What field capacity actually means.
Field capacity is the moisture content where your substrate holds the maximum water it can without releasing any when you squeeze it firmly. A drop or two should appear with a hard squeeze — but a steady stream means you’re overhydrated, and dry crumbs mean you’re underhydrated. Both kill colonization speed. The right ratio is species- and substrate-specific.
Approximate field capacity ratios (water : dry substrate by weight)
- Coir + vermiculite (CVG): ~5:1 to 6:1 — coir is thirsty. Pre-soak the brick for 30+ minutes, then mix.
- Hardwood sawdust: ~1.6:1 to 2:1 — Goldilocks zone, easy to over-shoot.
- Masters Mix (50/50 sawdust + soy hulls): ~2.2:1 to 2.5:1 — soy hulls absorb more than sawdust alone.
- Straw: ~3:1 — chop it short, it absorbs faster.
- Manure-based: ~1.5:1 — pre-composted manure already has high baseline moisture.
The squeeze test, properly done
Take a fistful of mixed substrate. Squeeze hard for 5 seconds. Open your hand.
- Drops trickle out steadily during the squeeze: too wet. Add ~5% more dry substrate by weight, mix, retest.
- A few drops form, then nothing: field capacity. Ship it.
- Substrate stays loose, no drops: too dry. Add water in 50–100 ml increments and remix.
Why hydration shows up in your contamination data
Overhydrated substrates are the #1 trichoderma vector outside of bad sterile technique — the standing water in pore space gives green mold a head start. Track your hydration ratios on every batch in Cynic & Spore and contamination patterns by recipe surface in the analytics tab. The grower who measures stops guessing.