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Vermicompost – Is It Really That Great?

Vermicomposting is a popular way of getting rid of kitchen scraps and proponents of this form of composting make all kinds of claims. Does vermicomposting really work? Does it produce a superior compost when compared to traditional bin composting or Bokashi? What do the worms actually produce and how does it compare to traditional compost?

This post will have a close look at vermicomposting and compare it to bin composting.

Vermicomposting worm bin - the worms are escaping!

Vermicomposting worm bin – the worms are escaping!

What is Vermicompost?

Vermicomposting” can be defined as the biological breakdown (decomposition) of organic wastes, via the joint action of (specialized) earthworms and microorganisms. It is a term that can be used interchangeably with worm composting.

Vermicompost is the materiel left behind after worms eat and digest kitchen scraps and other types of organic matter. It is aged worm poop, also referred to as worm castings.

This type of composting is a combination of digestion by the worms as well as a decomposition process that takes place after the material exits the worm. Worm poop sits for a period of time where bacteria and other microbes continue the decomposition process. The result is a black humus-like substance that, superficially at least, resembles the material produced by bin composting.

Commercial Vermicomposting

Commercial Vermicomposting

There are of course many recipes, but the basic formula for the home owner goes something like this. Take a plastic bin, add some shredded paper, food scraps and the right kind of worms. Let the worms do their thing as you continue to add more food scraps. Over many months the organic matter is converted into vermicompost. This process is well described in Composting With Worms (ref 7).

Although this post will focus more on home composting which is usually done in a small bin, it should be noted that vermicomposting can also be done on a larger scale.

Claimed Benefits Of Vermicompost

The following are some of the claims attributed to vermicomposting.

Claims That Are True

  • Easily done inside the home
  • Clean, odor free – true only if the process is well managed
  • Added to soil it increases the organic content and adds nutrients
  • Adds microbes to the soil – this is true, but are there any benefits to adding them?
  • Improves soil porosity – true for any added organic matter
  • Improves moisture holding capacity of soil – true for any added organic matter
  • Provides slow release nutrients – true for any type of compost
  • Will not burn like synthetic fertilizers – true, but then synthetic fertilizers only burn plants if used incorrectly

Claims That May Be True

  • Improves plant vigor – like any type of compost, if it is added to soil deficient of nutrients it will help plants grow. But if it is added to healthy soil that has a good organic content, it might actually make soil toxic to plants because of nutrient overloading. Vermicopsting is no different than other forms of compost in this regard.
  • Takes up less space than traditional bin composting – true, the worm bin takes up less space but it also produces less compost. It does allow you to compost small amounts of material. To compost larger amounts of material, both methods would take up the same amount of space.
  • Produces nutrient-rich compost – like all compost it does not contain high levels of nutrients.
  • Helps your plants resist/overcome attack from plant pathogens and pest organisms – like all composts healthier plants resists pests better. But too much compost results in weak plants that are more prone to pests.

Claims That Are Not True

  • Stimulates root growth – nutrients do not stimulate root growth. In fact the lack of nutrients stimulates root growth more because the plant needs to produce longer roots to find the nutrients.
  • Adds beneficial microbes to the soil – there is no evidence that the added microbes benefit the soil and soil already contains lots of beneficial microbes.

Summary of Claims

If you read through the list of claims you quickly realize that the claims are either false, or that they equally apply to most forms of compost. That surprised me. I expected to see a bunch of vermicompost specific benefits but did not find any, except for the fact that it is easily done in small batches in the home.

Vermicomposting – What Happens Inside the Worm?

Worms eat soil and organic matter which travels down a long digestive tube that consists of several key sections. The esophagus adds calcium carbonate as a way for the worm to rid itself of excess calcium. The food then moves on through the crop and into the gizzard. The gizzard uses swallowed stones to mash food into small particles. Enzymes are added to help digestion. The material then moves into the intestine where fluids are added to further digest the food. Similar to our own intestine, it absorbs nutrients that are needed by the worm.

This all sounds quite normal for an animal digestion system but there is one other key ingredient, microbes. The worm controls moisture and pH levels to favor the growth of microbial populations. These microbes play a major role in the digestion of organic matter.

