Hot Composting Summary

I'm not an expert on compost. These notes are compiled after researching truckloads of info and spending big bucks and too much time learning both the theory and practical sides of composting. Time also spent brewing compost teas and peering down microscope tubes has been very helpful too. I've made loads of dud piles including piles that neighbours would love to ask about but are too polite to do so and even piles that suffer the consequence of our antibiotic world we live in. However, I can now consistently (well, almost consistently) produce a high quality, safe to use compost. Compost that's almost good enough to eat.

For guiding me in this direction I thank the experts in this field of mass production of micro-organisms. If you too want to achieve similar compost results without the heart ache, find a comfy chair, grab yourself a coffee and read on.


Firstly, let's talk about how a plant feeds:
Nature is wonderful. The fact that plants survived for millions of years before the advent of garden centres is testimony to their ability to look after themselves. The way nature feeds plant life is to take carbon dioxide out of the air, moisture out of the soil and combine it with sunlight, to produce sugars in the form of carbohydrates. The plant sends about a third of these sugars underground to trade with the local microbes. Microbes eat the sugars on the plant root and leave behind the nutrients required by the plant. So, everyone is happy. The plant gets the nutrients and the microbes get their sugar fix. What compost does is boost the number and diversity of these microbes in the soil which in turn produces a smorgasbord of food for the plant. But good compost does more, a lot more. It can improve a plants resistance to disease, holds moisture and nutrients in the soil. It can tie up heavy metals, help stabilise soil pH and condition the soil by assisting the formation of a crumb structure. It promotes root growth which, in turn reduces soil erosion. It reduces the need for fertilisers by slowly releasing nutrients to the plant. In fact, it's blooming marvellous stuff!

WHY HOT COMPOSTING? (vs. other, less demanding types of compost eg static, sheet, cool etc.)

If correct temperature limits (see below) are observed, then the majority of weed seeds and plant / human pathogens (diseases) are eliminated. The high temperatures will also kill off root knot nematodes and other unwanted life travelling with the compost ingredients or "Feedstock".


For successful hot composting we must monitor temperature and moisture levels while maintaining adequate air to the whole pile.

Size is important.
Successful hot composting requires a minimum pile size of 1m3. This is the key to achieving and maintaining the temperatures necessary to kill weed seeds and pathogens (diseases).

Temperature is critical.
Above 70°C beneficial organisms will start to die. Below 55°C many pests, pathogens and seeds may survive. (See below for temp cycle)

Moisture level is critical
and should remain between 45-55% during the heat cycle. This is measured by taking a handful of compost and squeezing it hard:

De-composition of organic materials using "Hot Composting" must take place aerobically, ie, with sufficient oxygen present. If the compost becomes smelly it's a sure sign that anaerobic decomposition has taken place (insufficient oxygen available). "Smelly" probably means lost nutrients and the production of some nasty organic acids by anaerobic microbes. In severe anaerobic cases, the compost may become phytotoxic (kills plants). It is probably best to park this pile for at least a year or use the failed compost in a later compost pile. (Use it as "Brown" component in the next pile).


Remember, the reason you would make HOT compost as opposed to other forms is to:

If these are not your concern, then don't bother to make hot compost. It can be physically hard work. The common "cool" or "slow" compost may deliver a higher level of nutrients and disease suppressive organisms, however, it may also contain weed seeds and pathogens.


COMPOST BIN: - A cheap and effective compost bin can be made using a large sheet of wire mesh (approx 3 m x 1.6m) rolling it along the long edge. This will form a cylinder approx 1m diameter and 1.6m tall (when the ends are wired together) and have a volume approx 1 cubic metre. It is easy to untie the wire and re-locate the cylinder for re-filling when turning the pile. NOTE After each heating cycle the compost will probably be self-supporting, allowing the wire mesh to be removed and re-positioned. Alternatively, wooden pallets can also be used effectively.

Please note, black plastic compost bins are not suitable for hot composting for a number of reasons which we'll come to later but they are excellent for:

  1. pre-composting of materials before "Two-Phase" composting (see below)
  2. Storing or preparing ingredients for "compost-making day".
  3. Storing the finished compost (the bin just needs a good clean to ensure there is no contamination from raw manure or pathogens if you have been using them for this). Also, it's best to put them in the shade when storing compost.

