Mushrooms are a converter#
SLOP4761 · Week 3 · Substrate, yield and what to measure
Where we are#
Week 2 fixed how much ground you will water. Week 3 is the first biological module — and the first useful disappointment.
Still scoping. Quiz 1 runs at the start of the tutorial, on weeks 1 to 3.
This week’s question#
Your garden produces prunings. Straw is cheap. Oyster mushrooms eat both.
How much food does that actually make — and where does the spent substrate go?
Expect the first answer to disappoint you and the second to matter more.
By the end you can#
- Size a fruiting operation from the substrate a site actually produces
- Choose a biological efficiency from the published range and say why
- Account for both outputs, and put the spent-substrate arrow in the right place
Biological efficiency#
BE = fresh mushroom mass ÷ dry substrate mass × 100 %
| Substrate | Published BE range |
|---|---|
| Straw | ~60–80 % |
| Sawdust | 13–61 % |
| Supplemented | ~100 % |
BE can exceed 100 % because mushrooms are mostly water. It is not a thermodynamic efficiency.
These figures come from search summaries rather than papers read in full. They carry a medium confidence label, and nothing banded depends on them.
Species set the months#
- Pleurotus ostreatus — about 13–24 °C
- Blue oyster — about 7–18 °C
- Pink and golden oysters — about 18–30 °C
Between the cool and warm species you can cover roughly 8–30 °C of monthly mean temperature.
Worked example: Brisbane#
Monthly mean 14.9 to 25.0 °C, so some oyster species fruits in all twelve months with no heating or cooling.
20 kg dry straw × 0.70 BE = 14 kg fresh mushrooms 14 kg × 5 = 70 kg fresh spent substrate
At USDA composition figures (33 kcal and 3.31 g protein per 100 g):
14 kg → 4,620 kcal and 463 g protein → 0.13 % of energy, 0.7 % of protein
A tenfold scale-up still leaves you under 2 % of energy.
So what is it for?#
The substrate leaves heavier than it arrived. About 5 kg fresh spent substrate per kg picked, near 2.65 % nitrogen on a dry basis, C:N about 13.
That is a genuinely good soil input — and a genuinely bad insect feed.
Black soldier fly larvae convert pure spent mushroom substrate at 1.46 %. Effectively nothing. Draw the arrow to compost and soil, weeks 2 and 10.
Brisbane’s real constraint this week is strata rules, not biology.
Your tutorial task#
- Quiz 1, twenty minutes, on paper
- Estimate your own substrate supply — prunings plus a stated import
- Size both outputs, naming the BE you chose
- Work out together what 1.46 % means for a 100 kg substrate stream
Output: a substrate mass balance — dry substrate in, mushrooms out, spent substrate out with its mass, destination and nitrogen.
Before you leave#
- Is spent substrate on the sheet as an output, with a destination?
- Is there an arrow to the insect unit? There should not be
- Is your BE inside the published range, and written down?
- Does your sheet record the medium confidence label?
Full sources in the week 3 notes.