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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.

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.pore space

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.

In a followup post, I am going to be “cooking” my own vermicompost to test its moisture content. 

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!