Vermicomposting 101: What Happens In A Worm Bin?

New vermicomposters worry that their two-week old bin isn’t producing castings and that they may be doing it wrong. Things look fine and smell fine, but the worms aren’t “destroying” the food like we think they should.

If most beginners think of vermicomposting the way I did, then they think of a worm bin like salad with distinct ingredients each serving a distinct purpose with vermicomposting seen as a linear process where worms eat waste and then poop out worm castings.

And once there are enough worm castings, you simply harvest them by separating the castings from the rest of the bin.

End of story.

But a far better analogy for vermicomposting is that of a soup.

Yes, you certainly start with distinct ingredients like bedding, food, and the worms of course, but what you’re actually doing is creating a soup or a stew using a much more circular process that builds upon itself.

And what you end up with is a brew of bedding, food, and worms, but also worm castings, microbes, plant growth hormone, humus and other organic matter that didn’t exist when you started your bin.

Note: This article is part of a “Vermicomposting 101” series of posts aimed at helping the beginning vermicomposter. The read other “VC101” articles on how to start a worm bin, how to choose worm food, how to maintain moisture, and the differences between composting and vermicomposting, please visit the Vermicomposting 101 section of this site!

How the Vermicompost “Soup” Is Forming

In a nutshell, here is a decent beginner-level description of what is happening in your bin.

Your carbon-rich bedding and nitrogen-rich bedding are both breaking down, but at different rates. In this regard, it’s helpful to think of bedding as simply a different kind of food.

What is helping to decompose this material? Microbes…..pretty much only bacteria in a new worm bin.

Microbes: Where the Vermicomposting Process Actually Begins

In the decomposition process, more and more of the surface area of the organic matter is being exposed to oxygen and to microbes. In response, the microbe population, aka the number of mouths feeding on the food waste, expands.

An increased microbe population results in increased surface area, more food is now available for our microbes, which multiply in response to a food source that can sustain them. This further accelerates the decomposition in our worm bin.

This is what happens in an actual hot composting pile as the mesophiles, or mesophilic microbes, do most of the decomposing in the beginning before giving way to heat loving thermophilic microbes or thermophiles.

But because a properly-maintained worm bin is fed in small quantities (often thin layers), the thermophiles can never bloom because the organic waste is not piled deep enough to trap the heat needed to keep those heat-loving microbes happy.

Now you’ll notice that we haven’t even started talking about worms yet.

Enter the Worms

By the time that the worms are entering the picture, there should be a thriving microbe population which is why we recommend taking plenty of time to prepare a worm bin before you introduce worms. And what the worms are actually consuming are microbes that they are hoovering up off the surface of the decomposing waste. Yes, a little food itself enters the worms’ mouths but they’re also eating the microbes.

As these microbes travel through the worms’ digestive system and are exposed to the flora in the worms’ gut, their populations continue expand and condense, ultimately forming a bacterial mucus around the digested organic matter that will exit the worm in the form of worm castings.

Give what we know about microbes – that they consume organic matter and are themselves consumed by worms – what effect do you suppose these worm castings have on organic matter?

The castings themselves will promote the decay of organic matter and itself become food for the worms.

Yes, the worms eat their own poop!

And it’s glorious, not awful like when your cat does it.

Assuming the temperature, moisture, and pH remain within acceptable ranges, this virtuous cycle will continue until there is no organic matter left for the microbes to consume. When the microbes run out of food, their population will begin to decline.

This is why if you plan to harvest worm castings and store them for several weeks or months, it’s a good idea to add an organic food source like leaf mold or some other organic material for the microbes to munch on.

But the lack of a visible food source doesn’t mean the microbes are already starving. Plenty of anecdotal evidence exists of microbe populations continuing to thrive well after a worm castings harvest, even in less-than-optimal storage conditions.

Adding leaf mold on top of (or in) your worm castings is an excellent way to keep the microbes in your worm castings fed and thriving!

