than in thermophilically-composted animal wastes, and as good and usually better in vermicompost than in a commercial medium. In addition, early growth of ornamentals’ seedlings was as good or better in vermicompost/peat mixtures than in the commercial plant growth medium.
After the seedlings were transplanted into pots, the ornamentals grew better in vermicompost/peat mixtures than in the commercial growth medium. Plants that grew particularly well in the vermicompost mixture were aubergines, dahlias, coleus, capsicum, and polyanthus. In addition, several ornamentals (especially salvias, chrysanthemums, and petunias) planted in vermicompost mixtures flowered much earlier (the authors noted that this was possibly due to a hormonal effect).
Three types of ornamental plants grew better when a 50/50 mixture of swine and cattle vermicompost were diluted at several different levels with a commercial plant growth medium. Even 5 percent vermicompost in the vermicompost/commercial mixture had a profound effect on plant growth. Dilutions of the different mixtures grew better plants than 100 percent vermicompost, which tended to dry out quicker than the mixtures.
Although vermicompost produced from paper waste feedstock was lower in nutrients than that from animal wastes, it still had a positive effect on seedling germination and growth. Edwards and Burrows noted that the paper waste vermicompost was one of the best feedstocks tested, and they did not experience difficulties in processing or generating a standardized material.
The researchers concluded that vermicompost mixed with peat or other materials makes superb plant growth media; consequently, it has significant commercial potential. Vermicompost needs standardized production to ensure sufficient nutrient status, therefore uniform sources of organic feedstock must be available. Furthermore, vermicompost needs pH adjustment, and possibly sterilization to reduce pathogen problems and kill insects. If the goal is to reach a large market, mechanization of the production and packaging is in order, and the wholesale cost should be competitive with that of existing products.
Edwards, C.A. and Neuhauser, E.F. (1988) Earthworms in Waste and Environmental Management. The Hague, Netherlands: SPB Academic Publishing.