Regular composting, also known as cold composting, involves placing a variety of organic materials in a compost bin, enclosure, or even just in a large heap, and leaving it there until it breaks down several months later. It’s a very slow process and typically takes 6 to 12 months. It can be sped up by turning the compost, that is, moving around the material at the bottom of the heap to the top and vice versa to mix it up and get more oxygen in there, but it’s still a long wait. But there’s a better way to do composting…
The Difference Between Hot and Cold Composting
The other approach to composting is hot composting, which produces compost in a much shorter time. It will effectively destroy disease pathogens (such as powdery mildew on pumpkin leaves), weed seeds, weed roots (such as couch and kikuyu) and weeds which reproduce through root bulbs (such as oxalis). This process breaks down the material much better to produce a very fine compost.
By comparison, the slower cold composting methods will NOT kill disease pathogens or weed seeds and roots, so if this compost is put into the garden it may spread weeds and plant diseases, hence the common advice not to (cold) compost diseased plants.
The other issue with cold composting is that it produces a coarser compost, with lots of large pieces of the original materials left over in the compost when the process is completed, whereas hot compost looks like fine black humus (soil), and none of the original materials are distinguishable.
Hot composting is a fast aerobic process (uses oxygen), so given volume of compost materials produce almost the same volume of finished compost. In contrast, cold composting is slow anaerobic process (without oxygen), it’s a different chemical process, and as a result, nitrogen and carbon are lost to the atmosphere, which causes a reduction in the volume of compost to 20% of the original volume.
The Berkeley Hot Composting Method
The hot composting method, known as the Berkeley method, developed by the University of California, Berkley, is a fast, efficient, high-temperature, composting technique which will produce high quality compost in only 18 days.
The requirements for hot composting using the Berkley method are as follows:
- Compost temperature is maintained between 55-65 °C (131-149 °F)
- The C:N (carbon:nitrogen) balance in the composting materials is approximately 25-30:1
- The compost heap needs to be 1m x 1m (3′ x 3′) wide and roughly 1.5m (5′) high
- If composting material is high in carbon, such as tree branches, they need to be broken up, with a mulcher for example
- Compost is turned from outside to inside and vice versa to mix it thoroughly
With the 18-day Berkley method, the procedure is quite straightforward and can be summarised into three basic steps:
- Build compost heap
- 4days – no turning
- Then turn every 2nd day for 14 days
Detailed, step -by-step instructions of the Berkeley hot composting method are provided later in this article, but before we can begin composting , we need to get the right mix of materials into our compost!
Getting the Best Composting Material Carbon-Nitrogen Balance
In all composting, including the Berkeley hot composting method, the ratio of carbon to nitrogen in the compost materials needs to be between 25 to 30 parts carbon to one-part nitrogen by weight. This is because the bacteria responsible for the composting process require these two elements in those proportions to use as nutrients to construct their bodies as they grow, reproduce and multiply.
Materials that are high in carbon are typically dry, “brown” materials, such as sawdust, cardboard, dried leaves, straw, branches and other woody or fibrous materials that rot down very slowly.
Materials that are high in nitrogen are typically moist, “green” materials, such as lawn/grass clippings, fruit and vegetable scraps, animal manure and green leafy materials that rot down very quickly.
Many composting ingredients don’t have the ideal carbon to nitrogen ratio of 25-30:1. To make composting work, we get around this problem by mixing high carbon materials which break down very slowly, with high nitrogen materials which decompose very quickly, in order to create the right balance.
The nitrogen content of composting materials is denoted by the carbon to nitrogen ratio (C:N ratio) assigned to them, as detailed in the tables in the next section. Before we examine those, let’s have look some quick examples to understand how C:N ratios work..
- Materials high in nitrogen, which decompose very quickly, such as fish, which have a C:N ratio of 7:1, have a very low C:N ratio .
- Materials low in nitrogen, which break down very slowly, and need to be broken up to be used, such as tree branches, which have a C:N ratio of of 500:1, have a very high C:N ratio
The rationale for mixing ingredients is as follows.
If the C:N ratio in our composting materials is too high, meaning we don’t have enough nitrogen and too much carbon, we can lower the C:N ratio by adding manure or grass clippings, which are high in nitrogen.
If the C:N ratio in our composting materials is too low, meaning we have too much nitrogen, we you can raise the C:N ratio by adding cardboard, dry leaves, sawdust or wood chips, which are high in carbon.
When trying to understand C:N ratios, it may helpful to point out that all plants have more carbon than nitrogen in them (remember, they get their carbon from the carbon dioxide in the air) so that’s why the C:N ratios of plant material is always greater than 20:1.
