Hydration and the field capacity test

Water content is the most under-appreciated variable in cultivation. Too dry and colonisation stalls; too wet and you have created the anaerobic conditions that every common contaminant prefers over your mushroom.

And unlike temperature or humidity, you get one chance — once a substrate is treated and sealed, adding or removing water means compromising it.

Field capacity

The maximum water a substrate can hold against gravity without free water draining out.

At field capacity every particle is saturated, water fills the small pores, and air still fills the large ones. Mycelium needs both — it is an aerobic organism, and drowning it is as easy as drying it out.

The test

Take a handful. Squeeze it hard — as hard as you can.

What happens	Verdict
Nothing, or the material crumbles	Too dry. Add water.
A few drops appear between your fingers	Correct. This is field capacity.
A trickle runs down your wrist	Slightly wet. Usable, watch for pooling.
A stream	Too wet. Mix in dry material now.

Five seconds, no equipment, and it is more reliable than weighing for most home setups because it accounts for what the material actually absorbed.

Target numbers

If you prefer to weigh:

Substrate	Target moisture	Notes
Grain	45–50%	The narrowest window. Kernels dry to the touch.
CVG / coir	65–70%	Forgiving; holds water well
Straw	70–75%	Wetter than it looks; structure holds air
Master’s Mix	58–62%	60% is the standard figure
Plain sawdust	55–60%
Manure-based	65–70%

Moisture percentage is water as a fraction of total weight, not of dry weight. 60% moisture on 4 lb of dry material means 6 lb of water and 10 lb finished. The calculator does this properly.

Grain is the exception

Grain has the tightest tolerance in the whole process, and it is measured differently from everything else.

After soaking or simmering, spread the grain out and let it surface-dry until every kernel is dry to the touch. The water should be inside the kernels, not between them.

Then: tip the jar. If you can see liquid move, it is too wet. That is the whole test, and it predicts wet spot better than anything else.

Free water between kernels is a medium bacteria can swim through. Inside the kernel, it is not.

What too wet actually causes

• Anaerobic pockets. Water fills the air spaces, oxygen runs out, and the organisms that thrive there are bacteria, not your fungus.
• Rhizopus, which is more strongly associated with over-hydration than any other contaminant.
• Wet spot in grain.
• Stalled colonisation that looks like a dead culture. See stalled mycelium.
• Pooling, which concentrates the problem at the bottom where you cannot see it.

What too dry causes

• Colonisation stopping at the edge of the moisture and never resuming.
• Surface crusting, which prevents pinning and causes aborts.
• Poor yields — mushrooms are around 90% water and it has to come from somewhere.
• Blocks that give one flush and then have nothing left.

Dry is the less dangerous error. A dry substrate colonises slowly; a wet one contaminates.

Practical method

1. Weigh the dry material. Actually weigh it.
2. Calculate the water for your target percentage — or use the calculator.
3. Add most of it, not all. Hold back 10%.
4. Mix thoroughly and wait 15 minutes. Pellets in particular keep absorbing.
5. Squeeze test.
6. Add the rest only if needed.

Holding back that last 10% is the difference between a correction and a salvage operation.

If you overshoot

Before treatment: mix in dry material. Straw, coir, or more pellets.

After treatment, before spawning: spread it out on a clean sterilised surface and let it dry down, accepting the contamination risk that opening it creates.

After spawning: for bulk, increase fresh air exchange and let the surface dry. For grain, it is not recoverable — see wet spot.

The lesson each time is the same: it is far easier to add water than to remove it, so start dry.