90% SHADE CLOTH – for wind protection around the sides of the pile during composting. Shadecloth helps stop the outer edges of the pile drying due to wind / sun etc. The shade cloth also allows the pile to "breathe" which is very important.

SHEETING - or a well-supported tarp is required to keep rain off the pile. Ensure there is an air gap between the top and sides of the pile for air flow and that the sheet or tarp will not allow rain to drip on to your pile.

THERMOMETER (essential) – available from Ground Grocer.
It's worth getting one around 1m long to get well into the pile. I have seen an ingenious idea where a Bunnings el cheapo digital thermometer was used. The sensor was placed at the bottom of a long Alu tube with some heat-conducting paste. The difference in cost was about $30 vs $180!

RUBBER GLOVES – to test for moisture (squeeze test).

DUST MASK - is worth considering when turning the pile, as fungal spores become airborne and can lead to chest problems. I know this from first-hand experience!

PITCH FORK – sharp tines penetrate the pile much easier than a garden fork.

BIG SHOVEL - a nice lightweight aluminium job from a farm suppliers works well especially when the pile goes crumbly.

WATERING CAN OR HOSE with a fine rose to add water when turning the pile. (Note: chlorinated tap water should be left to aerate, preferably overnight to allow the chlorine to evaporate off. (the chlorine in tap water is there to kill microbes!).

NOTE: Tools used with raw manures should be cleaned before using on mature compost to avoid cross-contamination.


All compost ingredients contain carbon and nitrogen molecules and fall into three basic categories of Carbon to Nitrogen ratios, ie C-N ratios.

BROWN- C-N ratio is greater than 40 to 1
GREEN – C-N ratio is between 20 and 40 to 1
HI NITROGEN – C-N ratio is less than 20 to 1

The Carbon to Nitrogen ratio of each is always approximate and can vary considerably. (There's a stack of info available on the "Net" for C-N ratios).

WARNING: "Brown" refers to C-N ratio only, not the colour, ie pea hay, Lucerne and coffee grounds are all High Nitrogen even though they are brown in colour.

Typical recipe (by volume) for a hot compost pile is:

Approx 5% of the pile should be chunky material approx 25 mm dia unless using wheat straw. Weathered wood chips and straw are good for allowing air into the pile.

If you need help in calculating the C-N ratio of a new pile, then consider "Two-Phase" Composting (below) where we don't get too hung up on C-N ratios.

Moisten or soak straw before using (ideally with a weak solution of liquid kelp).

Use no more than 30% of any one individual material. The best composts are made with the greatest diversity of materials.

When making the pile you have the opportunity to add a few little extras: Consider adding rock dust (nutrients) or soft rock phosphate (helps reduce nitrogen losses during the hot phase), charcoal, or Zeolite. Crushed oyster or clam shells add a great calcium boost. Biodynamic preps can be added and seem to be very effective (but expensive). Add a bucket or two of forest topsoil (however, the majority of micro-organisms will not survive). Chuck in a few shovels of good compost, or add a variety of other composts – the key as we mentioned earlier is DIVERSITY. Note that pathogens are generally not a problem at this point as you are about to cook the crap out of them.

Add clay or a clay soil to the pile when constructing it. Clay is a very important addition. It will assist with moisture control during composting, greatly extends the life of your compost and most importantly, promotes the growth of Mycorrhizal fungi when the mature compost is added to soil. My personal preference is for kaolin clay over bentonite clay. There may be an upper threshold for the quantity of bentonite clay a soil can withstand. It is written that up to 10% of the pile can be clay, but I usually throw in around 1% by volume.

(NOTE: viruses are likely to survive even hot composting, so please don't add virus-infected plants to your pile).

If temperature (see below) is not achieved in the first 5 days of constructing the hot pile and the size and moisture content is good, add more Hi Nitrogen materials when you turn it. This should be distributed throughout the pile. One of the best sources of high nitrogen is lucerne hay (loose bale, not compressed) as it can add protozoa (these are excellent microbes to have in your soil).