Below are the average C:N ratios for some common organic materials used for composting
Carbon-Nitrogen (C:N) Ratios of Common Composting Materials
Here is a handy list of composting materials with their respective carbon to nitrogen, or C:N ratios.
The materials at the top of the list contain higher amounts of carbon, but are low in nitrogen, and are considered ‘browns‘.
As we move down the list, the nitrogen content increases, and the materials at the bottom of the list contain higher amounts of nitrogen, and are considered ‘greens‘.
| Browns = High Carbon | C:N |
| Wood chips | 400:1 |
| Cardboard, shredded | 350:1 |
| Sawdust | 325:1 |
| Newspaper, shredded | 175:1 |
| Pine needles | 80:1 |
| Corn stalks | 75:1 |
| Straw | 75:1 |
| Leaves | 60:1 |
| Fruit waste | 35:1 |
| Peanut shells | 35:1 |
| Ashes, wood | 25:1 |
| Greens = High Nitrogen | C:N |
| Garden waste | 30:1 |
| Weeds | 30:1 |
| Green Wood | 25:1 |
| Hay | 25:1 |
| Vegetable scraps | 25:1 |
| Clover | 23:1 |
| Coffee grounds | 20:1 |
| Food waste | 20:1 |
| Grass clippings | 20:1 |
| Seaweed | 19:1 |
| Horse Manure | 18:1 |
| Cow Manure | 16:1 |
| Alfalfa | 12:1 |
| Chicken Manure | 12:1 |
| Pigeon Manure | 10:1 |
| Fish | 7:1 |
| Urine | 1:1 |
What Materials Can Be Composted?
Anything that was once living can be hot composted – and I really do mean anything. All manner of things, including unusual items such as wool and cotton clothing, bones, leather boots (with leather soles).
Some farmers who use the hot compost method even place a fresh animal roadkill into their hot compost heaps (they have to go in the very centre of the hot compost heap to break down properly) because they are a high nitrogen source, and they find nothing but clean bones when the compost is ready. Not a good idea for urban areas though!
It’s best to use a variety of different ingredients in the compost, as this provides an input of a wider range of nutrients, and produces a richer compost.
There are many organic materials that can be composted, and there are also certain ingredients that should never be put into a compost bin. This is subject is a whole article in itself, so if you want more information, here is a link to a list of what materials should and shouldn’t go into your compost bin.
The Easiest Way to Mix Compost Materials for the Right C:N Ratio
Some gardeners are perfectionists and try to use some very complex mathematics to calculate the exact proportions of each ingredient they’re using to arrive at the ideal C:N ration of 25-30:1 by weight. This is totally unnecessary, and there’s a very simple alternative that works great, which is a measure by volume.
The One Bucket Greens, Two Buckets Browns Method
If ratios seem too complicated or confusing (which they are), you can work with volumes of ingredients instead to simplify things.
- Use 1/3 ‘greens’ (nitrogen containing) materials with 2/3 ‘browns’ (dry carbon materials).
Or to put it another way, which may be easier to understand:
- Add one bucket of nitrogen-rich material to every two buckets of dry carbon-containing material.
For example, using this method we could use 1/3 Manure and 2/3 dry carbon materials to start a hot compost pile and it will work. Alternating thin layers of greens and browns are laid down until the compost heap is 1 metre (3 foot) square and a bit taller than that.
There’s no real need to get caught up in the mathematics of precise C:N ratios for succesful hot composting. It’s more a matter of trying out the process by following the instructions below, and it really is quite easy.
Hot Composting in 18 Days, Step By Step Instructions
The following instruction detail the steps required to build a Berkeley hot composting system which will produce finished compost in around 18 days.
DAY 1 – Construct Compost Pile, Let it Sit for 4 days
- Mix together ingredients by laying then in alternating thin layers of “greens” and “browns”.
- Wet the compost heap down very well so it is dripping water out of the bottom and is saturated.
- Let the compost pile sit for 4 days (this day and three more days), don’t turn it.
- Tip: A compost activator such as comfrey, nettle or yarrow plants, animal or fish material, urine, or old compost, can be placed in the middle of compost heap to start off composting process.
DAY 5 – Turn Compost Pile, Let it Sit for a Day
- Turn the compost heap over, turning the outside to the inside, and the inside to the outside. To explain how to do this, when turning the compost, move the outside of the pile to a spot next to it, and keep moving material from the outside to the new pile. When the turning is completed, all the material that was inside the pile will be outside and vice versa.
- Ensure that moisture stays constant. To test, put gloves on and squeeze a handful of the compost materials, which should only release one drop of water, or almost drips a drop.
