Comparing Vermicomposts And Composts

Trials in soil and container media reveal some differences that may exist between vermicomposts and composts and point to a growing awareness of the potential importance of microbial activity in plant growth.

Scott Subler; Clive Edwards, and James Metzger

Reprinted with permission from BioCycle July 1998 pages 63-66.

Wild claims abound concerning the beneficial aspects of earthworm castings and vermicomposts when used on household potted plants or in the garden. Stories of the remarkable qualities of earthworm castings are firmly embedded in gardening folklore, and can still be found in modern web pages and cyberspace chat rooms. Even for scientists who are considered to be rational and objective observers of nature - experiences with the use of worm castings suggest that, more often than not, these stories ring true.

Although the anecdotal evidence may be abundant, scientific documentation of the responses of plants to the application of earthworm castings to soil or container media has been poor. There are a few summaries of the numerous horticultural trials conducted in the 1980s in association with Professor Clive Edwards' earlier vermicomposting research program in England. Other-wise, there have been only a handful of reports appearing in the scientific and technical literature in recent years, such as "Vermicomposts as components of potting media," by K.A. Handreck in October, 1986 Rio-Cycle. The conclusions of most of these sparse reports tend to support the concept that worm castings can have substantial effects on plant growth and may have real value as soil additives or components of horticultural container media.

Over the past few years, the Soil Ecology Laboratory at The Ohio State University (OSU) has been developing a comprehensive research program in vermicomposting, which includes studies into the effects of vermicomposts on plant growth. Recently, we have conducted a number of studies related to the effects of vermicomposts on the germination and growth of flowering plants such as petunias, marigolds, bachelor's button, and poinsettia, as well as on popular bedding plants such as bell peppers and tomatoes. Consistently, the addition of relatively small amounts of worm castings to standard horticultural container mixes, and even to commercially prepared premium quality container media, has resulted in dramatic improvements in plant growth.

Overall Trends

A consistent and interesting trend for trials with plants grown in container media is for the best responses to occur when worm castings constitute only 10 percent to 20 percent of the volume of the mix, with greater proportions of castings not always improving plant. growth as well. In some cases, even as little as five percent of castings in the mix is enough to cause a dramatic response, leading us to believe that the substantial growth effects that we observe are more than simply a function of the mineral nutrient content of the castings - perhaps related to enhanced micronutrient availability, the presence of plant growth regulators, or the activity of beneficial microorganisms in the castings.

In fact, some responses that we have observed for vermicomposts appear to be similar to those reported for some composts when used as components of container media - an area that has been studied much more extensively than vermicomposts. Since the differences in the composting and vermicomposting processes are quite distinct, particularly concerning the optimum temperatures for each process and the types of microbial communities that are predominant during active processing (i.e., thermophilic bacteria in composting, mesophilic bacteria and fungi in vermicomposting), we speculated that there would be general differences in the qualities of vermicomposts and composts with regard to their utilization as soil amendments or as components of plant growth media.

When comparing nutrient and chemical analyses for a number of composts and vermicomposts we have found that there may be some tendency for vermicomposts to have slightly lower pH values, and slightly higher nutrient concentrations, particularly of nitrogen, than composts. It is also quite common for vermicomposts to have very low concentrations of ammonium-nitrogen and very high concentrations of nitrate-nitrogen, whereas the opposite is often true for many composts. Overall, however, there is a great deal of overlap between the nutrient contents of the two types of materials, and it is not always possible to tell one from the other based on such analyses. In fact, it is often very difficult to predict the relative growth responses of plants grown with different materials based solely on these types of test results.

