Substrate Batch Sheet

The arithmetic

Wet weight

dry ÷ (1 − moisture)

Water to add

wet weight − dry weight

At 60% moisture

dry is 40% of the finished weight

5 lb dry at 60%

12.5 lb wet, 7.5 lb water

Spawn

wet weight x spawn rate

Wet-rate → dry-rate

divide by (1 − moisture)

10% of wet at 60%

= 25% of dry

Water

about a pint per pound

⚠ Moisture target and spawn rate

from YOUR supplier

This batch

Date

Species

Substrate

Bags

Dry per bag

Target moisture

WATER PER BAG

WET PER BAG

Spawn rate

SPAWN PER BAG

TOTAL SPAWN

Squeeze test result

Pasteurised / sterilised

Batch record

Mixing day

• Substrate weighed DRY, not as delivered
• Moisture target taken from the supplier or the species
• Spawn rate checked: against WET or DRY weight?
• Water weighed rather than judged
• Mixed thoroughly - dry pockets colonise badly
• Squeeze test done: a few drops, not a stream, not nothing
• Adjusted and the adjustment written down
• Pasteurised or sterilised as the method requires
• Cooled fully before spawning
• Sterile technique at spawning - the arithmetic cannot save this

10% of wet is 25% of dry

At 60% moisture those are the same quantity of spawn described two different ways. Suppliers use both conventions, so check which yours means before ordering. Under-spawning gives slow colonisation and a longer window for contaminants to get established, which is the costly direction to be wrong in.

The squeeze test is the real check

Squeeze hard: a few drops, not a stream, not nothing. Absorbency varies between batches of the same material, so the arithmetic gets you close and your hand confirms you arrived. Too wet goes anaerobic and sour; too dry just colonises slowly. Adjust at mixing, because neither is fixable once the bag is sealed.