- On the next day, let the compost pile sit, don’t turn it.
- TIP: If the compost pile gets too wet, spread it down, or open a hole about 7-10cm (3-4”) wide with the handle of the pitchfork, or put sticks underneath for drainage.
DAY 7 & DAY 9 – Measure Temperature, Turn Compost, Let it Sit for a Day
- Measure the temperature at the core of the compost heap.The compost heap should reach its maximum temperature on these days. As an simple guideline, if a person can put their arm into the compost up to the elbow, then it is not at 50 degrees Celsius, and is not hot enough. Best to use a compost thermometer or a cake thermometer for this purpose.The hot composting process needs to reach an optimum temperature of 55-65 °C (131-149 °F).At temperatures over 65 °C (149 °F), a white “mould” spreads through the compost, which is actually some kind of anaerobic thermophilic composting bacteria, often incorrectly referred to as ‘fire blight’. This bacteria appears when the compost gets too hot, over 65 °C and short of oxygen, and it disappears when the temperature drops and aerobic composting bacteria take over once again.Temperature peaks at 6-8 days and gradually cools down by day 18.
- Turn the compost heap over every second day (on day 7 and again on day 9).
- Allow the compost to rest for on the next day after turning it.
- TIP: If the compost pile starts coming down in size quickly, there is too much nitrogen in the compost.
- TIP: To heat up the compost faster, a handful of blood & bone fertiliser per pitchfork when turning speeds it up.
- TIP: If it gets too hot and smelly and goes down in size, it has too much nitrogen, need to slow it down, throw in a handful of sawdust per pitchfork when turning.
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DAY 11, 13, 15 and 17 – Turn Compost, Let it Sit for a Day
- Continue to turn the compost every 2nd day (on days 11, 13, 15 and again on day 17).
- Allow the compost to rest for a day after turning it.
DAY 18 – Compost Completed, Ready to Harvest
- Harvest completed compost, which will be warm, dark brown, and smell good.
- Congratulate yourself for a job well done!
- TIP: When the earthworms move into the compost, it’s a sign that it’s finished and ready, because it’s cooled down enough for them and they’re in there because it’s full of nutrients!
Some important points to note:
- Locate the compost heap in an area which is protected from too much sun to prevent the compost from drying out, or from heavy rain to avoid water-logging, as both extreme conditions will slow down the composting process.
- Space required for for your heap should be about 1.5 x 1.5 metres (5′ x 5′), and enough space in front of it to stand when turning the compost.
- Water each layer until it is moist as you build the heap. After three or four days, give the compost air by mixing and turning it over, then turn every two days until the compost is ready, usually in 14-21 days. Remember, frequent turning and aeration is the secret of successful composting.
- Turn the compost using a garden fork, or even better, a long-handled pitchfork.
- In cold or wet weather, cover the compost heap with a tarp or plastic sheet, to prevent the rain cooling it down, since the water will penetrate into the core of the compost pile. Even though cold outside air will cool the surface, but not the core of the compost heap, by covering it, this prevents some heat loss from the surface to cooler outside air, and retains the heat within the compost heap better.
Is My Garden Too Small for Hot Composting?
A full–sized hot compost pile can be made successfully in a small courtyard, I know from experience!
The first time I tried hot composting was assisting a friend with only a small courtyard in a rental property, who had never tried this process before. For composting materials, he gathered a wheelie bin full of fallen leaves from his local street, a second wheelie bin full of weeds from his garden, and he also purchased a small straw bale for the sake of it. I also helped him collect a few garbage bags of cow manure from an urban farm. It took us under an hour to pile up all the materials in reasonably thin layers of less than 5cm (2″) to build the compost heap.
Even though it was his first attempt at hot composting, and in around 18 days, he had over 1 cubic metre of rich, dark, compost to use in his garden. None of the original ingredients could be identified in the final product either, it had a very fine consistency. Best of all, it cost him next to nothing – the straw bale was the only item purchased, and that was more of a gratuitous addition, as the hot compost would have worked just as well without it.
Considering that a hot compost pile doesn’t really reduce in volume, the biggest issue in small yards and gardens is figuring out what to do with such a large volume of high-quality compost!
Ways to Use Compost in the Garden
Wondering what to do with over a cubic metre of freshly made compost?
- It can be used to improve your soil by digging it through your garden beds.
- Don’t like digging? Use the compost to start a no-dig garden with the no-dig gardening method, which is my personal preference!
- Compost should be always mixed into the soil to improve drainage in heavy clay soils, and to improve water retention in sandy soils when planting new trees.
These are just a few ideas to get things started. Happy composting!