How to Measure Moisture Content in Compost and Vermicompost

I just stumbled across two ways to measure the  moisture content in compost and vermicompost. Granted, these methods, especially the first one, would measure the moisture in nearly ANY material, but I am super thrilled that I found this information.

I have been on the moisture warpath lately as I feel most new vermicomposters tend to run their bins way too wet

In a system like the Urban Worm Bag – even the Version 2 with the interior fabric liner – a gross amount of excess moisture can still foul up the zipper if the moisture makes its way through the bottom of the fabric liner and up into the zipper area where it can calcify. I really want to prevent this through education until I can adapt my product to handle this leachate.

pore space

In all bins, excess moisture can create anaerobic conditions, which destroys much of the goodness of vermicompost. The pore space in compost allows for aerobic conditions. If this air is displaced by excess moisture, then anaerobic conditions can result as the microbes in the compost consume the oxygen in the moisture.

And the compaction of the wet compost will prevent it from being reoxygenated. That’s a problem.

Now, these methods were under my nose for quite awhile now. As many of you already know, I purchased the plans and operating instructions for O2 Compost’s Micro-Bin aerated static pile system from Peter Moon. Peter included an appendix to his plans that explains how to measure moisture content. The first one is accurate and elegantly simple while the second requires a bit for feel but is cheaper and still gets you in the ballpark for estimating moisture.

With Peter’s blessing, I am publishing these methods.

So let’s cover both methods below.

Method #1: Lab-Worthy Moisture Test Results in Your Kitchen

Just follow the steps below to measure the moisture content in your worm bedding, vermicompost, or compost.

  1. Weigh an ovenproof container to determine its tare weight.
  2. Place a small sample of your bedding or vermicompost in the container and weigh it.
  3. Subtract the tare weight to determine the wet weight of the sample.
  4. Put the sample and container in an oven at 220°F and let it “cook” for 24 hours to completely dry it out.
  5. Weigh the dried sample and subtract the tare weight. This is your dry weight.
  6. Determine the moisture content of the wet sample using the following equation.

((Wet Weight — Dry Weight)/Wet Weight)) x 100 = Moisture Content

I failed high school algebra (seriously), so let me put this in plain English. Take the wet weight and subtract the dry weight. Then divide that difference by the wet weight. This will give you a number far less than 1. Then multiply that number by 100 to find the moisture content in percentage terms.

 Method #2: Hand-Squeeze Test

Pretty much everyone who has read the basics of vermicomposting has heard that their vermicompost should yield a drop of water when squeezed.

But what exactly does that mean?

Thankfully, Peter has quantified what are qualitative moisture tests. The following tests are simply rules of thumb and rely on your own observations about what you feel. I’ll add that it also depends on how hard you are squeezing the vermicompost.

So here goes, according to Peter Moon.

Procedures for the Hand-Squeeze Test

  1. Reach into your bucket or bin and grab a handful of vermicompost. It should be a representative sample of the majority of your bin.
  2. Squeeze the material very tightly and check for drops of water.
  3. Release your grip and allow the moisture to stay in your hand. Rub some compost between your thumb and finger.
  4. Inspect the material and your hand .
  5. Use the rules of thumb below for estimating moisture content.

Material feels dry and dusty: Less than 42%

Material feels mostly dry with a hint of moisture: 42-47%

Material feels tacky and sticks together: 47-52%

Material feels moist, but no water comes out when squeezed: 52-58%

Material leaves a wet sheen on hand: 58-63%

One or two drops of water comes out and water beads on fingers: 63-68%

Many drops of water come out during squeezing: 68-73%

Stream of water emerges when squeezed: 73% or more

Final Thoughts on Moisture in the Worm Bin

If I had to guess based on Peter’s estimations, I would say most people’s worm bins have 68% or higher moisture content.

Most vermicomposters are likely faithful to the hand squeeze test early on when they are first establishing their bin, but then fail to track their moisture content along the way, unaware that they are adding plenty of water by simply feeding the bin food waste this is already high in water content.

Adding water to an already wet enough worm bin is the fast track to anaerobic conditions. As excess moisture is pulled lower by gravity, it displaces air in the pore spaces of the worm compost. Still hungry for oxygen, the microbes in the vermicompost consume the available oxygen in the pore spaces, now filled by the water, resulting in a wet anaerobic vermicompost.

If anaerobic vermicompost weren’t bad enough, it’s also a serious pain in your “castings maker” to harvest.

