Mushroom Substrate Calculator

Most gourmet mushroom species grow best in substrate with a moisture content (MC) of 60 to 65% by weight. This is also called field capacity. At 60% MC, a handful of substrate squeezed tightly should release just a few drops of water. Substrate that is too wet promotes mould; too dry and mycelium growth stalls. Oyster mushrooms tolerate up to 75% MC.

Moisture content by weight (%) = (Wet weight - Dry weight) / Wet weight x 100. For example, if 1 kg of dry sawdust absorbs 1.5 kg of water, total wet weight = 2.5 kg and MC = 1.5 / 2.5 x 100 = 60%. Most substrate ingredients have a known starting moisture: hardwood sawdust is typically 10 to 15% MC when dry.

Water to add = Total wet weight - Dry weight, where Total wet weight = Dry weight / (1 - Target MC). For 1 kg dry hardwood sawdust targeting 65% MC: Total wet = 1 / (1 - 0.65) = 2.857 kg. Water = 2.857 - 1 = 1.857 kg (approximately 1.857 L). Adjust for any moisture already in the dry ingredients.

A simple high-yield substrate for oyster mushrooms (Pleurotus spp.) is 80% hardwood sawdust and 20% wheat bran by dry weight, adjusted to 65% MC. More nutritious supplements like bran increase yield but also increase contamination risk. USDA ARS research supports hardwood sawdust as the primary carbon source for Pleurotus cultivation.

Pasteurisation (60 to 82 degrees C for 1 to 2 hours) is sufficient for oyster mushrooms on straw: it kills most competing organisms while leaving beneficial bacteria. Sterilisation (121 degrees C for 2.5 hours under pressure) is required for more contamination-prone substrates like manure/compost mixes or supplemented sawdust. King Oyster and Shiitake on sawdust typically require sterilisation.