Unpopular opinion: Squeezing your substrate to find the ideal field capacity is garbage advice.

Don’t get me wrong. This fast and dirty method will generally get the job done. But frankly, it’s poor practice, confusing for new cultivators, and terribly inefficient when using different substrate materials due to their varied capacity to hold water, i.e. coir, verm and gypsum will hold water differently than a manure and straw substrate.

Learning how to find and get a feel for the ideal moisture content of bulk substrate is foundational knowledge for home mycologists and deserves some respect.

So let’s all agree that the squeezing is best left for kegel exercises, and use our brains to find ideal moisture capacity and apply it to your using your specific substrate recipe in the future.

How do we get precise with our substrate moisture?

It’s simple!

Step 1: Make a batch of bulk substrate like usual.

Step 2: Use physics and math to determine if our original recipe needs to be adjusted.

Step 3: Manhandle the sub to know what your substrate feels like when properly hydrated.

It seems that most mushroom species thrive in substrates that hold a 60% moisture content. So we will use this as our goal for proper moisture content, i.e. “field capacity”.  

You visual learners can click here to skip past all of the words and watch T.R. from Earth Angel Mushrooms explain the process. 

How moist should your substrate be?

A common misunderstanding is that your substrate should be hydrated to field capacity, which is not true. 

What is Field Capacity?

Field capacity can be explained as the maximum amount of water a soil medium will hold against gravity – whereby adding a drop more will result in drainage. So – soil 100% hydrated.

It seems that most mushroom species thrive in substrates that hold a 60% moisture content. So we will use 60% as our ideal moisture content, or ideal field capacity percentage.

For this writeup I will be using a beginner friendly Bulk Substrate Recipe – Damion’s Coir Tek – as a hypothetical example.  

Simple Coir and Vermiculite Bulk Substrate


  • One (650g) brick of coir – like this > Coconut Coir Growing Medium
  • Two quarts of Vermiculite – found at your local hardware store or here
  • Five-gallon bucket – Like so 
  • 5 quarts of spawn – Provided by you
  • 1gal of water – Bought by the gallon
  • Pot to boil water in – Ask your mother

Damion’s Coir Tek Instructions:

  1. Pour 4 quarts of water in a pan, on the stovetop over high heat, bringing to a boil.
  2. Place one 650g brick of coconut coir into the 5 gal bucket.
  3. Pour 2 quarts of vermiculite into the 5 gal bucket with the coir.
  4. After boiling for a few minutes, pour the boiling water over the vermiculite and coir in your bucket.
  5. Place the lid on the bucket – important, as this keeps the heat and steam in.
  6. Wait 30-60 minutes, then remove the lid and use something to mix up the substrate.
  7. Place the lid back on and wait for another 3-6 hours until the substrate is cool.

Now that you have your bulk substrate made, let’s see how much moisture it is holding.

Step 1) – Weigh, Evap Away, Weigh Again

  1. Grab a handful of your hydrated substrate. 
  2. Place this on a scale and take note of the weight. 
  3. Remove from the scale and place in a microwave-safe container. 
  4. Microwave for 2.5 minutes. 
  5. Remove from the microwave and let it cool down.
  6. Microwave for another 2 minutes
  7. Remove from the microwave and let it cool down.
  8. Place the substrate sample on your scale and note the weight. The difference between the original weight is the water that has evaporated. Pretty cool right!?
  9. We are going to continue this process at one-minute intervals, each time making note of the weight until the weight no longer changes, i.e. there is no more moisture in the substrate. 

Step 2) – Math, Nerds

The is the expression we will use is:  X / Y * 100 = Moisture Content in %

Using hypothetical numbers for a pre-microwave weight of 3 oz (Y) and a final weight number of 1.3 oz. The difference between those two numbers is the weight of the water we evaporated (3 oz minus 1.3 oz = 1.7 oz) – This is the variable (X), still with me?

So if  X / Y * 100 = %; then our moisture content would be: 1.7 / 3 * 100 = 56.66… or 56.66…%.

Based on this information, our substrate is 56% water.

Once you know this number you can increase or decrease moisture by either  adding water or using vermiculite to absorb excess moisture. 

But even more importantly use this exercise is to familiarize yourself with the feel of ideal hydration of the substrate of your choice.

So touch it, feel it, massage it, get to know it, and rub it on your face. Love it… 

Step 3) – Modify your substrate recipe accordingly

Finally, make any changes to your substrate recipe for next time so that you don’t have to go through all of this every time. Repeat twice a year to see if you’ve gotten better. 

For all of you visual learners out there

Big thanks to T.R. from Earth Angel Mushrooms who produced the original video which inspired this post.