Vermicompost – Is It Really That Great?

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Robert Pavlis

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!, source: Toby Hudson

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.

Plant Science for Gardeners by Robert Pavlis
very long bins contain brown material
Commercial Vermicomposting, source: Agri-Farming

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 as well as my book Composting Science for Gardeners.

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 and other forms of organic matter.

Re-composting Inside the Worms

I was speaking to Rhonda Sherman, (affiliate link) a world expert on vermicompost and she mentioned that after casts are first produced, worms tend to eat and digest the poop material an average of 5 times. It sounds surprising, but remember that manure is a common feed source for commercial and agricultural vermicomposting. Worms love eating shit!

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.

NPK 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”. 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 you find that the pH, and nutrient levels are quite similar. Actual values depend very much on the input ingredients. Here are some of the values I found.

0.85%N, 0.62%P, 0.82%K (backyard vermicompost)

1.94%N, 0.47%P, 0.70%K (New Mexico State University)

0.85%N, 0.15%P, 0%K (kitchen waste)

2.11%N, 1.22%P, 1.45%K (vegetable market waste)

1.5% – 2.2% N, 1.8% – 2.2% P, 1.0% – 1.5% K (the story of organic gold: A review)

The average of these is 1.5%N, 0.9%P and 0.8%K, which converts to an NPK of 1.5-2-1

Average NPK values:

– bin compost is 1.4-1-1.3

– vermicompost is 1.5-2-1

Vermicompost compared to bin compost, has higher levels phosphorus and calcium. 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.

Getting Started

If you are interested in giving this method a try, start with a vermicomposting system like the Worm Factory 360 (affiliate link).

Vermicompost vs Traditional Bin Compost

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


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 looked at 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. 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. Measurement of CO2 production is higher in vermicomposting than conventional aerobic composting, also indicating that the end product is more decomposed.

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. This study compared the two for growing lettuce in lab conditions and found little difference.

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.


More Composting Information

What is the Best Composting Ratio for Browns and Greens?

27 Composting Myths That Will Save You Time and Money

Bokashi vs Composting

Composting – Which Method is Best?

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Robert Pavlis

I have been gardening my whole life and have a science background. 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!

57 thoughts on “Vermicompost – Is It Really That Great?”

  1. Why the article says NPK value of vermicompost is 1.8-3.8-1.3 when a linked SCI articles itself says 2.11, 1.22 and 1.45 for Kitchen waste feed?

    Secondly regarding the statement that there’s no evidence that added microbes benefit the soil, the mere fact that it keeps off most pests it a good reason which you have already mentioned. But I will give an article link:
    Refer the same for the list of some microorganisms in vermicompost and the benefits of each of those strains (some strains add nutritive value as well)

    • I’ll have to check where the NPK value comes from.

      For the second point. “the mere fact that it keeps off most pests” – not sure what that means. Microbes in soil does not “keep off most pests”?

      There is a lot of research looking at the effect of adding microbes to soil, and so far few have shown any consistent benefits.

    • I checked the numbers. I am fairly sure the numbers were an average from several articles but I no longer know which ones.

      So I redid the numbers and also included some new articles I found with data in them. I also listed each one so you can see the data more clearly.

  2. Very interesting read. Thank you. My vermiculture is in good working order. My query – since the compost is very nitrogen rich, will it not simply generate leave propduction and not fuit or flowers? I have found that. Many thanks.

    • It is all about dose. How much nitrogen is the vermicompost adding at any given point in time, how much nitrogen is in the soil, how much nitrogen is the plant using. A high excess can lead to more green growth.
      Remember that the “high” nitrogen in any organic matter is released slowly over time, so it does not mean a lot is made available to the plant at one time.

  3. Good read, thanks.
    A couple of observations.
    Traditional bin compositing also generates nitrous oxide and methane.
    A bit more emphasis on speed, at the outset of the article, might be helpful. If you start a traditional composting bin, you’ll probably need to wait a year or so before you start using the compost. With a vermicomposting bin, 2-3 months is enough to start having ‘usable’ compost (yes the term is elastic and nebulous, but still).
    Also, physical effort is worth mentioning, to be exhaustive. Not everyone will take well to the collecting of leaves and trimmings, nor to the regular work of turning. Worm composting can be less physically arduous. With vertical migration bins, there is no separating of worms and compost to be done, and the only real work is shuffling the trays after harvesting – hardly worth mentioning.

  4. Really appreciate your article and approach to discussion. Related, I added my first vermicompost product to raised garden bed this spring and early results are terrible compared to bed amended with chicken manure at same time. Thus I am searching for what is wrong with my supposed black gold. Your article may be the answer.

  5. I just set up my worm composter and got my worms via the post office. The worms showed up dead. A couple were moving, yet by far most were simply not moving and were in a major mass. They additionally smelled truly unpleasant. I put them in the composter in any case. Would it be a good idea for me to have done that or toss them out?


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