Along with the soil and organic matter, worms also ingest large amounts of microbes. In fact, the microbes are their primary source of food, not the organic matter.

The whole digestive system is not very efficient and only 5-10 percent of the ingested food is absorbed by the worm. The rest is excreted as mucus coated particles called vermicasts or worm castings. The worm casts contain undigested plant material, nutrients, soil and a large amount of microbes. The microbial activity in worm casts is ten to twenty times higher than it is in soil (ref 1).

Vermicomposting – What Happens Outside The Worm?

The worm is not really doing much composting. It does some digestion, but its main contribution to the process is that it breaks organic matter into small pieces and mixes it with microbes. A point that is not emphasized enough is the fact that much of the composting process takes place after the casts exit the worm. This external processing may in fact be the most significant part of the composting process.

I can only assume that in a worm compost bin, the casts get re-eaten by other worms?

In a commercial setting, which is maximized for productivity, usable vermicompost is ready in as little as 6 weeks. In worm bins receiving minimal management (most home systems), vermicompost is ready in 4 to 6 months. It is not quite clear how the term usable is defined.

Chemical Analysis of Vermicompost

“Earthworm castings in the home garden often contain 5 to 11 times more nitrogen, phosphorous, and potassium than the surrounding soil. Secretions in the intestinal tracts of earthworms, along with soil passing through the earthworms, make nutrients more concentrated” (ref 3). Although this quote refers to earthworms in the garden similar statements can be found for vemicomposting. These types of statements lead one to believe that the product is very high in nutrients, but keep in mind that the process does not create matter. These same nutrients would be added to soil if the un-composted organic matter was added directly to soil.

When you compare the nutrient levels of vermicompost to traditional compost (ref 3) you find that the pH, and nutrient levels are quite similar. Keep in mind that actual values depend very much on the input ingredients.

Average NPK values:

– bin compost is 1.4-1-1.3

– vermicompost is 1.8-3.8-1.3

For the same input material, vermicompost compared to bin compost, will have higher levels of nitrogen, phosphorus and calcium. The higher nitrogen can be beneficial, but the higher phosphorus levels can create problems in garden. High levels are toxic to plants and mycorrhizal fungi. Most gardens have enough phosphorus and many that have been heavily fertilized with either organic or synthetic products, including compost, are showing high P levels. The higher calcium levels will be of little benefit for most gardens since most soil has enough calcium.

From a chemical analysis point of view compost is better for the garden unless the soil is low in phosphorus or calcium. The benefit of a small amount of extra nitrogen would not outweigh the potential harm of excess phosphorus.

Vermicompost vs Traditional Bin Compost

In addition to the chemical differences discussed above, the following sections discuss other differences between the two processes.

Temperature

Conventional composting relies on the right ratio of carbon to nitrogen and the action of microbes to carry out the degradation process. When these conditions are right, the process produces a lot of heat, hence the term hot composting.

In vermicomposting, the process is carried out at cooler temperatures by the action of the worms and the microbes.

Nitrogen Levels

A hot environment releases more nitrogen into the air than a cooler process. Consequently, vermicomposting  usually results in a compost with higher nitrogen levels. Since the nutrient most likely to be deficient in any soil is nitrogen, vermicomposting has an advantage.

Killing of Seed

Hot composting kills weed seed, while cold composting such as vermicomposting does not. However, in most home systems, weed seeds are not introduced as food for the worms, so this may not be a major issue. It  does become a more important consideration in large scale production of vermicompost when it uses input ingredients that contained weeds. This is more likely the case for purchased vermicompost.

Killing of Pathogens

Any composting process will result in a decrease of pathogens, but that decrease happens faster in a hot environment. Vermicomposting happens cold, so one would expect it not to be effective in removing pathogens, but that is not the case.

A recent study (ref 4) looked the change in microbes after ingestion by four different kinds of earthworms and found no nematodes, a significant decrease in the number of coliforms, and a decrease in the number of protozoa. Other studies have found that although earthworms ingest a lot of microbes, the population that exits the worms is quite different than the one that enters them. This is all new science, but the worm digestive system seems to reduce pathogens, at least to some extent.