Within reason, any organic material can be used but it may be best to avoid:





This is my favourite way to hot compost for the following reasons:


Ah! I'm so glad you asked that question. Firstly, get yourself at least half a cubic metre of woodchips and let it start to weather, preferably somewhere near where the Hot Pile will be built.

Two-phase composting entails a pre-composting phase of the "Greens" and "Hi Nitrogen" materials as a separate "Cool" composting activity. This can be done using a large plastic compost bin with a lid that allows some air to circulate (I like the 400 lt Gedeye bin). While this is happening, a pile of woodchips is being weathered.

Pre-composting can be done over an extended period (mine is usually about six months to a year. By using the plastic bin and adding small quantities, all manner of materials can be added without going seriously anaerobic or attracting cats, dogs and rats, all wanting to dig up the tasty bits which may include chicken carcases, cray shells, fish heads, guts, meat bones etc, etc, etc.).

The starting point for the "cool compost" is a plastic bin with around 300 mm deep compost from an old compost pile. This has already got a great suite of microbes raring to go.

As you slowly collect materials for this plastic bin, chop them with with a sharpened spade before adding to the plastic compost bin and turn these into the existing compost. (I chop mine in a 20 lt bucket or a 60 lt tub with 15 mm thick wooden board in the bottom)

Keep adding materials over an extended period until the bin is full, making sure not to add too much hi-nitrogen material in one go, ie the temperature in the bin should always be "cool" which is generally well below 50°C. If too much green or hi-nitrogen is added, it can go smelly as there is insufficient air getting into this type of bin. If you have added a bit too much then, "stir" the bin with your pitch fork to let air in and maybe add some straw. If you have larger quantities of material, consider several bins (I often have 3 or 4 bins on the go) and feed them evenly.

When sufficient pre-composted material is available, ie at least ½ m3 and your wood chips are well-aged (ie at least six months), you are ready to start building your hot pile.

For typical hot compost, add approx the same quantity of pre-composted material to the aged woodchips and around three-quarters of a bale of lucerne hay per cubic meter of hot compost materials. The best way to do this is to build the pile in thin layers about 15- 25 mm thick ie:

Moisture levels can be checked at any time by doing a "squeeze test" ie, squeeze a handful of the compost pile and no water should drip out. Squeezing the compost will sound like squeezing a damp sponge.

While building the pile in this manner, you get chance to introduce clay, zeolite, charcoal or other materials that are to be included but do not generate heat themselves.


All parts of the Hot Pile need to be subjected to sufficient temperature for sufficient time to kill most seeds and pathogens. This is known as the "Thermophylic Phase" and is between 55°C and 72°C. For the whole pile to be subjected to the correct temperature for an adequate duration, the pile requires turning. As a guideline, turn after the following time / temperatures:

NOTE: It may not be wise to extend these durations at temperature as the pile can become anaerobic quite quickly. However, forced air can be very helpful if the pile cannot be turned in time (See below).

If the correct mix of materials is used, the pile should reach temperature in 2-5 days.

Turning allows you to re-introduce oxygen that is rapidly being used by the aerobic microbes. It is also likely that additional moisture will be required at each turn of the pile. This is because air is travelling into and out of the pile and taking moisture out. This sometimes looks like steam coming from the pile.

NOTE: If the compost is a bit "hot", ie too much Hi Nitrogen or Green material has been added, it is not unusual to have to turn the pile every day or two during the first week or two. Turning frequency should reduce after the third week. More "Browns" can be added to increase the C-N ratio but this may lead to a doubling or trebling of the pile size to bring temperature down and still not have the desired effect!

To ensure the whole pile is subjected to the high temperatures, it must be turned a minimum of 3 times but ideally more during the thermophilic phase, each time putting the cold outer edges of the pile into the centre when re-building the pile.

When the temperature no longer rises significantly after turning the pile, the compost is now classified as "FINISHED" (and the temperature of the pile is now likely to be well below 40°C), there should be no real reason to turn the pile for the next three months. At these lower temperatures, fungi are doing some serious de-composition work. Turning the pile cuts them to pieces and is therefore best avoided unless the pile is drying out and moisture needs to be added. Three months after the compost has "FINISHED", the compost is now considered "MATURE". It should now be safe to use on your compost-loving plants and the temperature of the pile is close to, or just above ambient soil temperatures.


