Is Your Mushroom Substrate Too Wet? Here's How to Tell (And Fix It)
Learn how to tell if your mushroom substrate is too wet, why it matters, and how to fix it using the field capacity test. Avoid contamination and grow successfully.
5/17/20251 min read
Is Your Mushroom Substrate Too Wet? Here's How to Tell (And Fix It)
Getting the right moisture level in your substrate is crucial to a successful mushroom grow. If your substrate is too wet, you're asking for contamination, stalled growth, or worse—total failure.
Here’s how to tell if your substrate is holding too much moisture, and what to do about it.
1. Signs Your Substrate is Too Wet
It feels soggy instead of fluffy
You can squeeze water out with your hand
Puddles form in the tub or on the surface
It smells swampy or sour
Mycelium looks weak, patchy, or doesn’t colonize at all
If you're noticing these signs, don’t panic—just adjust before spawning or early in colonization.
2. The Field Capacity Test
One of the easiest ways to get your substrate moisture right is the field capacity test:
Grab a handful of substrate
Squeeze it firmly
You should only get a few drops of water—no stream
The handful should stay together, but not drip
If it's pouring or mushy, it’s too wet. If it crumbles or feels dry, it’s too dry.
3. How to Fix It
If already mixed and too wet:
Add more dry coco coir or vermiculite
Let it sit uncovered to evaporate some water
Mix thoroughly to even it out before spawning
If you're just prepping it:
Squeeze out small batches until they hit field capacity
Use your hands—it’s more accurate than measuring by the book
Final Tip:
Don’t trust the bag or tub—trust your hands.
Every block of coir or verm can hold moisture differently. Doing the squeeze test every time will help avoid beginner mistakes and boost your success rate.