Thermal Composting

Thermal Composting

Thermal Composting 1280 960 Cornelia Holten

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, herb stalks etc.

At KoruKai Herb Farm, we do 4 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, make thermal (hot) compost and build 1-2 Johnson Su bioreactors. With such a variety of methods we are creating an enormous variety of microorganisms, which benefit the health of our soil and contribute to resilience in our system. All of our inputs are checked under the microscope and are combined and used in various ways to suit a variety of crops from annual vegetables to herbs and fruit trees.

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 Bio-Complete Compost

The major benefit of bio-complete 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, bio-complete compost will improve the soil structure, draw carbon and nitrogen from the atmosphere into the soil, aerate and loosen up heavy clay soils, store more water, supply nutrients 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.

A note on store-bought compost

Unfortunately what is labelled and sold as compost is not always good compost, but “putrefied organic matter” as Dr Elaine Ingham would call it. The key is in the material going into a pile and the ratio of those materials. You don’t want to add to much manure or fresh grass clippings as the pile may reach too high temperatures and risk going anaerobic. As soon as the pile goes anaerobic, pathogens and anaerobic bacteria take over and our good guys go to sleep or die. The pH drops to very low levels and alcohols are produced. The nutrients within the plant material go up into the air as gasses and are lost from the compost.

When shopping for compost or potting mix you want to be able to do a sniff test. If that material smells revolting you do not want to touch it or use it for your plants. If the sales person tells you that you cannot use the compost as a potting mix for plants as the plants will die, then don’t buy it. Why would plants die if the product sold to you is good compost? In my 10 years of gardening I have never had one plant die from the compost produced on the farm, they only thrive and the more you give them the better they perform. That’s good compost!

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 building process

The compost materials (high nitrogen, greens and browns) get layered like a lasagna, 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. 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.

Reading your Thermometer & Heat development in a 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: BioKai Compost Thermometer.

57°C on day 5 of constructing the pile. Reaching the “green zone”! Ticking along nicely.

You should measure the temperature once a day in 3 areas of the pile especially the center (see video below) 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?

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.

AMBIENT (BLUE): 20-30 °C

LOW (YELLOW): 30-40 °C


HOT (GREEN): 55-68 °C

TURN (RED): 68 – 74 °C

The BioKai Compost Thermometer has 5 color coded temperature zones. When purchasing a thermometer, you will get a leaflet explaining the 5 zones and helping you in your decision making if turning a pile is necessary or not. The leaflet also lists the three groups of composting materials.

After about 1-3 weeks, when the thermophiles run out of food, the temperature decreases and mesophilic organisms wake up again and take over. This process takes a few weeks and the temperature may decrease quite slowly. As long as it’s decreasing you are all sweet and can stop taking temperatures every day. 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.

Compost Classes Online and On-farm

We run composting workshops on the farm every autumn and also offer training online so you can create you own bio-complete compost and grow amazing plants. We teach you about measuring water content, suitable compost materials, carbon/nitrogen ratios, visual signs of a good compost, how to brew your own compost tea and make compost extract, worm farming and much more.

Check out our website for upcoming classes.

From “Mess” to Bio-Complete Compost in 3-6 months 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.

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