What is field capacity?

Field capacity matters because substrate that's too wet colonizes slowly. GroCycle, a company that sells growing kits and courses, names overhydrated substrate as the most common cause of slow colonization (GroCycle). Excess moisture also helps bacteria grow, as our pink oyster grain spawn guide notes, and our contamination ID guide shows what bacterial contamination looks like.

The term comes from soil science. University of Minnesota Extension defines field capacity as the water left in soil "after all excess water at saturation has been drained" (UMN Extension).

Test for field capacity with a squeeze test: grab a handful of the wetted substrate and squeeze.

• At field capacity: a light squeeze gives no water, and a hard squeeze gives only a few drops (GroCycle). Midwest Grow Kits says the same of its pellets: "a firm squeeze in your fist should release only a few drops" (Midwest Grow Kits).
• Too wet: more than a drop or two on a light squeeze. Add some dry substrate and mix (GroCycle).
• Too dry: no water, and the handful doesn't hold together. Add a little water and test again (GroCycle).

For repeatable batches, weigh instead. Moisture content is the water's share of the total wet weight. A UC Cooperative Extension handout hydrates 5-lb sawdust blocks to 65% water, which is 3.25 lb of water to 1.75 lb of dry ingredients. Its recipes subtract the water already in the dry ingredients before adding the rest (UC Cooperative Extension).

Field capacity has no single number, but published recipes for sawdust and straw aim for about 60–65% water. The UC handout gives Stamets' enriched sawdust recipe at 60–65% and expects pasteurized straw, drained well, to hold about 65% (UC Cooperative Extension). GroCycle hydrates Master's Mix to 60% (GroCycle) and gives 50–70% for substrates in general (GroCycle). The squeeze test works whatever the material, which is why growers use it.

• Saturation. Saturated substrate has every pore full and drips on its own. Field capacity is what's left once that free water drains away. For straw, UC drains the soaked straw overnight on hardware cloth (UC Cooperative Extension).
• Humidity. Field capacity is water in the substrate. Relative humidity is water in the air around the block.
• Casing moisture. A casing layer is watered up to field capacity a little at a time, before pins form (Penn State Extension).

Hypha keeps your substrate recipes with your supplies in its inventory, so the batch that hit field capacity is easy to repeat.