Pot Study with Raspberry Plants

To begin to address the question of how vermicomposts and composts compare when used as amendments to soil or container media, we conducted two greenhouse studies. In the first, we joined efforts with Professor Richard Funt, in the Department of Horticulture at OSU, and Research Associates Peter Bierman and Tom Wall, with OSU's Piketon Research and Development Center, to evaluate some vermicomposts and composts as soil amendments in a pot study with raspberry plants. We grew Heritage raspberry plants for three months in the greenhouse in one gallon containers containing soil mixed with 20 percent (by volume) of various vermicomposts or composts. In all, there were 13 different treatments including soil mixes with two vermicomposts (vermicomposted pig solids provided by Vermicycle Organics, Inc. in Charlotte, North Carolina, and vermicomposted food wastes from Oregon Soil Corporation, in Portland, Oregon) and four composts (leaf, yard waste, and hardwood bark composts from Kurtz Bros., Inc. in Groveport, Ohio, and chicken manure compost from Nature Pure, Inc. in Mansfield Ohio), various combinations of vermicomposts and composts, and unfertilized and fertilized controls that did not receive any organic amendments. Two weeks after the soil and organic amendments were mixed together, we took soil samples for analysis of chemical and nutrient characteristics, as well as for various measurements of microbial activity, and transplanted the raspberry cuttings. At the end of three months, we measured raspberry plant growth in each pot and analyzed the plant tissues for nutrient content.

At the end of the study, the growth of the raspberry plants differed dramatically among the different treatments. Plants grown in the pig solids vermicompost did as well as those grown in soil that had received a complete fertilizer treatment. Treatments with food waste vermicompost and one ounce of chicken manure compost had less effect, doing no better than the unfertilized soil. Amendment with the yard, leaf, and bark composts actually produced poorer growth compared to the unamended, unfertilized control soil. This may have been related to the stability or maturity of these composts. Although small quantities of the chicken manure compost had little effect on raspberry plant growth, larger amounts were quite detrimental to their growth, killing most of the plants in the five and ten ounce treatments. This was probably due to extremely high ammonia contents in the chicken manure compost, which severely burned the plant roots.

An intriguing result was that, although amendment with five ounces of the chicken manure compost killed the plants in most of the treatments, when it was mixed with 20 percent pig solids vermicompost the detrimental effect diminished and the plants grew about as well in this mix as they did in the unfertilized control soil. This suggests that this vermicompost may have some beneficial buffering capability or some other quality, not found in the composts, that ameliorated the damage caused by too much of the chicken manure compost.

Overall, few of the initial soil chemical or nutrient characteristics that we measured correlated well with raspberry plant growth. The best correlation was with the percentage of the soil cation exchange complex that was saturated with magnesium ions, suggesting that micronutrient nutrition or overall nutrient balance may have had an important influence on plant growth. Interestingly, magnesium availability in the soil did not correlate with total or available magnesium in the organic amendments. This suggests that the improved growth with the pig solids vermicompost was not simply a function of the nutrient content and that other factors, possibly including enhanced microbial activity, may have been responsible for increased micronutrient availability and plant growth.

Container Media Trial

In a second greenhouse study that we conducted, along with Ph.D. students Rola Atiyeh and Gary Bachman (now an Assistant Professor at Tennessee Technological University), and Postdoctoral Associate Qishni Zhang, we compared vermicomposts and composts as components of container media for the germination and growth of the common bedding plants, tomatoes and marigolds. We germinated and grew tomato and marigold seedlings for three weeks in small plug trays with various mixes of vermicomposts and composts and a standard, premium quality, commercial medium (Metro-Mix 360, The Scotts Company). The media treatments included Metro-Mix 360 as a control, separate 10 percent and 20 percent mixes (by volume) of pig and food waste vermicomposts as before, with the Metro-Mix, and separate l0 per-cent and 20 percent mixes of biosolids and leaf composts with the Metro-Mix. The biosolids compost, 'Com-Til', was provided by the city of Columbus, the leaf compost was the same as mentioned above. The yard trimmings and hardwood bark composts used in the raspberry study were not used here because their average particle sizes were too coarse for bedding plant media. We measured plant root and shoot weights, leaf area and chlorophyll content, and available nitrogen concentrations and microbial activity in the media.

For the marigolds, we began to observe differences between the vermicompost and compost treatments in leaf chlorophyll concentrations after only one week. However, these differences tended to diminish as the plants grew. By the end of the three-week study, there were few differences in leaf area between the treatments and the control media, except that the leaf compost treatments had reduced leaf area. In contrast to leaf area, the total weights of the marigold plants were increased significantly over the Metro-Mix control by 10 percent mixes of both of the vermicomposts and of the biosolids compost after three weeks of growth. However, the leaf compost mixes resulted in poorer growth than in the control medium. The results for the marigolds indicated that both vermicomposts, and at least one of the composts, could be used to obtain significant growth improvements over the commercially available medium.

