Mycelocity
Get hydration wrong and almost everything downstream suffers: colonisation stalls, yields drop, and contamination risk climbs. Get it right and the rest of the grow has a fighting chance. Hydration is genuinely the first lever to pull.
Measuring moisture content
Moisture content is the fraction of a substrate’s weight that is water:
moisture% = ((wet − dry) / wet) × 100
To measure it, weigh a representative sample wet, dry it fully (a low oven or food dehydrator works), then weigh it again. The difference is the water that was in it. The Substrate Moisture & Hydration calculator does this arithmetic for you and — just as usefully — runs it in reverse: tell it a target moisture and it returns exactly how much water to add.
Field capacity: the target
Most bulk substrates fruit best at field capacity — fully hydrated but not waterlogged. For supplemented sawdust and many bulk mixes that lands around 60–70% moisture. The classic bench test is the squeeze: a firm squeeze of a handful should release a few drops, not a stream and not nothing.
- Too dry (< ~55%): slow, patchy colonisation; reduced yield.
- Too wet (> ~72%): anaerobic pockets, pooling at the bag base, and a standing invitation to bacteria and mould.
Why hydration drives contamination
Excess free water is the link between this guide and the Contamination Loss calculator. Waterlogged substrate holds anaerobic, oxygen-poor zones where bacteria thrive and where mycelium cannot compete. Dialling moisture back to field capacity is one of the cheapest contamination controls available — no new equipment, just better weighing.
Hydration and the dry weight you report
Because biological efficiency is quoted against dry substrate weight, you cannot honestly state BE without knowing your moisture content. Measure moisture first, back out the dry weight, and only then compute BE. Skipping this step is the most common reason a grower’s “BE” number drifts batch to batch — they are really just measuring inconsistent hydration.
A simple routine
- Mix and hydrate to your target moisture (use the calculator’s water-to-add mode).
- Confirm with the squeeze test before loading bags.
- Record the dry weight per block — you will reuse it for BE and yield.