Field Capacity Calculator

Determine ideal substrate moisture levels using the squeeze test method. Proper field capacity prevents contamination and ensures optimal colonization rates.

Substrate Information

Dry Substrate Weight (lbs)

Weight of dry substrate before hydration

Substrate Type

Type of substrate material

Understanding Field Capacity

What is Field Capacity?

Field capacity refers to the optimal moisture level where substrate holds the maximum amount of water while still maintaining proper air-filled porosity. This balance is crucial - too wet promotes bacterial contamination, while too dry slows colonization.

The Squeeze Test Method

The squeeze test is the gold standard for determining field capacity:

Grab a handful of hydrated substrate
Squeeze firmly in your fist
Observe the water release
Perfect: 2-3 drops fall
Too wet: Water streams out continuously
Too dry: No water releases

Substrate-Specific Ratios

Coco Coir

5-6:1 water ratio - Highly absorbent, requires more water than other substrates. Holds moisture excellently.

Straw

2.5-3:1 water ratio - Less absorbent, requires moderate water. Good drainage properties.

Hardwood Sawdust

2-2.5:1 water ratio - Dense material, less water needed. Excellent for shiitake and hardwood lovers.

Why Field Capacity Matters

Prevents Contamination: Excess water creates anaerobic conditions favoring bacteria
Optimal Colonization: Mycelium needs both moisture and oxygen to spread
Fruiting Success: Proper moisture supports healthy mushroom development
Prevents Stalling: Too dry substrate stops mycelial growth

Troubleshooting Moisture Issues

Too Wet Substrate:

Spread substrate on clean surface to evaporate excess
Mix in dry substrate material to absorb moisture
Use a fan to speed drying (maintain cleanliness)
Add vermiculite to improve drainage

Too Dry Substrate:

Mist surface lightly with spray bottle
Add small amounts of water and mix thoroughly
Re-test frequently to avoid over-correction
Consider casing layer to retain surface moisture

Frequently Asked Questions

Can I add water after spawning if substrate is too dry?

It's difficult but possible. Lightly mist the surface or add a thin casing layer. Avoid injecting water as this can create contamination pockets. Prevention is better - always check field capacity before spawning.

Does substrate lose moisture during colonization?

Yes, some water evaporates during the colonization phase (typically 5-10%). This is why starting slightly above perfect field capacity can be beneficial, especially in dry climates.

What if different parts of my substrate have different moisture levels?

This indicates poor mixing. Always mix water thoroughly into substrate. Use your hands or a large spoon to ensure even distribution. Test multiple spots with squeeze test.

Should I use tap water or distilled water?

Tap water works fine for most substrates. If your tap water is heavily chlorinated, let it sit overnight or use dechlorinated water. Distilled water is unnecessary and expensive for substrate hydration.

How do I maintain field capacity during fruiting?

Maintain proper humidity (80-95% RH) in your fruiting chamber. If substrate surface dries, lightly mist or dunk between flushes. A good fruiting chamber setup prevents excessive moisture loss.

Can I rehydrate substrate between flushes?

Absolutely! This is called "dunking." Submerge your substrate block in water for 12-24 hours between flushes. This rehydrates the substrate and signals the mycelium to fruit again.

Does temperature affect field capacity?

Warmer water is absorbed faster, but final field capacity is the same. Hot water (160-170°F) is often used for pasteurization, which also aids absorption. Always cool to room temperature before spawning.