Thermal Composting

For me as an organic gardener, the single most important thing is compost. And not just any compost. I am talking about bio-complete compost, buzzing with microorganisms – fungi, bacteria, protozoa, nematodes and worms. Chocker full with plant available nutrients, soft, dark, rich, sweet and earthy! This is only achieved, in my opinion, by making it on the farm with a wide variety of ingredients. I am yet to find a commercial compost that meets my high standards.

We build two or three 2-3 m3 heaps every autumn (April/May in New Zealand) to process the “autumn mess” of plant residue, grass clippings, autumn leaves and woody herb stalks from the herb farm. Autumn storms provide washed up seaweed from the beach and the chicken house bedding gets replaced and the old one used in the compost. The compost matures over winter and is ready in spring. In addition to that we also make smaller heaps once a month to deal with ongoing food scraps, humanure, garden waste etc.

At KoruKai Herb Farm, we do 3 types of composting. We do Bokashi composting (most contents go into thermal compost pile after the 2 weeks of fermentation in the bucket), have a large worm farm and make thermal (hot) compost. We do not use cold piles, where organic matter is simply piled on top of each other throughout the year without the creation of heat. Three reasons for this a) seeds stay intact in static piles, b) plant diseases and animal pathogens are not destroyed, and finally c) it takes a lot longer than thermal composting.

In this article I will explain why compost is so beneficial for the garden and farm. Suitable materials for a thermal compost pile and their carbon/nitrogen ratios will be discussed. I will also explain how the heat develops in a pile and finally visual signs of a finished compost.

Benefits of Compost

The major benefit of compost is that it contains massive amounts of microorganisms that can restore poor soil and bring back a functioning soil food web. It also contains food for soil microbes and supplies plants with nutrients. The addition of high quality compost and therefore a functioning soil food web will improve the soil structure, draw carbon from the atmosphere into the soil, aerate and loosen up heavy clay soils, store more water, supply nutrient for the crops 24/7 and provide better drainage. Healthy, nutrient-dense plants that are resistant to diseases and pests are the end result of good compost.

Suitable Composting Materials and Carbon to Nitrogen ratios

The more variety the better! Each and every compost ingredient contains its own set of bacteria, fungi, protozoa, dormant cysts, spores and eggs – invisible to the naked eye, but clearly visible at 400x magnification with a microscope. The more variety of materials you use the more variety of food supply there is and the more variety of microorganisms will be growing in your pile.

There are three big groups of compost materials: high nitrogen, greens and browns.

  • High nitrogen are materials like seeds, meat, fresh manure, peas and bean plants and other legumes. Their carbon (C) to nitrogen (N) ratio is around 10:1
  • Greens are materials that have been cut green like grass clippings, green leaves, seaweed, food scraps and flowers. Their C:N ratio is 30:1
  • Browns are your woody materials like wood chips, bark, brown autumn leaves and straw. The C:N ratio of brown material is above 60:1

Be aware that the carbon/nitrogen ratio is not stable in plants throughout the year. The first spring flush of grass for example, is very high in nitrogen and other nutrients. This gets diluted in late spring/early summer as the plant grows and photosynthesizes adding carbon to its structure. When the grass flowers and sets seeds it concentrates the nutrients in the seeds, while taking them from other parts of the plants. As the season progresses and the plant has finished seed production, nutrients get concentrated in the roots and the rest is now very high in carbon.

The below table is a mere guide and states the carbon/nitrogen ratios of a range of compost materials. The gardener needs to consider seasonal fluctuations as explained above.

Again, I want to point out that the more variety of materials, the better compost you will produce. So aim for a variety of high nitrogen materials and not just manure and a variety of brown materials from different plants, shrubs and trees and not just straw. Commercial compost is often made with manure and straw or bark and makes a poor compost due to the lack of diversity of materials.

The high nitrogen materials are the party food, they will get the party started and heat up the pile. Use low amounts of these materials to not get the pile too hot and risk it going anaerobic and burst into flames. Around 10% is a good place to start.

The green materials contain simple sugars and are bacterial foods. Use between 20 and 35%. High carbon, woody material is predominantly fungal food because they have the necessary enzymes to break up these materials. Use around 55-70% in your pile.

Building a compost pile

The compost materials are kept separate and the heap gets constructed in one go. Most materials can be collected and stored over a few weeks or months and some need to be fresh like grass clippings. So mow the lawn the day before or the very same day of compost construction.

We start by breaking up the base layer of the compost area with a fork, to get it nice and loose to hold water and to encourage earthworms up into the heap once the thermal phase is over. Then we make a 20-30 cm base layer with branches and twigs to help with drainage and to get oxygen into the bottom of the pile.

Untreated pallets make a great composting bay.

Then the compost materials (high nitrogen, greens and browns) get layered like a lasagne, making sure the materials have a range of sizes and shapes to get adequate airflow happening in the pile to keep it aerobic at all times. You want materials to be chunky as well as small but not as small as sawdust. It is best to break up and chop dense materials so they can decompose over the 3-5 months that it takes for the compost to be ready.

Aim for a water content of 50-60% moisture and soak dry ingredients in buckets with water before using. To test your water content, grab a handful of compost material after the construction of the heap and make a fist, squeeze the materials really hard, you should get about 1-2 drops out, then you are at 50% moisture level. Activity stops when there is not enough water and you will get pockets with non composted materials and worst case scenario, nothing will happen at all. Also don’t overly wet the pile because we need oxygen to be available for the organisms at all times.