(There is less chance of compost going anaerobic once "MATURED" but it can be leached by heavy rains).


Q. My pile temperature just hit 70°C and looks like rapidly heading further north. What do I do?

A. Apart from hitting the panic button, there are several things you can try:


Q. I have a "You Beaut" black plastic compost bin. This is good, yes?

A. You can make compost with or without a bin but you will find it almost impossible to get proper temperature control in a plastic container less than 500 lts (½ m3). A common size for large plastic compost bins is less than 500 lts but plastic containers of any size are unlikely to allow sufficient air flow into a large pile. However, the "You Beaut" plastic bin is great for Two-Phase composting (see above) and storage (see below).

Q. My compost will not heat up

A. Are your piles big enough? (Minimum pile size: 1m³, preferably bigger). Is it too dry? Do you have too much "Brown" material or are your piles is too lose? Too much brown may not stop the compost reaching temperature but a very loose pile will. If you have used lots of branches or long stemmed, woody materials there may be a need to stand on the pile in the initial stages. If the branches are shredded, then definitely not.

Q. My compost stinks.

A. Don't use it on plants! Instead, if it's wet (very likely), store it somewhere to dry it a little and use it as the brown component in a future pile. OR: keep it for a year or so (until all the nasties have dissipated and it smells quite nice again). If the pile got very hot you've probably lost some of your nitrogen and other major nutrients. It could even contain alcohol or some nasty organic acids. Don't risk putting this on your plants for at least 6-12 months. If it got seriously hot and looks like ash, just don't risk using it at all. Maybe not even in another pile, it may be seriously alkaline. If in doubt, TEST IT! (see "Testing" below).


Q. My compost looks and smells so good I could eat it for breakfast.

A. Don't do it. It tastes like sh*t even if you add sugar.

NOTE: even good-looking compost can be harmful to plants. TEST IT! (see below)


This is definitely worth doing or you can set your garden back a long time with bad compost! (Been there, done that too!). There are a few ways to do this:

  1. Quick method: Put some MATURE compost in a sealed bag for a day or two. If it smells of ammonia when you re-open the bag then don't use it yet, it is not yet mature. Leave it a few months and re do the test.
  2. Sprout seeds: Get yourself two jars and some paper kitchen towel. Run some distilled water through your "MATURE" compost and collect the extract. Soak some seeds (cress is good as it's sensitive to bad compost) for an hour or so in the liquid extract then place these seeds on some tissue moistened with the extract. Next, do the same again but this time with distilled water only and compare germination rates.
  3. Grow something in it; Get a punnet of cheap tomato plants and pot up a couple in potting mix. Do the same for a couple more plants but add some mature compost to the potting mix ie about 30 -50%. See if the compost ones grow well.


Once fully matured, it's ready to go but this may take around SIX MONTHS or more from the day you built the pile ie approx 3 months to reach the "FINSHED" stage and another 3 months to reach the "MATURE" state.

Q. How much should I put on?

A. This is impossible to answer as it depends on many variables including your soil type, the plants you're growing, the organic matter in your soil etc. However a very rough guide would be 5 to 50 mm per year. Generally sandy soils require more compost than clay soils.

Q. When should I apply it?

A. If possible, apply 1 month before planting, in spring-time or immediately after harvest. Either lay it on the soil surface or rake lightly into the top 50 - 80 mm of soil. Both are effective but my preference is to incorporate it.

Q. Should I mulch over freshly laid compost?

A. Yes, the compost should be protected from drying in the sun. Leaving compost exposed to the sun is the best way to trash it. A good blanket of porous mulch is ideal.

Q. I'm about to plant my trees. Should I add compost to the planting hole?

A. Maybe not. It's possible the tree roots will want to stay in the hole with the yummy compost. This could eventually cause the tree roots to "girdle" and choke the tree. Instead, apply compost around the tree out to the drip line and mulch over it. (And preferably keep it off the trunk)

Q. Can I store my MATURE compost?

A. Yes, but don't forget to let it breathe (it's still very much alive) and keep it moist. A large pile can be stored using a wire frame with porous weed mat on the walls. Smaller piles can be stored in black plastic composting bins, preferably in the shade. Keep the rain off or nutrients will be leached from the pile before benefiting your soil. Also, make sure the storage bin and tools used are not contaminated with fresh manures.


