Week 3: Mushrooms are a converter, not a crop
Week 3 — biological efficiency, species temperature bands, and what happens to five kilograms of spent substrate for every kilogram you pick
8 March 2027
The question#
Your garden produces prunings. Straw is cheap. Oyster mushrooms will eat both. How much food does that actually make, and — the question people forget — where does the spent substrate go?
Expect the first answer to disappoint you and the second to matter more.
The method: biological efficiency#
BE = fresh mushroom mass ÷ dry substrate mass × 100 %Straw substrates run about 60–80 %; sawdust ranges 13–61 %; supplemented substrates reach roughly 100 %. These figures come from the mushroom literature through search summaries rather than papers read in full, so they carry a medium confidence label and week 3 is the one week of the course whose yield figures were not re-verified against primary sources.
Fruiting temperature decides your species and your months. Pleurotus ostreatus fruits around 13–24 °C, blue oyster around 7–18 °C, pink and golden oysters around 18–30 °C. Between the cool and warm species you can cover a mean temperature range of roughly 8–30 °C.
Worked example: Brisbane#
Brisbane’s monthly mean sits between 14.9 and 25.0 °C, so some oyster species fruits in all twelve months without heating or cooling. Take 20 kg of dry straw at 70 % biological efficiency:
20 kg × 0.70 = 14 kg fresh oyster mushroomsAt USDA composition figures, 33 kcal and 3.31 g protein per 100 g:
14 kg → 4,620 kcal and 463 g proteinAgainst the household’s 3.49 million kcal and 67.5 kg of protein a year, that is 0.13 % of energy and 0.7 % of protein. A tenfold scale-up still leaves you under 2 % of energy.
So mushrooms are not a calorie strategy. What are they?
The part that matters: the substrate leaves heavier than it arrived#
Every kilogram of mushrooms leaves behind roughly 5 kg of fresh spent substrate, at about 2.65 % nitrogen on a dry basis and a C:N ratio near 13. That is a genuinely good soil input, and it is the real output of this week.
Which raises the obvious idea: feed the spent substrate to the insects in week 4.
Do not. Black soldier fly larvae convert pure spent mushroom substrate at 1.46 % — effectively nothing. The lignin the mushroom left is the part the larvae cannot use either. Adding spent substrate at up to about 25 % of a mixed larval feed does no harm, but as a feedstock on its own it is a dead end.
Spent substrate goes to compost and soil, weeks 2 and 10. Draw that arrow correctly now and week 10’s mass balance will close.
Five sites#
| Site | Fruiting months | The constraint that actually binds |
|---|---|---|
| Brisbane | 12 | Space and strata rules. A fruiting chamber on a townhouse courtyard is a neighbour-amenity question before it is a biology question |
| Darwin | 12 | Warm species only; contamination pressure is highest here |
| Alice Springs | 12 | Temperature is fine, humidity is not — 25 % relative humidity at 3 pm means a humidified chamber, and that chamber is a counted energy and water load |
| Adelaide | 12 | The easiest site for this subsystem: mild, dry enough to manage contamination, warm enough year round |
| Canberra | 9 | Three winter months below the cool-species band. Either accept the gap or heat the chamber and pay for it in week 12’s energy budget |
What this week hands you#
A substrate mass balance: dry substrate in, mushrooms out, spent substrate out, with the spent substrate’s nitrogen carried forward into your soil plan. Both arrows, or the balance does not close.
The yields above are published figures with a stated confidence, so the number you carry forward is a scoping estimate. What your client’s own prunings supply is a measurement question, and it belongs in Assignment 1.
Teaching team
- Noor Bakhshi — tutor