Are there benefits to adding vermicompost to my organic potting mix?
Answer: Vermicompost (or worm compost) will play a similar role to thermophilic compost in a potting mix, though less is generally needed, due to a higher concentration of nutrients and beneficial microbes. Unlike compost, vermicompost does not go through a thermophilic (or hot) stage and relies on the processing of worms, usually red wigglers (Eisenia fetida), to produce a finished and stable product. In a potting mix, vermicompost improves moisture retention and contains nutrients such as N, P, K, Ca, and Mg in readily available forms, with concentrations of nitrate-N that can be twice as high as those in themophilic compost (Leonard and Rangarajan, 2007; Wallace, 2012). Studies have shown vermicompost to improve seed germination and enhance rates of seedling growth and development due to plant growth hormones and humic acids (Edwards and Arancon, 2006).
Vermicompost has even greater disease-suppressive qualities than thermophilic compost and has been well-documented to suppress Pythium damping off of vegetable seedlings. Including vermicompost in a potting mix or applying a vermicompost liquid extract were both shown to be effective means of suppressing damping off of seedlings in research conducted at Cornell University (Jack and Nelson, 2010). A non-aerated vermicompost extract can be produced by mixing vermicompost with water at a ratio of 1:60 in a large container, with water circulated or mixed twice a day. Such a liquid extract can serve as a liquid fertilizer to provide P, K, and micronutrients, in addition to providing disease control.
Although vermicompost has a higher N content than thermophilic compost, additional fertilizers are often needed to meet the full nutrient requirements of transplants. Researchers at Cornell have found that the addition of a fertility mix of blood meal, green sand, and rock phosphate (at 7 pounds per cubic yard) to a peat-based mix containing 10% vermicompost can provide enough fertility to grow high-quality tomato and cabbage transplants without supplemental fertilizer (Leonard and Rangarajan, 2007; Rangarajan et al., 2008). Tomato and cabbage transplants grew nearly twice as large in potting mixes that contained 10% vermicompost plus the fertility mix, compared to potting mixes that just contained 10% vermicompost without the fertility mix. For more information on vermicompost, see ATTRA’s Vermicomposting: The Basics.
Learn more in the ATTRA publication Potting Mixes for Certified Organic Production. It covers considerations for an organic grower selecting a potting mix to use for transplant or containerized plant production or for someone wishing to blend organic media, and discusses individual components of potting media in addition to organic and biological amendments to improve plant performance. Several organic potting mix recipes are included in the appendix.
Edwards, C.A., and N.Q. Arancon. 2006. The science of vermiculture. Proceedings of the International Symposium on Vermi Technologies for Developing Countries. Philippine Fisheries Association Inc., Los Baños, Laguna.
Jack, A.L.H., and E.B. Nelson. 2010. Suppression of Pythium damping off with compost and vermicompost: Final report to the Organic Farming Research Foundation.
Leonard, B., and A. Rangarajan. 2007. Organic Transplant Media and Tomato Performance. Department of Horticulture, Cornell University, Ithaca, NY.
Rangarajan, A., B. Leonard and A. Jack. 2008. Cabbage Transplant Production Using Organic Media at Cornell University. Department of Horticulture, Cornell University, Ithaca, NY.
Wallace, Janet. 2012. Perfecting the Potting Mix. The Canadian Organic Grower. Summer. p. 52-55.