There are also studies that show vermicasts introduced plant pathogen when mixed with soilless mixes, killing seedlings (ref 6). Plants in the garden have to deal with plant pathogens anyway, but house plants and seedlings started indoors are exposed to a lessor degree. It is probably best not to use worm castings on indoor plants.

Speed of Decomposition

Conventional hot composting is a relatively fast process which is followed by a slower process in which the compost goes through a finishing step. Vermicomposting also consists of two steps, one inside the worm, followed by a slow maturation process.  Compared to hot composting it is a slower process.

However, it is difficult to compare the so-called finished compost from the two processes. At best, some gross level tests can be done to measure the degree of completeness, but these do not give an accurate comparison.  When the two processes were compared using cattle manure the final C:N ratio was lower for vermicomposting, indicating that it was a more complete process (ref 5).

The other thing to keep in mind is that in the home garden, conventional composting rarely gets hot enough and therefore bin composting is a much slower process. For home gardeners it is impossible to know which process is faster – you can’t ‘see’ the completeness of the process.

So-called finished compost from either source is still a long way from being completely decomposed- which is good news since it means they will feed the garden for years.

Starting C:N Ratio

Conventional hot composting depends very much on having the right C:N ratio of 30:1. The starting C:N ratio is not as critical for vermicomposting, making it easier for home owners to use.

Plant Growth

All that is nice science but which compost results in better plant growth? Some studies show one or the other as superior, but the consensus is that neither compost produces better plant growth than the other. A lot depends on the specific conditions of soil, technologies used and input ingredients.

It is incorrect to say that vermicompost produces better compost unless your soil needs more phosphorus and calcium. The high phosphorus levels actually mean that for most soils bin compost would be a better choice.

Production of Greenhouse Gases

Every composting process produces some greenhouse gases, like CO2. Because vermicomposting happens at least partially inside the worm, it is an anaerobic process which produces both nitrous oxide and methane. Both of these gases are many times more harmful to the environment, as far as global warming goes, than CO2.

From a global warming perspective, traditional bin composting may be a more environmentally sound process. I say ‘may be’ because such calculations are very hard to do, and I doubt anyone has done them.

Vermicompost Tea

You guessed it – worms also make tea and proponents of this process think this stuff is great. If the input ingredients in a vermicompost bin get too wet, the excess liquid collects at the bottom and this is called vermicompost tea, worm tea, castings tea, worm compost tea or leachate.

Some people will argue that it should not be called tea because the castings were not separated and brewed as a tea. Some go on to say it needs to be brewed aerobically. I have discussed compost tea before and since chemically and microbially, compost and vermicompost are relatively similar the same arguments against the benefits of tea would also apply to vermicomposted anaerobic tea.

The leachate can contain more nutrients than the compost itself, but since this is usually not aerated, it can also contain plant and animal pathogens as well as other phytotoxic chemicals. It is probably fine if added to garden soil, but can harm plants if it is sprayed onto leaves. Leachate does not have magical powers anymore than other types of compost tea.

Pros And Cons of Vermicomposting

Vermicompost is much like other kinds of compost. There is no scientific evidence to show that it is a superior product, as so many claim. On the other hand it is a very suitable organic source to add to gardens. Like all organic sources it will provide some immediate release of nutrients, and then go on to provide more nutrients long term. It will also improve soil structure since it adds organic matter, but too much organic matter can also be a problem.

Pros of Vermicomposting

  • Easily done in small batches.
  • Works during cold weather if done indoors.
  • Less nitrogen loss.

Cons of Vermicomposting

  • You have to buy and deal with the worms – do you need another pet?
  • Worms have to be separated from the compost before it is added to the garden.
  • Not easily managed by home owners for large amounts of yard waste.
  • Does not kill weed seeds.
  • Produces nitrous oxide and methane – harmful greenhouse gases.
  • Leachate may spread plant pathogens.

