Hot Composting

I first discovered the hot composting process a few years back when my local tree lopper dropped off a load of eucalypt mulch at the end of our street.  After a couple of days the 2 meter high pile was steaming with a pleasant aroma of eucalypt.  The pile was so hot that I had to be careful not to burn myself when handling the mulch a few feet deep!

hot compost

What is hot composting and how does it differ from other forms of composting?

Regular composting, which can also be referred to as cold composting is simply adding a mix of organic materials such as leaves, lawn clippings and kitchen scraps and leaving them heaped on top of each other (usually in a compost bin) for a number of months until the mixture breaks down.  This can be a very slow process of making your own compost and if the mix of materials is not right, it can also lead to a very smelly compost bin that attracts rodents and fruit fly. The process of breaking down the material can be hastened by mixing the heap up a little to help get air into the mix, but if the temperatures do not heat up enough then it can still take a long time to break down the materials.  Often the final mix of compost will have large pieces in the mixture and will break down in volume significantly from the original mix added.

Hot composting produces usable, fine compost in a significantly shorter time than cold composting (sometimes as short as 3-4 weeks).  The final mix is only slightly less volume to what was added initially.  It works on the process of providing the right mix of carbon to nitrogen and constantly mixing the pile to aerate the mix which aids in the process of breaking down the organic matter. As a result, the high temperatures generated in the heap kill off weed seeds and also plant diseases. Cold composting does not destroy the weed seeds or diseases and this is why it is often recommended not to add any weeds for diseased plants to your compost bins.

How do you make a hot composting pile?

Firstly, you need to have all the ingredients ready to mix together, as adding compost material over time can delay the process.  A good way to collect materials is to buy some straw, collect some manure, keep your lawn and garden clippings in a pile and store your kitchen scraps in some buckets.  This should give you enough ingredients to make a decent sized heap.

The recommendations for the Carbon / Nitrogen ratio vary from 20 to 30 parts carbon to 1 part nitrogen by weight (materials will be discussed later).  If mixed and maintained properly, this mix will generate temperatures of up to 65 degrees Celsius.

Keep the pile about 1 meter wide by 1 meter length by 1.5 meters height.

Make sure all the materials you are adding are well broken up.  For example, clippings and braches from your garden should be chopped up into less than 1 inch lengths.  Paper and cardboard should be shredded and chopped up to avoid clumping of the materials.

Build the compost heap in layers (5cm thick) of brown material such as leaves, straw, wood chips and green material such as kitchen scraps, coffee grinds, manure, lawn and tree clippings.  Make sure you water each layer lightly as you go.  Once you have reached the height of 1.5m, water well until water is dripping from the bottom of the pile.  Leave the pile to sit for 4 days to cook the inside of the heap.  After the 4 days turn the mix (outside of the pile to the inside) every couple of days for a few weeks.  After this time you should notice the inner temperature of the heap has dropped and the material should have broken down into a fine soil like mixture ready to be added to your garden.  The whole process should take about 3 weeks.

Some things to keep in mind:

Locate the compost bin in an area than does not get too much sun to prevent the pile from drying out too much.  You also do not want too much water, so keep the pile protected (such as covered with a tarp) if there is plenty of wet weather.  Use a garden fork to turn your heap over and take notice of the temperature and consistency of the mix when you are turning it over.  If the mixture looks too wet and cloggy, then add some brown material.  If the mixture looks too dry and is falling apart too easily, add some more green material.

Materials you can use in your hot composting heap

Brown materials tend to be high in carbon content and will break down slowly.  Green materials tend to be high in nitrogen content and break down more rapidly than brown materials.  The table below shows some common composting materials and their C/N ratios. 

Carbon-to-Nitrogen Ratios – Estimated
Browns C:N
Ashes, wood 25:1
Cardboard, shredded 350:1
Corn stalks 75:1
Fruit waste 35:1
Leaves 60:1
Newspaper, shredded 175:1
Peanut shells 35:1
Pine needles 80:1
Sawdust 325:1
Straw 75:1
Wood chips 400:1
Greens C:N
Alfalfa 12:1
Clover 23:1
Coffee grounds 20:1
Food waste 20:1
Garden waste 30:1
Grass clippings 20:1
Hay 25:1
Manures 15:1
Seaweed 19:1
Vegetable scraps 25:1
Weeds 30:1

The aim with your compost heap is to provide the right mix of low C/N ratio materials that break down quickly and high C/N ratio material that break down slowly to achieve at C:N ratio of 20-30 to 1.

One of the keys to making rich broken down compost which is high in nutrients is to add a small amount of manure (horse, sheep or cow) to each layer.  The advantage of this over adding the manure directly to the garden is that any seeds than remain in the manure will be killed off in the cooking process.  One of the issues I experienced earlier on when setting up my garden beds was the masses of weeds that were popping up out of the horse manure I was adding to the garden soil.  My plan now is to add this manure to the hot compost heap to kill off the seeds, prior to adding the mixture into the garden.

This method is definately cheaper than having to buy your own compost from the nursery and is a great way to recycle your unwanted kitchen scraps, paper and leaves.

Here are some pics of my own hot compost system.  This compost was ready after about 10-12 weeks.

IMAG1409 IMAG1410 IMAG1411 IMAG1412

I use the aerobin 400 for my hot composting system and fill with a good mix with brown and green material along with a few bags of horse manure.  My method is slightly different to the above mentioned methods.  I like to continue to add my bokashi mix to the top of the compost heap right up until a few weeks before the mix is ready.  The reason for this  is because the bokashi mix tends to break down really quickly (in a couple of weeks) and helps accelerate the composting process for the rest of the mix.