Maintaining Ideal Worm Bin Moisture
A healthy worm bin is a blend of organic matter, water, and air.
Table of Content
Balancing Moisture Levels is Critical to Success
Earthworms Need Oxygen and Water to Survive More Than Nutrition
Humans can survive months without food (someone fasted for a year actually!), weeks without water, but can only live for a matter of minutes without air.
The worms have a similar priority list. Providing nutrition is far less important than focusing on their hydration and ability to breathe. I myself have found earthworms in folded up bubble wrap that had been without nutrition for months, but since they had oxygen and moisture they were still alive. In rough shape, but still alive.
How Worms Breathe and "Drink"
As humans, we breathe and drink as separate processes. But worms are a bit different. They are always "drinking" and breathing from their surroundings through their skin.
So, to keep hydrated and also to breathe, environment needs to be moist enough that their skin is constantly in contact with moisture and air simultaneously.
This is the main balancing act you'll need to learn to worm compost successfully.
Understanding Moisture in the Worm Bin
Understanding Soil's Capacity to Hold Water (or Air)
Imagine filling a bucket of soil up to the brim and closing a lid over it on top.
You might say that the bucket is full of soil, but it's really a mixture of soil, and air and water.
Each particle of soil won't perfectly mesh with the other particles around it - leaving an open space.
However, unless you're in the vacuum of space, it would be wrong to call that "open space" as "empty."
Just like how fish probably don't realize they are in water, we (or at least I) often forget we are "submerged" in the earth's atmosphere of air, and we sometimes call space that's occupied by air "empty" even though it's not.
An easy way to visualize this is by rapidly filling up the bucket of soil with water. You will see many bubbles appear and pop at the surface. All of that was air that used to be mixed into the "soil matrix," and that air is an important ingredient in your worm bin.
Air is a Crucial Ingredient in Your Worm Bin Recipe
Just as much as your worm bin needs to be holding food and water for the worms, it needs to be holding air too.
This image (forgive my lack of graphic design skills) illustrates our worm bin with a worm in it. We have placed worm bedding in it, but haven't added water yet.
It is full of a mix of organic matter and also air.
Let's say about 50/50 organic matter and air, which is visualized by the pie chart.
Now let's add some water to our worm bin. As we pour water in, it pushes air out. Organic matter will "grab" the available water, but if we didn't add more water than the organic matter can soak up, there will still be some gaps of air throughout the worm bin.
This is ideal worm bin moisture. Worms can wiggle through the worm bin, coming in contact with plenty moisture to keep their skin moist, but also enough air that they can still breathe.
We've added more water. The organic matter is holding as much water as it possibly can, but worms are still able to find a little bit of air as they tunnel through the material.
This is definitely too wet, but won't be lethal to the worms quite yet.
However, once the organic matter decomposes and loses volume, the excess water will flood any available air pockets.
Uh oh! There is more water than the organic matter can hold and the water has had to fill up all the "empty" spaces that otherwise would've been filled with air. Now the worms are at risk of dying.
The Ideal Moisture Content in a Worm Bin
The ideal moisture content is around 50-80%, which means that the water/air holding portion of your organic matter is 50-80% occupied by water, and 50-20% occupied by air.
This is often described as a "wrung out sponge" feel and is best visualized by the 2nd of my diagrams above.
This can be achieved/measured by a handful of different ways.
Assessing Worm Bin Moisture
The Squeeze Test
The most common method to test worm bin moisture is the squeeze test. Grab a handful of the material, squeeze it with your hand and assess how much water comes out. 1-3 drops is a good area. Any less and it could be too dry, and any more it could be too wet.
| 0 Drops, material does not stick together at all | 0-30% moisture |
| 0 drops, material sticks together but falls apart quickly after releasing | 30-40% moisture |
| 0 drops, material holds form after squeezed but falls apart when poked | 40-50% moisture |
| 1-2 drops of water | 50-60% moisture |
| 3-5 drops of water | 60-80% moisture |
| 5+ drops of water | 80%+ moisture |
Plastic Bag Visualization
Place some of the material into a plastic bag. Looking at it from the side, how much of the side of the plastic bag is open air, and how much of it is wet organic matter? You want about 10-25% of it to be open air.
Direct Measurement
Take a small sample of your worm bin, weigh it, and then oven dry it at 220F for 24 hours. Please make sure you do this safely. After 24 hours, weigh the sample again.
(Wet weight - dry weight) / Wet weight = moisture %
Sight and Smell
A healthy moisture level
- Smells like a healthy forest floor
- Crumbly, fluffy, visible structure
- Minimal pests
Too Wet
- Smelly
- Muddy, no structure
- Insect Pests
Get Your Worm Bin Moisture Right the First Time
How to Achieve, or Fix, Moisture in Worm Bin
Worm bin too dry?
If your worm bin is too dry the fix is simple enough: add water!
The best way I have found is to lightly spray water on top, or mix it in. If you pour water directly in the bin it will go straight to the bottom and pool up there.
If you are consistently having problems keeping your worm bin moist enough, consider using a "worm blanket" such as a plastic cover to prevent excess evaporation. Don't cover the entire surface though - worms still need to breathe!
Worm Bin too wet?
If things are too wet, simply add more dry bedding that can absorb the excess moisture.
If you are consistently having problems with high moisture, consider adding dry bedding with every feeding. Fruit and vegetable waste are around 90% water, so each time you are adding some of this waste, you are adding mostly water.
Are moisture meters effective?
The few times I've used them they seem to max out pretty quickly even at lower worm bin moisture levels. I can't say that I'm a fan.
Is water dripping out of my worm bin ok?
This is very common misinformation - if your worm bin is dripping water it is too wet. It means there is more water than the organic matter can hold, so any available air is being displaced by water and the worms, and beneficial microorganisms, could suffocate in that environment.
Do I need to "fluff" my worm bin?
This isn't really necessary. If you're finding you need to fluff it to keep it from going anaerobic, then you need to dry out the bin more.
Scott Kent
Owner of Hungry Worms. Worm farmer, soil microbiologist, and educator since 2014. Read more about me here.