The results for the tomatoes were even more interesting. Both of the vermicomposts at the 10 percent rate, and the pig solids vermicompost at the 20 percent rate, led to significantly greater tomato seedling weights after three weeks of growth than with the Metro-Mix alone. Amendment of the commercial medium with leaf compost led to few changes or even to slight reductions in plant weight, but the greatest biomass increase was seen with the biosolids compost treatments.

The large growth response of tomatoes to the biosolids compost may be explained partially by the levels of mineral nitrogen in the medium. Initial ammonium-nitrogen concentrations were extremely high, reaching nearly 1000 ppm, and the nitrate-nitrogen concentrations were even higher - up to 8000 ppm! Mineral nitrogen levels in the other treatments were considerably lower. It is also interesting to note that, although the vermicompost amendments increased growth significantly relative to the commercial medium, this did not appear to be correlated with differences in initial mineral nitrogen concentrations.

This study also provides a good example of "more is not always better" since, even though the biosolids compost amendments led to the greatest plant biomass increase, the growth form of these plants was quite clearly not as desirable, from a horticultural production perspective, as that of plants grown in the vermicompost amendments. Probably due to the excessive nitrogen concentrations, they grew too rapidly, and became overly succulent and spindly - not at all desirable for subsequent transplanting or sale. This was evident in the root-to-shoot ratios of the tomato plants; those in the biosolids treatments had significantly lower ratios (more allocation to shoot relative to root) than the other treatments.

Microbial Activity

The evidence that we have from these two studies, and from a number of other plant growth studies that.. we have conducted recently at OSU, indicates that microbial activity, and possibly the makeup of the microbial communities existing within earthworm castings and vermicomposts, play an important role in the plant growth responses that we have consistently observed. As part of the greenhouse study with the tomatoes and marigolds, we measured the ability of the microbial communities in each media treatment to utilize 92 different carbon substrates for their growth. Some of these substrates, such as simple sugars, are readily utilized by many different microbial species, but other, more complex substrates may only be broken down and utilized by particular species. Therefore, the entire microbial community in a sample will be able to utilize each individual substrate to a greater or lesser degree, depending on the overall species composition and activity of the microbial community.

It is possible to take the sum of all of the activities for the substrates and use it as an overall index of microbial activity. In our horticultural media study, the vermicomposts were clearly distinguishable from the composts, in that they had significantly greater cumulative microbial activity than the composts. We can also use all of the data from the 92 different substrates to develop a profile or fingerprint of carbon substrate utilization by the microbial communities of each sample. These profiles provide strong circumstantial evidence that there are distinct differences between the microbial communities found in vermicomposts and composts. On reflection, this makes relatively good sense, given that the nature of the microbial processes are quite different in vermicomposting and composting -the active phase of composting is characterized by thermophilic bacteria, whereas the active phase of vermicomposting is characterized by mesophilic bacteria and fungi, which are stimulated and encouraged by the activity of earthworms.

Although these recent studies provide only a small piece of the puzzle, their results contribute to our growing awareness of the potential importance of microbial activity and of specific microorganisms in determining the influence of organic amendments on plant growth, and may help us understand better some of the differences that may exist between vermicomposts and composts.

We have found that, just like composts, vermicomposts have the potential for improving plant growth when added to soil or container media. Furthermore, it appears that there may be important differences between specific vermicomposts and composts: both in the nature of their microbial communities, and in their effects on plant growth. From the studies that we have described here, and from others that we have conducted, it is apparent that the pig solids vermicompost we tested consistently outperformed the other vermicomposts and composts. We are still attempting to identify the biological mechanisms responsible for the consistent performance of this material, as well as for the unique and remarkable plant growth responses that continue to be widely observed and reported for other vermicomposts and earthworm castings.

Subler and Edwards are with the Soil Ecology Laboratory, Columbus, Ohio. Metzger is in the Department of Horticulture and Crop Science at the Ohio State University, Columbus, Ohio.

JULY 1998 BIOCYCLE pages 63-66