 

References:

  1. Vermicomposting – The Story of Organic Gold; http://file.scirp.org/Html/4-3000330_24396.htm#ref39
  2. Bacteria And Earthworms; https://microbewiki.kenyon.edu/index.php/Bacteria_and_earthworms
  3. Vermicomposting; http://aces.nmsu.edu/pubs/_h/H164/
  4. Microbe Changes in The Gut of Earthworms; http://www.sciencedirect.com/science/article/pii/S0929139307001783
  5. Comparing Composting And Vermicomposting; http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.457.2951&rep=rep1&type=pdf
  6. Vegetable Seedling Diseases Associated With Earthworm Castings; http://www.plantmanagementnetwork.org/pub/php/research/2012/earthworm/
  7. Composting With Worms; http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/23949/em9034.pdf
  8. Some opinions of researchers in the field; https://www.researchgate.net/post/Vermi-compost_vs_thermo-compost_which_one_is_better_as_a_soil_amendment
  9. Image source – main; Toby Hudson
  10. Image source – commercial vermicomposting; Agri-Farming

 

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Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

22 Responses to 'Vermicompost – Is It Really That Great?'

    • Interesting link. Unfortunately the article is behind a paywall.

      The soil compost mixtures were made up based on volume and there is no indication that they were normalized for nutrient content. It is not clear how easy the results can be interpreted. The abstract says “Results showed that MSW vermicomposts consistently outperformed equivalent quantities of composts in terms of fruit yield, shoot, and root dry weights, which can be attributed to the contributions of physicochemical properties and nutrients content (N, P, and K) in the potting experiments. Consequently, it seemed likely that MSW vermicompost provided other biological inputs such as plant growth regulators (PGRs) and plant growth hormones (PGHs), which could have a considerably positive effect on the growth and yields of P. vulgaris as compared to composts”

      It says the differences can be attributed to “physicochemical properties and nutrients content (N, P, and K)” and then goes on to speculate (ie “seemed likely”) that growth regulators might be at play.

      Seems to me that they are not sure what caused the differences.

  1. John Law says:

    Worm compost is often blended with other organic material that may have a lot of silt sized particles. This can reducing drainage as the organic matter decays. A common example in CA in rice hulls.

    Always important to distinguish between permanent landscape that will not have regular additions of amendment and gardens that are tilled. Plant exudates and root debris will dwarf any exogenous OM additions on a permanent landscape

  2. Hildegunde Tasner says:

    Mr. Pavlis, My post should be eliminated since I realized only after posting it that I had missed the subject completely, I.e., worm composting. I was induced to write because of your comment that you throw scraps on frozen compost which I do as well. I just want to tell you here briefly how much I enjoy your articles (and videos), how much common sense you have, and how much de-mystification you employ on a subject where people often make things more complicated than they are. Best wishes for you and thank you.

  3. Mark leighs says:

    Last year I had 3 types of compost processing going at once , bin, vermicomposting, and my usual compost heap.the vermicomposting looked great but I’d added rotten tomatoes which I would normally add to my heap , unfortunately when I used the vermi compost around plants outdoors I had tomatoes seedlings growing out of it. So I made an aerated tea out of it ! Not worth the mither no fantastic results at all. My overall winner was my old compost heap which I keep well turned and produces every 6 months . my worm bin has now been put to better use and the worms put in my compost heap where they should be .

  4. Daryle in VT says:

    Vermicomposting does make it easier to find the worms when going fishing …

  5. Patrick Dolan says:

    Hi Robert. My understanding is that the claim about increased root growth has to do with the presence of higher levels plant hormones in vermicompost (e.g., ndole 3-acetic acid, Gibberellic acid, Kinetin) compared to traditional compost. Claims about plant hormones in vermicompost would make a great part 2 on this topic. Thanks!

    • If the hormones are in vermicompost, is there evidence that they increase root growth in soil?

      • Mary Ann Smith says:

        “Substitution of humic acids into soil-less MM360 at
        rates ranging from 250 to 1000 mg kg−1 planting
        medium (MM360) (Fig. 2A–C) increased root dry
        weights of marigolds, peppers and number of fruits of
        strawberries significantly (P ≤ 0.05)”
        You can read the entire study at: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.486.2522&rep=rep1&type=pdf
        Actually there are numerous studies by Norman Arancon on vermicompost and vermicompost tea. You would change some of your claims if you studied them thoroughly.

