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Lesson 4 / 18 · Compost Operator

Microbiology

Nobody heats a compost heap. The microbes do, and the thermometer on the stack is really a report on how well they are being fed.

  • By the end you can explain heat in a heap as microbial output, not input.
  • By the end you can say what selective compost means and which stage builds it.
  • By the end you can read temperature, smell and weight loss as evidence.

Compost temperature from day 0 to spawning

Long-method heap 65 to 70 °C with turnings, then pasteurisation at 58 to 59 °C for 4 to 6 hours, conditioning at 45 to 52 °C, cooling to 25 to 30 °C for spawning. Source: ICAR-DMR manual 2011.

30°40°50°60°70°80°turnturnturnPhase I: outdoor heap, 65 to 70 °CPhase II: 58 °C, then 45 to 52 °Cspawn at 25 to 30 °Cday 0day 22
Phase I heapPhase II tunnelPasteurisation band

Read numerical examples with their source, method and crop context. They are not universal operating instructions. Historical prices are not current quotations.

Peak heap temperature70°Clong method, within 24 to 48 hoursSource: ICAR-DMR manual, 2011
Core temperature ceiling80°Cabove this the core goes anaerobicSource: ICAR-DMR manual, 2011
Conditioning band45-52°Coptimum for thermophilic flora in Phase IISource: ICAR-DMR manual, 2011
Conditioning floor40°Cbelow this mesophilic fungi set inSource: ICAR-DMR manual, 2011
Weight loss30 in Phase I, 20-25 in Phase II%short method of compostingSource: ICAR-DMR manual, 2011
Conceptual compost heap cutaway showing straw structure and internal spaces.
AI-generated teaching illustration · not field evidence
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Structure and air

Notice
The heap has structure, depth and internal variation.
Understand
One central reading does not describe every part of the material.
Try it
Record representative readings and the actual preparation sequence.
See where this fits in the full workflow →

01Who does the work

Composting is a relay. Mesophilic organisms, which live at everyday temperatures, start on the easy sugars as soon as straw is wet and nitrogen is available. Their own heat then pushes them out and hands the heap to thermophilic bacteria, fungi and actinomycetes, which prefer high temperatures and break down tougher material.

That is why a heap which will not heat is a feeding problem, not a heating problem. DMR notes that when nitrogen is kept below 1.5 per cent the heap may not pass 55 to 60 degrees Celsius, because there is too little microbial activity to make the heat.

02Heat is the meter reading

In the long method the heap rises quickly and may reach 70 degrees Celsius within 24 to 48 hours. The DMR farmer folder describes 65 to 70 degrees in two or three days and calls it a good sign.

There is an upper limit. DMR states that above 80 degrees in the central core is not desirable, because the core goes anaerobic and the friendly thermophilic flora is lost. A heap hot in the middle and cold at the edges is not composting evenly; it is two heaps in one, and the turning schedule exists to swap them.

03Selectivity is the point

Good compost is not just rotted straw. It is food that Agaricus bisporus can use and most of its competitors cannot. That selectivity is built in Phase II, where thermophilic organisms take up the free ammonia and lock it into their own bodies as microbial protein.

DMR keeps the mass at 45 to 52 degrees Celsius for that conversion, and warns against conditioning below 40 degrees because mesophilic fungi set in and make the compost unsuitable. The conditioning band is not comfort; it is the range where useful organisms beat the others.

04What the biology costs

Microbial work burns material. DMR reports about 30 per cent weight loss in Phase I and a further 20 to 25 per cent in Phase II. The carbon to nitrogen ratio falls from 25 to 30 at stacking to around 16 to 1, and nitrogen per cent rises as carbon leaves as carbon dioxide.

This is why finished compost weighs far less than the straw you bought, and why nitrogen readings climb without anyone adding nitrogen. Expect the loss, plan for it, and do not read a rising nitrogen figure as proof of supplementation.

Plot one heap instead of spot checking it

  1. Fix three probe points: core, mid depth and edge.
  2. Read all three at the same hour every day.
  3. Draw the three lines on one sheet of graph paper.
  4. Mark each turning and watch how fast heat returns.
  5. Flag any core reading above 80 degrees the same day.

Write in your farm diary: Date, hour, three probe readings, turning dates, and any smell change such as sharp ammonia.

Mistakes that cost a crop

  • Treating a cold heap as a thermometer fault instead of checking nitrogen and water.
  • Letting the core pass 80 degrees and calling it a strong heap.
  • Conditioning below 40 degrees to save fuel, which lets mesophilic fungi in.

Check yourself

Three questions, instant answers

  1. A heap has not crossed 50 degrees after three days. Suspect:

  2. Holding compost at 45 to 52 degrees is meant to:

  3. Phase I loses about 30 per cent of its weight because:

An editorial self-check, not a certificate. Answers are not stored, not even on this device.

Words used in this lesson

Mesophile
An organism growing at everyday temperatures, roughly 20 to 40 degrees Celsius.
Thermophile
An organism growing best above roughly 45 degrees Celsius.
Selective compost
Compost that feeds the mushroom well and its competitors poorly.
Actinomycetes
Thread like bacteria that give good compost its grey white patches.
References & further reading
Button mushroom compost knowledge system

A lesson cannot replace batch measurements or plant-specific procedures.

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