Compost - beyond 'hot' and 'cold'
Composting is truly a revolutionary act. It helps us turn 'waste' into an invaluable garden resource and it builds soil health like nothing else.
Many of us are composting in our backyards using different systems and with different levels of success. At Patch of Plenty we have experimented with various systems in our gardens, and we've come across a different way of thinking about compost on the backyard scale which we think demystifies the whole thing. First of all - what is compost? Well it's not just a pile of waste left to rot (as happens in landfill).
According to legendary South Australian organic garden expert, Tim Marshall, it is ‘‘...the product of the biological decomposition of organic wastes under the correct conditions." (Marshall, Tim, ‘Recycle your garden, the essential guide to composting’, ABC Books, 2003).
'Biological' means it’s done by living organisms – bacteria and fungi (mycelia), worms, slaters, millipedes, mites and protozoa among many others.
'Correct conditions' means it has the right carbon to nitrogen (C:N) ratio (about 30:1), and adequate moisture and oxygen.
Compost results in humus – a dark, sweet smelling substance. It is a fantastic additive for soils. Among many things it:
helps soil hold on to water
holds on to nutrients and makes them available for plants
prevents erosion and leaching
improves soil structure – binds light sandy soils together and lightens heavy clay soil
helps soil to warm and is insulative
feeds the soil food web.
So how do we achieve this magic substance at home, instead of just always buying it in? (By the way, this is still a good option, as SA Composters' compost is made locally from your own green bin waste).
We often hear about 'hot' and 'cold' composting, but all compost is hot during some stages and cold at other stages. The level of heat is determined by which kinds of bacteria proliferate, and how fast compost is made.
However, all kinds of compost piles are decomposed by bacteria – just different kinds. We think ‘all-at-once composting’ and ‘gradual composting’ is a more useful way of thinking about it.
You build a pile of pre-collected materials in a heap or bin and you don’t add to it once it’s full. Generally made of garden wastes rather than kitchen scraps.
If the pile is large enough and built with the right materials, it will heat up. The optimal temperature range is about 45-65 degrees celsius. After heating up, it will cool down and can be turned where it will heat up again. Eventually it won’t continue to heat up, and can then be left to mature.
Because of heat: faster decomposition, more uniform compost, it kills pathogens and weed seeds but allows bacteria to thrive.
You get all of your humus at once, rather than a gradual harvest.
You need lots of space.
You need lots of material.
You may need to turn the heap.
You can’t add your kitchen scraps gradually.
A pile or bin which is made over time as you add small amounts of material. This bin will still heat up, but will not reach as high a temperature or sustain that temperature. Often called ‘cold’ composting.
Manage waste as it is produced
Harvest as you need
Don’t need to turn it as long as you have another way to get oxygen into the mix
Need to stop at some point to let pile mature properly – so need at least two piles/bins
Can you do both?
Yes, absolutely. You can have a dedicated bin or two for all at once composting and another bin or tumbler that you use for gradual composting.
Other tips for success with composting:
Don't get too hung up on the carbon:nitrogeon ratio. Just add something dry and brown with anything green.
Make sure you chop up your materials as small as possible before adding to your pile.
Keep your pile moist in summer, and not too wet in winter.
Turning your pile will speed things up.
If you don't turn your pile, you will need to aerate it some other way. You can use a slotted pipe stuck through the pile to do this.
There's so much to know about compost. Here's some more resources:
‘Recycle Your Garden - The essential guide to composting’, Tim Marshall, ABC Books
Hannah Moloney’s ‘Home Composting Guide’
Composting with Gardening Australia