What is the effect of vermicompost on pepper, tomato, lettuce and marigold seedlings? Four Ohio State University researchers started with two standard commercial greenhouse potting media, each consisting of 15% (by volume) of perlite mixed with 85% (by volume) of either sphagnum peat moss or coir. Coir, a fibrous, peat-like material is a waste product of cutting and sifting coconuts. It has been used successfully in nurseries and greenhouses as a substitute for peat, largely because of environmental opposition against peat harvesting in wetlands and rising costs for this material. These researchers also used a third standard commercial soilless medium, Metro-Mix 360 (Scotts Co., USA), made of vermiculite, Canadian sphagnum peat moss, bark ash, sand, and a starter nutrient fertilizer. The purpose of using Metro-Mix as a yardstick was to see if the addition of earthworm castings to soilless media might provide a suitable medium for use in container-grown flowers and vegetables.
Eleven different container mixes were prepared and used for the study on the four kinds of plants. In addition to Metro-Mix 360, there were two controls: a coir-perlite mixture and a peat-perlite mixture. The remaining eight mixes were blends of the two soilless media combined with earthworm castings from two sources (pig waste and food waste). One set of blends consisted of the coir-perlite combination mixed with either10% or 20% earthworm castings from pig manure, and a coir-perlite blend with 10% or 20% earthworm castings from food waste. Another set of blends consisted of peat-perlite blended with 10% or 20% of the two types of castings. By using these different combinations, researchers were able to measure the differences in effects of using earthworm castings considering such factors as the parent material (pig waste or food waste), percentage of castings used (10% or 20%) and the soilless medium used (coir-perlite or peat-perlite).
The researchers examined the root and shoot dry weights of four different plants grown in the eleven different container media. After the seeds germinated, half of the plant trays for each group were watered daily with a nutrient solution and the other half given only tap water. This lasted for fourteen days, after which the plants were randomly harvested and the average plant shoot and root dry weights per mixture were determined.
Plant growth in the coir-perlite and peat-perlite controls was less than that found in Metro-Mix 360, the standard commercial potting medium. However, when the two controls were substituted with 10% or 20% earthworm castings, “the growth of seedlings improved significantly, and was equal to or greater than the growth in the commercial medium.”
Investigators found that blending castings with peat yielded better results than blending castings with coir. There may be a beneficial synergistic effect when the two materials are combined. They observed that while the initial low pH of the peat-perlite blend (3.8) inhibited germination and growth, the addition of earthworm castings seemed to serve as a pH modifier or buffer, raising the overall pH to a less-acidic level. Since the coir-perlite blend had a less acidic base point (5.6) than the peat-perlite blend, the addition of castings did not produce the same effect.
Researchers in this study speculated that while the nitrate content in castings could explain some increases, the effect that castings has on plant growth “could not be solely nutritional.” They surmised that the presence of beneficial microorganisms or biologically active plant-growth-influencing substances might be involved. “The optimal plant growth,” they concluded, “was found in peat/perlite-based media substituted with pig manure vermicomposts, in conjunction with regular liquid fertilizer application.”
Atiyeh, R.M., C.A. Edwards, S. Subler, and J.D. Metzger. 2000. Earthworm-processed organic wastes as components of horticultural potting media for growing marigold and vegetable seedlings. Compost Science and Utilization, 8(3):215-223.