Note: Your ability to feel when your compost is just right will improve over time. One way to accelerate this learning is to do Method 2 (the hand-squeeze test before Method 1. Note how it feels, then throw it in the oven! Adjust your moisture from there as necessary.

I’d be very interested to see if any of you would be willing to test your vermicompost with Method #1 above! If you can let me know if – before the test! –  you thought your vermicompost was too dry, too wet, or just about right, and what your final results are! I’ll compile and publish the results at a later date

23 thoughts on “How to Measure Moisture Content in Compost and Vermicompost

  1. A handy, though non scientific approach Heather taught us. Take a handful of compost and squeeze tightly. It should hold together. Then “crumble” it and it should easily fall apart.

      1. I’ve been measuring my pH with a fancy direct insertion probe, pre-freezing and blending food waste, weighing additions of food, cardboard, and water, trying to be on top of everything because I’m new to this.
        Imagine the surprise to discover the upper-middle layer of my bin is 78% water content!!
        Just goes to show, getting a quality measurement can be game changing.

        Thanks for posting this method Steve, it rocks!

  2. I have a wooden box/worm bin.
    My compost runs about 45% moisture.
    Am I harming my worms by keeping it this dry?

    1. Hi Susan,
      I wouldn’t go much drier than this. Do you think it’s this dry because the wood wicks away the moisture? Or is this moisture level being done kind of on purpose?

  3. Steve: The engineer in me is coming out… When you weigh the sample, be sure to go to 2 decimal points (0.12). The scale I use for food prep only goes to 1/8 oz. in Imperial units, so if I use a small sample, I might not be able to tell that there was any moisture loss. Find a scale that measures in decimal pounds. My food prep scale also measures in grams, which is sensitive enough – but you have to think in the metric system. The process is exactly the same regardless of the units you are using – just be consistent.

    Another way is to get a larger sample, say, more than a pound.

  4. Method one is very similar to the way we measured moisture content for lumber during the kiln drying process (the only difference is that we divided the difference in weights by the dry weight). You are correct that it is very accurate. I would keep the oven dry sample in an air-tight container during the cooling cycle if possible. Ambient air always contains a significant amount of moisture and bone dry compost will absorb that moisture very efficiently. In a small sample this will definitely skew your results.
    I like the idea of using method two before oven testing. I believe that would help to quickly develop a sense of how moist your bin is. Great article and video!

    1. Thanks Eddie! That is great advice about keeping the lid on during cooling. Never thought about the compost reabsorbing any ambient humidity.

  5. I had a very wet bag for the first few weeks because I was transferring from a homemade pass through system. Whenever I opened the bottom compartment a liter of water would fall out and it smelled awful. I tried the obvious things like leaving it open and adding paper/cardboard. The bag is mostly full, so neither of those options worked well. Leaving it open brought flies, and not feeding isn’t really an option for me. What worked wonders was putting a 20″ box fan on top (facing downward) and closing the lid 90%. I checked on it every few hours and turned it off at night, but within a couple of days I had nice, slightly moist compost. It’s been about a month since I dried it out. During that time I’ve been giving it heavy, regular feedings and the moisture content has stayed stable.

    1. Kevin, thjis is very interesting and may be the basis for its own post on how you did this! Could you e-mail me any images you have to [email protected]? It seems that a lot of folks are in your same shoes, trasnferring contents from one system to another, so they may be interested in how you did this. An image of the fan itself inside the Bag would be illuminating as well! 🙂

  6. Wanted to thank you for all the videos and support you give to newbies like myself trying to learn the art of raising composting worms. As a newbie, I am a little confused about the feeding of a worm bin. I read and hear so much about overfeeding and poisoning the bin.
    I don’t understand the concept of how piling a device full of food and closing it up for days works without sowering the worm living habitat.
    Thanks again for all the information you generously share with us.

    1. Thanks William! You would definitely be souring a worm bin by piling up tons of food waste. I hope you didn’t get that idea from me. 🙂

  7. How do I get the worms to not be in my sample before the method 2 squeeze test? Or am I supposed to squeeze the worms too? I would think that leaving them in would hurt them but sifting them out before would also affect the moisture content by the time I squeeze…?

    1. It should be easy to grab any of the larger worms and pull them out. Squeezing the material for a moisture test shouldn’t harm smaller worms.

    2. Hey Amber. Sorry for the late response. We missed some comments. It shouldn’t hurt to have a worm in a handful of material as you squeeze it.

    1. If it still works for you Gene, nothing wrong with it! Just giving folks the most accurate way to do it.
      Sometime, the probes get corroded.

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