Back to the Soup Vs. Salad Analogy

The time it takes you to make a salad equals the time it takes you to throw the ingredients together.

But a soup will take longer as the ingredients all need time (and normally heat of course) to interact with one another to create a concoction via more slowly-developing chemical and physical changes.

This is why a new worm bin owner is disappointed that their 2-week old worm bin isn’t breaking food down quickly enough. That bin is still a salad. Even with good conditions, there haven’t been enough of these cycles to produce a rocking and rolling ecosystem.

Wrapping It Up: Is Your Bin a Soup or a Salad?

There is no magic point at which your bin turns from salad to soup. And I would even argue that in less-than-optimal conditions, your bin may become a salad on top of the soup below as the microbes and worms are in survival mode more than thrive mode. But this new material will be better absorbed into the soup because of the community of microbes below.

Well I hope this has been helpful, especially for you beginners who may be initially discouraged by the lack of apparent progress in your bins. Again….if things look fine, smell fine, and your worms aren’t trying to escape en masse from your worm bin, things probably are just fine!

Let me know if you think your bin is a soup or a salad, or if you think I’m smoking crack by even making this comparison!

22 thoughts on “Vermicomposting 101: What Happens In A Worm Bin?

  1. Steve, you make this comment above “I would even argue that in less-than-optimal conditions, your bin may become a salad on top of the soup below as the microbes and worms are in survival mode more than thrive mode”.
    Can you elaborate on how to avoid this, and/or what can be done to rectify it?
    To answer your question, I like to think of my worm bin/bag a crock-pot… it is kind of a slow simmer that breaks down the food wastes faster than a cool system, but slower than a fully thermophilic compost pile. I get some heating in the center, to about the 100-115 range at times, and then as it cools the worms move in droves.

    1. Hi Dan,
      Thanks for this. My wording didn’t actually communicate what I meant to say. I have edited for clarity to say that, keeping the analogy going here, is that your salad is far better off by being in close proximity to the microbe soup. The decomposition will happen much faster because of the existing microbe population.

      And can you describe your setup? Is this outdoors or in a very large system?

      1. Hi Steve,
        Thanks for clarifying!
        I have a Worm Inn Mega. Inside, semi-heated basement. Bedding is shredded cardboard and coir. We eat a lot of fresh veg, so lots of material goes in. I also tend to stir in the top 8-10” every other week or so so get a lot of C to N contact and air added. This yields my minor heating “events” for a short period and breaks down the food waste faster… the crock pot effect ?

  2. I will say the soup or salad analogy you use is a great comparison. The most ideal worm friendly environment is where the food waste material has been pre-composted for the worms. You avoid the dangers of overheating and anaerobic conditions. All the same I am having success making a food waste “sandwich” in my Mega-Inns. After removing the dry shredded cardboard from the surface I will extract about a gallon of the processed top material, worms and all. I then add my food waste w/shredded cardboard. I then add the material I removed completing the “sandwich”. I have microbes on both the top and bottom of the food waste to expedite processing. More food waste surface area exposed to microbes. At that point I may drizzle on a bit of water if the food waste isn’t water rich and cover with the dry shredded cardboard. It doesn’t seem to have any negative effects on the worm population and the food waste is quickly processed. I am doing this process bi-monthly and it seems to be worth the effort.

    1. Thanks Steve! And yes, precomposting is a wonderful idea. Pretty much a necessity for anything larger than a home worm bin. I like your idea! Seems like a labor of love! But you can’t quibble if you’re getting results!

  3. Great analogy Steve. One thing that I’ve noticed in my bins are tiny, individual castings on top of a cabbage leaf/melon rind, etc.; almost like I’ve sprinkled it with pepper. If I see those, I know that the worms are feeding, even if otherwise it seems like nothing is happening. Another good point you make is that if the worms aren’t trying to escape, the bin is in reasonably good shape. Actually, this is probably the best indicator that you’re on the right track. Earthworms haven’t survived for untold millennia by hanging out in toxic environments, so if they’re staying put, the bin’s OK. If nothing else, vermiculture will teach you patience.

