What Is Field Capacity? How to Get the Right Moisture Levels in Your Substrate

If you’re new to mycology, you’ve probably come across the term field capacity. It’s one of the most important concepts for successful grows, but it often gets overlooked by beginners. In this guide, we’ll break down what field capacity means, why it’s critical for mushroom health, and how you can test it in just a few seconds.

What Is Field Capacity?

Field capacity refers to the ideal moisture content in your substrate—wet enough to support strong mycelial growth, but not so wet that it drowns the fungi or promotes contamination.

Think of it like the “sweet spot” between soggy and bone-dry. At field capacity, your substrate should feel hydrated and fluffy without pooling water.

Why Does It Matter?

1. Healthy Mycelium Growth – Mycelium spreads fastest in properly hydrated substrate.

2. Prevents Contamination – Too much water can create anaerobic conditions (lack of oxygen), which invite bacteria and mold.

3. Better Yields – Balanced moisture supports consistent fruiting and larger flushes.

How to Test Field Capacity

The most common way is the squeeze test:

1. Take a handful of prepared substrate.

2. Squeeze it firmly in your fist.

   • If a few drops of water come out, it’s at field capacity. ✅

   • If water pours out, it’s too wet. ❌

   • If nothing comes out and it feels crumbly, it’s too dry. ❌

3. Adjust as needed:

   • Too wet? Add more dry substrate and mix well.

   • Too dry? Mist in small amounts of clean water until it balances.

Tips for Beginners

• Always test after mixing, not before. Moisture distributes unevenly until mixed thoroughly.

• Use clean, filtered water where possible. Tap water is fine for most grows, but avoid heavily chlorinated water.

• Don’t obsess over perfection—the squeeze test is more than enough for hobby-level grows.

Conclusion

Mastering field capacity is one of the easiest ways to boost your success as a mushroom grower. Once you know what the “right moisture” feels like, it becomes second nature. With practice, you’ll be able to tell at a glance whether your substrate is ready for colonization.