Can I run this batch without the blood meal?

Yes, but expect a slower start and a longer cure. Swap it for 1.5 kg of fresh grass clippings or 2 L of urine diluted 1:4, both of which push the same nitrogen without the ammonia spike risk.

The tumbler stalled at 40 °C in week three — is the batch dead?

No. A tumbler loses heat fast because the shell is thin and the volume is small. Warm it by moving the whole charge into one chamber, adding 1 kg of fresh greens and tumbling. If it still does not move within 48 hours, the moisture is wrong — check the squeeze test before adding anything else.

Oak leaf and coffee chaff in a 190 L tumbler

01The sheet

Feedstock as charged, with the carbon accounted row by row
material	kg in	C:N	water	role	carbon
Oak leaves (shredded, 12 months old)	22.0	52:1	22%	brown	21.6
Coffee chaff (roastery husk)	6.0	20:1	14%	green	5.7
Spent grounds (café collection)	4.5	20:1	62%	green	4.3
Blood meal (C:N balancer)	0.3	4:1	8%	balancer	0.3
Water added over the batch	11.0	0:1	100%	balancer	—
Blended	43.9	32:1	44%	31.9 kg C-eq

10:1 120:1 32:1

Blended C:N as charged 32:1 · target 28:1 · moisture 44%

02Batch facts

Bin: tumbler
Charged: 08 Jun 2026
Finished: 21 Aug 2026
Thermal class: Thermophilic (crossed 55 °C)
Turns: 7
Days to finish: 74
Finished volume: 96 L
Target C:N: 28:1 (as charged 32:1)
Moisture at build: 54 % (as charged 44 %)
Odour risk: low
Space needed: One 190 L dual-chamber tumbler on a paved pad; both chambers in rotation.
Time to finish: 74 days from first charge to screened output in a warm summer.

Raw feed5 materials, 43.9 kg charged

Hot corepeak 63 °C (day 8), held 55 °C for 4 consecutive readings

Cured74 days to finish, 96 L out

Rest and screenresting since 21 Aug 2026

Cross-section: tumbler — Section through the bin

03Why oak leaves alone never heat up

Oak leaves carry a C:N near 52:1 and, more importantly, a waxy cuticle that sheds water instead of absorbing it. A pile of whole oak leaves will sit at ambient temperature for a year, because the leaf surface never wets and the microbes never get a continuous water film. The fix is mechanical, not chemical: shred to 20-30 mm and pre-wet the shredded leaves for 12 hours before they go anywhere near the bin. In this batch the leaves took up 8 of the 11 L of added water in that pre-soak, which is why the first tumble landed at 54 % moisture instead of the 30 % a dry charge would have given.

04How the C:N landed at 28

The arithmetic is simple enough to do on the back of a seed packet. 22 kg of leaves at 52:1 contributes 22 × 1000 × (52/53) = about 21,580 g of carbon and 415 g of nitrogen. The 6 kg of chaff at 20:1 adds 5,714 g C and 286 g N; the 4.5 kg of grounds adds 4,286 g C and 214 g N. That totals roughly 31,580 g C against 915 g N, or 34.5:1. The 350 g of blood meal at 4:1 contributes 280 g C and 70 g N, pulling the ratio down to about 32:1 — still high, so 2 kg of the grounds were held back and added on day 16 after the first heat had passed. Real batches rarely land exactly on target; what matters is that you can show the working.

05The week-two moisture wobble

By day 6 the pile was at 63 °C and visibly dry at the vent — a tumbler in full sun loses 2-3 L of water a week to evaporation. The mistake here was adding all 3 L at once on day 4; the water ran straight through the centre channel and pooled in the lower chamber. The correction was to add the remaining water in two passes with a 10 minute gap, tumbling between them, which let the chaff absorb it rather than shed it. Squeeze test afterwards: a handful from the middle of chamber A held together and left a faint sheen on the palm, no drips.

06What the temperature curve is telling you

The 63 °C peak on day 8 is the useful part of this batch. Three consecutive days above 55 °C is the working thresholds for knocking back weed seeds and most tomato and pepper pathogens, and this batch held that band from day 6 through day 10 — four days. The long plateau from day 16 to day 28 at 44-48 °C is the second, slower wave as the chaff and grounds break down. Below 35 °C from day 33 onward, the pile is curing rather than composting: keep it damp but stop trying to make it hot.

07Screening and what came out

74 days after the first charge, 96 L of finished compost came off a 12 mm screen, plus 14 L of oversize leaf stems and avocado stones that went back as starter. Moisture at screening was 41 %, so the batch lost about 13 points of water over the run. It went onto a rhubarb bed at 3 L/m² in late August and onto the asparagus bed as a 4 cm mulch.

08Turn schedule

day 0 Charge chamber A, tumble 20 revolutions to mix, leave lid cracked 2 cm for the first 36 hours.curve 21 °C · 08 Jun 2026 · done
day 2 First full tumble, 15 revolutions. Note the smell at the vent — should read like wet forest floor, not vinegar.curve 34 °C · 10 Jun 2026 · done
day 4 Tumble 15 revolutions, add 3 L water in two passes with a 10 minute gap.curve 46 °C · 12 Jun 2026 · done
day 8 Tumble 15 revolutions. Peak heat reached on day 8 at 63 °C.curve 63 °C · 16 Jun 2026 · done
day 16 Tumble and move the whole charge to chamber B; rinse chamber A and let it dry in the sun.curve 48 °C · 24 Jun 2026 · done
day 28 Tumble every 4 days from here to the finish — heat has dropped below 40 °C.curve 38 °C · 06 Jul 2026 · done
day 74 Screen at 12 mm, return the oversize to the next batch as starter.curve 22 °C · 21 Aug 2026 · done

09Temperature curve, as it ran

Temperature curve: Oak leaf and coffee chaff in a 190 L tumbler — Readings taken 30 cm into the core before and after each turn. Squares mark turn days; the dashed line is 55 °C.

day 0–2Phases · mesophilicday 4Phases · warmingday 6–13Phases · thermophilicday 16–24Phases · warmingday 28–74Phases · mesophilic

Smell notes

Day 5 the vent smelled faintly of ammonia for about six hours after the water addition — a sign the blood meal was releasing fast while the pile was still below 50 °C. Two extra tumbles that evening cleared it. From day 9 onward the smell was sweet and earthy, and the finished batch smelled like damp oak floorboards with a mineral edge.

10Questions this batch raised

Can I run this batch without the blood meal?: Yes, but expect a slower start and a longer cure. Swap it for 1.5 kg of fresh grass clippings or 2 L of urine diluted 1:4, both of which push the same nitrogen without the ammonia spike risk.
The tumbler stalled at 40 °C in week three — is the batch dead?: No. A tumbler loses heat fast because the shell is thin and the volume is small. Warm it by moving the whole charge into one chamber, adding 1 kg of fresh greens and tumbling. If it still does not move within 48 hours, the moisture is wrong — check the squeeze test before adding anything else.