  4. Colour me skeptical on the notion that only “a little” of the original food material enters the worm’s mouth. How would that even work? Surely the differences in scale would make this very difficult. Everyone likes to remark that worms don’t have teeth to bite off chunks of solid food, but they also don’t have fingers to separate the micro-organisms from the softened parent material.

    I don’t doubt that the worms get most of their nutrition from micro-organisms, but I find it hard to believe they don’t ingest a large portion of the original, decaying material. The presence of the gizzard in the worm’s intestinal tract would seem to add further credence to this notion.

    Last summer I kept a worm bin in my back yard which I fed only aged dog sh*t and leaves from the previous fall. I was amazed at the speed which the worms transformed the disgusting parent material into a neutral smelling, friable substance. The only way I can credit this seemingly complete and rapid transformation is that the original soft fecal material passed through the gut of the worm and became coated with that microbial mucus you spoke of before being expelled as castings.

    Of course, I’m no scientist and could be completely off base. If you have sources for further reading I could do, or, if there are actual scientists among us, I would be very interested in learning more.

    BTW, I love my UWB!

    1. Hey Dean! Firstly, thanks for the kind words about the Urban Worm Bag! And you may be right about the food vs microbe question and it is a source of debate in some of the Facebook groups I’m a part of. But I see it a little like this….. the initial decomposition of food takes place with mesophilic microbes….mostly bacteria which is covering the food waste. By definition, the worms must eat the layer microbes before they get to the food waste and those microbes are much more likely be sucked into the worm’s mouth than the much larger food particles.

      1. I mean it– I love that thing. I’m seriously thinking about buying another (haven’t told my wife yet).

        Facebook groups? You mean there are other nerds on the internet who ponder these sorts of things?

    2. im not sure if this is helpful full when I was much younger and collected night crawlers there were times I caught them with pieces of grass hanging out of their mouths

      1. Those are not composting worms. Anecic worms like nightcrawlerscan take whole leaves back into their burrows with them. Food needs to be prepared through bacterial decomposition before composting worms can really have a go at the food!

  5. Hi I am not sure of the function of bedding. Should the bedding be edible and eaten? If not, isnt it just buried beneath casings? I shred newspaper and then saturate it with cooked shredded food, (rice, oatmeal, amaranth, flour, brocolli, coffee grounds, egg shells, sand, and more).. Edible bedding. Is bedding needed other than food? I also have cardboard but only at first. Thanks

    1. The bedding for worms, unlike bedding for most other animals, will be eaten. I call bedding a “slow food” as it takes awhile to break down, unlike regular food waste. And cardboard is a great bedding/food, especially the corrugated stuff as the worms really like the proteins in the glues that hold together the corrugations.

  6. I am setting up a worm bin in my classroom for the first time. It has been a very slow start as we started into winter about the time our bin was ready to have worms. I haven’t had a worm been at home so I am learning on the fly. The bin has been in a good place for about 2 months now, it smells like soil, the worms are staying put. I feed it about once a week and then cover it with a little more substrate to keep the fruit fly populations down.

    I keep looking up more information. Your soup and salad analogy was very helpful. I know I set the bin back a ways today because I wanted to let my students examine what was happening. We were careful to keep the worms moist as we counted mature and immature worms, found cocoons and looked for castings. We found very few castings.but lots of healthy worms and the kids were really excited. I came looking for the reason for the lack of castings. Thanks for helping with my education! I will pass it on.

    1. Hi Heather, I think your bin is still in the “ramp up” period if it’s only two months old. When you start a new bin, everything is kind of sterile; you don’t have a huge population of bacteria or worms yet. I think you’ll see a big difference in about 2 months!

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