        • From the study “Experiments were designed to evaluate the effects of humic acids extracted from
          vermicompost and compare them with the action of commercial humic acid in combination with a commercial plant growth
          hormone, indole acetic acid (IAA) which is a commonly found in vermicomposts.” – the study did not look at using vermicompost directly, nor did it compare to regular compost. And all work was done in pots in the greenhouse, using soil-less mix.

          We can’t assume the same results would be found in our gardens, even if gardeners decide to extract the humus – which they don’t.

          Did a quick look at the study. When food waste vermicompost was tested, they found that the humic acid extracted from 100g of vermicompost had no statistical effect on the number of fruits in strawberries. When 200g was used, the number of fruits doubled. 200g per Kg of soil is quite a lot of compost and much more than any home gardener could possibly produce for their garden.

          Problem is we don’t know what effect this has on real soil or how it compares to regular compost.

          This study changes none of the claims in my post.

      • Patrick Dolan says:

        I’m not sure if the claims are well supported, and was suggesting this as a possible part 2 on vermicompost (if you’re so inclined).

  6. Jackie Burkey says:

    Worm-composting is fantastic for those of us in climates where our bins freeze solid for about 6 months of the year. Much of my kitchen scraps that would have gone into the landfill are now converted into something useful.
    I would question your notion that the inside of a worm is anaerobic. Most biological processes require oxygen and I assume the breakdowns going on in the digestive tract are no different.
    Finally, don’t worry about taking care of another pet. You can ignore a worm bin for months and still have plenty of composting going on. Yes, they eat and re-eat the castings. In fact, the more times they are “processed” the more beautiful the final product! I thought my initial worms had died so I didn’t feed or check on them for months – and was shocked by the wonderful compost, and now, 7 years later, their descendents are working 2 bins for me and countless bins for others I have given or sold worms to.
    I don’t make exotic claims for the castings. Just that they turn kitchen scraps into useable compost – and that’s all good!

    • If you have some references that say the digestion inside a worm is aerobic, please post them.

      I just put my kitchen scraps on the frozen compost bin in winter. I think that is easier.

      • Hildegunde Tasner says:

        We never used compost bins – unheard of in my youth in 1950’s in Upper Bavaria. My father’s compost pile in our backyard was massive, many meters long, fairly high. Everything went on there: ratios were unknown (or unobserved), the pile was never turned (would have taken 3 men to do so anyway – exaggeration). This had gone on for generations in our family backyard. Father: excellent gardener; compost of a quality I have not encountered since. AND – no one ever did a bloody thing except throw stuff on there. I am copying that and yes – I throw my scraps and paper on it even in the snow. Works for me.

  7. North of the limits of glaciation, Earthworms are invasive aliens – but while some species or other is probably already in any plot called a garden, an additional caution would be that you don’t want to add invasive species which are new to your area. And in my experience, the benefits of vermicomposting are mainly whooped by those who are selling worms or bins to do the composting in

  8. Patrick says:

    Any thoughts on how compost from soldier fly grubs stacks up versus worm composting? My backyard bin has been a roiling mass of soldier fly grubs the last two summers, and they break my kitchen waste down amazingly fast.

  9. Lisa says:

    Excellent analysis

  10. I built an indoor worm bin and I like having it for a few reasons.

    1) I can compost all my food scraps indoors, away from pests that would get to it in my outdoor compost.
    2) I’m in zone 4 so outdoor compost comes to a complete halt in the winter, so being able to make more inside is a plus.
    3) I really just think it’s cool to see the worms do their thing, I know that might not be a great reason but part of the reason we garden is to enjoy it too!

    But one claim I hear a lot that I didn’t see in this article is humic acid in worm castings, and the benefits it has for starting seeds. This video takes a pretty level-headed, science based approach to that questions https://www.youtube.com/watch?v=ORu_mdB7cng but I’d be curious if you found any info on that subject.

    Great post as always!

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