  1. Consider running your mulch through a composting quick "hot" cycle. It will get rid of the nasties before they get to your garden. It can then be dried and bagged after the heat cycle if you wish to store it. (I wish I'd done this to a huge quantity of bio-dynamic hay I spread on my garden – masses of slugs and snails hatched out after it rained!).
  2. You can tailor the compost to your plants needs. An increase in "Browns" will bias the compost toward "fungal". This is more beneficial for perennials or trees. A bacterial-biased compost (ie one made with mostly "hi nitrogens" and "greens" will benefit your veggies. This is a complex but fascinating subject so you may wish to Google Elaine Ingham, she's a world authority on compost and compost teas. NOTE: The time to "maturity" of bacterial-biased compost can be much less than half that of fungal-biased compost.
  3. Use aerated compost tea to moisten your pile instead of water. This will reduce the time to get fully matured compost, especially bacterial-biased compost. (ref Dr Elaine Ingham of Soil Food Industries).
  4. Moisten your wood chip pile using diluted fish Hydrolysate (available from Sampi in Fremantle). Hydrolysate is a fungal food made from whole oily fish (often tuna) and will accelerate the breakdown of woody materials.
  5. The addition of soft rock phosphate will reduce the loss of nitrogen from the hot pile. It's a good idea to be aware of the phosphate content of your soil first. A mineral imbalance has the potential to lock up nutrients in your soil which means they will not be so accessible to the plant.
  6. The addition of clay maintains moisture in the pile, promotes the growth of mycorrhizal fungi (when the compost is used in the soil) and greatly extends the life of the compost.
  7. Larger piles mean you can incorporate a higher component of "browns" and still reach temperature. A higher "browns" content could mean better quality humus is produced.
  8. When building the pile, add 1% brown coal or 1 lt of humic acid per cubic meter of compost to supply around seventy trace minerals and a rich lode of humic and fulvic acid. (If you don't have this stuff or want to know what it is and how to use it, check out Nutri-Tech Solutions (NTS) in Queensland for Nutri-Mate Organic Humates™).
  9. Include up to 10% basalt crusher dust in the compost. Some experts suggest that basalt has a high paramagnetic property which means the Earth's magnetic field is enhanced in the soil which is said to be beneficial to soil life. Nutri-Tech Solutions in Queensland (NTS) indicate the paramagnetic effect can have a massive impact upon compost quality and has been shown to increase microbial subdivision by up to 400%. The compost will be more bioactive as a result and the paramagnetic effect of the rock dust will be transferred to your soil. Trace minerals in the rock dust are released more rapidly as a result of the enhanced bioactivity. All too technical for me! By the way, if this presses your buttons, NTS offer a free testing service if you have rock dust you would like checked for paramagnetic properties.
  10. Keep about 5-10% of your old pile to start the next one (Consider the Two-Phase composting method above). This will inoculate the next batch with very beneficial microbes.
  11. Swap composts with friends. Remember DIVERSITY is the key to building a comprehensive suite of microbial life. The diversity of these microbes will help feed your plant and protect it from disease.

If you really want to know more, apart from Dr Elaine Ingham's work (highly recommended), you may also wish to check out the following books:

"Teaming with Microbes" by Lowenfels and Lewis (ISBN 13: 978-1-60469-113-9) is a brilliant book that takes you into the underworld of microbial life and is an easy and fascinating read.

The "Rodale Book of Composting: (ISBN-12 978-0878579914) is stuffed full of good info but the last re-print was about 20 yrs ago and it's not a cheap book.

For an interesting and very comprehensive (but not too technical) book on compost, I found "The Complete Compost Gardening Guide" very good. It's by Pleasant and Martin (ISBN 978-1-58017-702-3).

And there are two URLs for Dr Ingham on the "Quick Links" page in the "More Info" section of the Rare Fruit Club's website.

Note: If you wish to do further research on composting make sure it is from a reputable source, eg universities and peer reviewed tech papers are good. You will come across a lot of folklore and stories not supported by research facts and often these can be misleading or confusing.

Good luck – You've now got the key essentials for successful hot composting.

Haydn Gunningham

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