To get adequate heat development in the pile aim for a minimum size of 1 m3.

Finish off with a thick layer of brown materials like autumn leaves and shape it like a dome. This will provide insulation and sheds water. We have very wet winters in our region of New Zealand and need to also cover the top with a waterproof cover.

A completed and covered pile ready for some heat development.

Heat development in a pile

Initially you will get ambient temperature for 24 hours, then you will start to see activity happening as the temperature goes up from 10-20 °C on day one to 30-40 °C after 48 hours. Mesophilic organisms are starting to multiply and get active in the pile.

Within 4-6 days the temperature should go up to around 50-65 °C as thermophilic organisms like actinomycetes and spore forming bacteria start to take over and rapidly consume carbohydrates and proteins. Their frantic feeding and multiplying cycles creates the heat in the pile. You will get 1 million bacteria from one single bacterium within 24 hours as they divide every 20 minutes (1, 2, 4, 16, 256, 65.536 …. every 20 minutes). One gram of starting material already contains 600 million bacteria and they grow into 600 billion overnight, which releases heat, that we can measure.

You should measure the temperature once a day in 3 areas of the pile especially the center and take notes. It is crucial for learning to take place. Ask yourself these questions and note down observations:

  • How fast did the temperature go up?
  • How high did it get? And on what day was the peak?
  • How many days was the pile above 55 °C?

After about 1 week, when the thermophiles run out of food, the temperature decreases and mesophilic organisms wake up again and take over.

Do not allow your compost to go higher than 68/70 °C. You are burning off too much nitrogen and carbon can be lost as gas. The center of the pile is also getting anaerobic in those temperatures. It also kills beneficial organisms and can create unwanted chemical by-products. If your pile reaches this temperature, then turn the heap or open up the center asap. When it gets above 77 °C act immediately! This means you have used too much high nitrogen and green materials in your pile and not enough brown materials. Take action by turning it, take a note and learn from it for the next heap. A pile over 80 °C needs to be turned with utmost care as alcohols will have been created in the pile and the pile can catch fire when exposed to oxygen. Add woody materials as you turn the pile.

Constructing a thermal compost pile can be dangerous and is impossible in my opinion without a thermometer with a long probe. Without it the learning effect is zero and you won’t have a chance to improve your formula for next time. You don’t know if it got too hot and all your nitrogen, potassium, sulphur and carbon have been lost as gases, while strong acids have been produced along with alcohol and preservatives. You also don’t know if it has been heating up high enough for long enough to kill weed seeds, animal pathogens and plant diseases. For this you need to be above 55 °C for at least 3 days.

We struggled to find a compost thermometer in New Zealand when we started out 10 years ago. So we decided to get thermometers made to our specifications and are now selling them on our website to supply kiwis and help them with their composting adventure. You can order one here: KoruKai Compost Thermometer. Specifications, more info and a video on how to use it are on the website.

41 °C on day 3 of constructing the pile. Ticking along nicely.

The heap will cool slowly after the thermophilic phase and the temperature peak for 2-4 weeks and will slowly decrease its temperature until it is back at ambient. Then you get earthworms, mites, beetles, millipedes, bacterial and fungal feeding nematodes, protozoa and other critters moving into your pile to shred coarser plant material, consume bacteria and fungi and get nutrient cycling started by eating and getting eaten. The compost is ready after 3-5 months at ambient temperature.

To Turn or Not To Turn

Turning a heap needs to be done if temperature in the center goes above 68/70 °C.

We used to turn our heaps, but have observed that most of the time the heat is lost and it didn’t go back up after turning the pile. It is also a lot of work. We prefer to go easy on the high nitrogen materials so that it stays at around 60 °C. Our heaps are above 50 °C for 3-5 weeks with a peak of around 60 °C after about 7 days. The addition of lots of brown material and coarse materials throughout the heap ensures that it doesn’t get too hot and that there is enough oxygen throughout the pile.

We make sure that everything we need to get hot is in the middle area. We create a buffer zone of clean, disease and seed free material along the outside of it and prefer the temperature to stay above 55 °C for 3 weeks than turning it and loosing the heat altogether. With every turn you crush organisms and destroy fungal hypha, so we keep it to a minimum or none at all.

Below are some arguments for and against turning a pile.

Signs of a finished compost

I love it every time I open up the cover in spring and have a peek. The compost needs to have a dark, rich, colour, be friable, spongy and has a crumbly feel to it. It needs to be cool and not warm, and smell sweet and earthy like a forest floor. There should not be many original plant materials be visible, but you may find chunkier material or thicker branches. They can be put aside for the next pile or simply used in the garden to break down there.

From “Mess” to Compost at KoruKai Herb Farm, New Zealand

The “Mess”
The heap in autumn right after construction.
The finished compost in spring with the insulation layer still intact.
Beautiful, rich, dark, friable compost with heaps of worms present as well as some sticks still present,
but all other materials are humus.


  • Alda
    3 August, 2020 at 9:20 pm

    Thіs iѕ a topic that is cloѕe to my heart…

  • Daniel Eb
    28 May, 2020 at 7:21 am

    Very helpful, thanks Cornelia!


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