River Cottage Australia

The Ultimate Guide to Hot Composting

A rich compost will not only increase the flavour of your produce, but also the quality. This guide from Paul West will show you how to turn your fish waste into a lush nutrient-rich compost in no time, through hot composting.

One of the bonuses of using hot composting is that the high temperatures reached in the process will kill any diseases or pathogens in your waste. Paul is keen to use fish waste as the nitrogen source, and weeds and cut hay as the carbon source, when he makes hot compost.

What is it?

Aerobic, or 'hot' composting is the conversion of organic matter into humus. Garbage such as fish parts (the nitrogen) is mixed with plant waste (the carbon) including sawdust, peat, wood chips, leaves, branches or bark.

Microorganisms in the pile feed on the waste and over a period of several months convert it into a rich humus. In the process, the microorganisms called thermophilles generate a great deal of heat, which pasteurises the product- eliminating odour and destroying weed seeds and disease organisms. 

Thermophiles are heat loving, with an optimum growth temperature of 50 degrees celsius or more, a maximum of up to 70 degrees celsius or more, and a minimum of about 20 degrees celsius.

The resultant product usually is used as a soil amendment or soil enhancer.

Microorganisms such as bacteria, fungi, and actinomycetes account for most of the decomposition that takes place in a pile. Of all these organisms, aerobic bacteria are the most important decomposers. They are very abundant; there may be millions in a gram of soil or in decaying organic matter. Bacteria utilise carbon as a source of energy (to keep on eating) and nitrogen to build protein in their bodies (so they can grow and reproduce). They obtain energy by oxidising organic material, especially the carbon fraction. This oxidation process heats up the compost pile from ambient air temperature. If proper conditions are present, the pile will heat up fairly rapidly (within days) due to bacteria consuming readily decomposable materials.

Mesophilic bacteria rapidly decompose organic matter, producing acids, carbon dioxide and heat. Their working temperature range is generally between 70º to 100ºF. When the pile temperature rises above 100º F, the mesophilic bacteria begin to die off or move to the outer part of the heap. They are then replaced by heat-loving thermophilic bacteria.

As microorganisms break the fish down, they generate lots of heat, which serves to pasteurize the resulting fish compost- in turn eliminating any odour and killing disease organisms and weed seeds. After several months, the resultant product is rich humus lauded as a nutrient wealthy fertilizer for soil amendment.

It’s quite astounding that the compost can reach temperatures up to 80 degrees! If you don’t have a compost thermometer, try putting your arm straight into the pile to make sure the compost is progressing, as it should. If you can hold your arm in for longer than a few seconds, the heap is not hot enough and should probably be turned, and have more nitrogen rich material added- or at the very least be built better next time. If, however, you can't even hold your arm in at all, then the heap is too hot. (An overly hot heap loses excessive amounts of nutrients, and may even catch fire!)

Backyard composting speeds up the natural process of decomposition, providing optimum conditions so that organic matter can break down more quickly. As you dig, turn, layer and water your compost pile. You may feel as if you are doing the composting, but the bulk of the work is actually being done by numerous types of decomposer organisms.

Why do it?

It only takes a month to create, and is no more difficult than brewing beer or baking bread, two other processes that take advantage of a different kind of microorganism.

Homemade compost always will contain more beneficial micro herd than most commercial brands.

Fish scraps are great as it is high in protein, a good balance, and contains lots of nutrients in it - more than manure or even plants. And as a bonus, the fish is usually a free by-product given away from co-ops.


It’s a simple recipe that involves 3 parts carbon (wood chips, bark, sawdust, charcoal if you have it) to 1 part nitrogen (fish scraps), as well as water and oxygen.

The size of your compost pile will vary in accordance to available space; however, a minimum recommendation for productive decomposition is about a square cubic metre.

1. Add a few layers of carbon and nitrogen and turn, then cover.
2. Allow the mixture to heat up for 3-5 days before being turned to ensure even decomposition and the destruction of weed seeds throughout the pile.


Temperature is a key parametre in determining the success of your composting operations! Compost heat is produced as a by-product of the microbial breakdown of organic material. The optimum temperature you want is between 55 and 65 degrees.

How to take the temperature

If you can put your hand in and leave it there without scolding- then it’s at the optimum temperature. If it burns then it is too hot, which results in the reproduction being too rapid- so turn and add water.
Over 70 degrees and then anerobs start breeding, which consume oxygen and could then result in a spontaneous combustion!

Keep in mind that the physical characteristics of the compost ingredients such as moisture content and particle size- affect the rate at which composting occurs.

Decomposition occurs most rapidly during the thermophilic stage of composting (40-60°C), which lasts for several weeks or months depending on the size of the system and the composition of the ingredients. This stage is also important for destroying thermosensitive pathogens, fly larvae, and weed seeds. In outdoor systems, compost invertebrates survive the thermophilic stage by moving to the periphery of the pile or becoming dormant.

'Hot' tip

Ground up charcoal is a great source of carbon for your hot compost. You can use it to fill around 25 per cent of your compost pile.  Charcoal not only extends the life of your compost, but it will also keep the soil fertile for up to 1000 years! 

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