What is Field Capacity? (And Why It’s Important for Mushroom Growing)

If you’ve been learning how to grow mushrooms, you’ve probably heard the term “field capacity” thrown around when prepping substrate. But what does it actually mean, and why does it matter?

In simple terms, field capacity is the perfect moisture level for your substrate—wet enough to provide hydration for your mycelium but not so wet that it drowns it. Getting this right is crucial because too much or too little water can slow down colonization and hurt your yields.

I’ve personally had both problems—a substrate that was too wet, which led to contamination, and a substrate that was too dry, which caused mushrooms to either not fruit at all or abort before fully developing. Learning to dial in the right moisture level has made a huge difference in my success with gourmet mushrooms.

How to Check for Field Capacity

The easiest way to check if your substrate is at field capacity is to do a squeeze test.

1️⃣ Grab a handful of your hydrated substrate.
2️⃣ Squeeze it firmly.
3️⃣ Look for these signs:

• ✅ A few drops of water should come out. That’s perfect.

• ❌ If water streams out, it’s too wet. You need to let it dry out a little.

• ❌ If nothing comes out, it’s too dry. You’ll need to add more water and mix again.

📸 (Insert your squeeze test photos here!)

Why Field Capacity Matters

• Too wet? Your substrate can become anaerobic (lacking oxygen), which can slow or even kill mycelium growth.

• Too dry? Your mycelium won’t have enough moisture to colonize properly, leading to weak growth.

• Perfect field capacity? Your mycelium will colonize fast and strong, leading to healthy fruiting conditions.

Final Tips

✔ Always mix your substrate thoroughly after adding water to ensure even moisture distribution.
✔ If your substrate is too wet, let it air out or add more dry material (like vermiculite) to absorb the excess.
✔ Field capacity is key for bulk grows like monotubs, dub tubs, and trays. Master this, and you’ll get much better results.

Understanding field capacity is a simple but important step in gourmet mushroom growing. If you found this guide helpful, check out my [TikTok/Instagram] for more tips!