Composting isn’t rocket science. Left to its own devices, anything that was once alive will rot down, it’s just a matter of time. We try to speed up that process, and to hone it so that we produce lovely, weed-free compost we can spread on the garden, and knowing something about how composting works makes it far more likely that we will be successful in doing so.
To start with, it’s important to think of a compost bin as a living thing, rather than a rubbish bin where you put your plant waste. In fact it’s not so much a living thing as an ecosystem in its own right – the key to successful composting is to ensure that the various organisms that live in the compost bin have what they need to do their job.
The organisms in your compost heap may not be building the Pyramids, but they still need to be well fed if they’re going to work hard for you! In fact, the most important thing for making good compost is to ensure those organisms get a balanced diet. They need a good source of carbon to give them the energy to get to work, and a source of nitrogen to enable them to reproduce – in the same way that we need carbohydrates and protein in our diet.
Savvy composters talk about the materials they add to their heap in terms of ‘browns’ – carbon rich materials – and ‘greens’ – things that are high in nitrogen. There’s an ideal ratio for composting, which is around 25:1 – 25 parts carbon to 1 part nitrogen. But without knowing the chemical composition of everything you throw on the compost heap, how do you make that work?
The answers is that you aim to put two parts ‘green’ waste (in terms of volume) into the heap for every ‘brown’ one, and that’s a good enough approximation.
Greens and browns
So what counts as green and what counts as brown? Well, plant material that is still green and fresh (such as cut grass) is an obvious green, and dead stems and bark are obvious ‘browns’, as is cardboard.
Other ‘greens’ include: kitchen waste – vegetable peelings, tea bags and coffee grounds – seaweed (if you live near the coast), fresh animal manure from chickens and vegetarian animals and human urine.
For ‘browns’ you can also add kitchen paper, shredded or scrunched paper, bedding from vegetarian pets, straw and hedge trimmings.
If you have something else you want to compost then think about how long it would take to rot down – if you think it would be quick then it’s more likely to be a green, and slower things would be more brown. It’s best to avoid cooked food waste, meat and dairy products in compost bins as they can pong and attract rats.
There’s plenty of help available on what you can and can’t add to the compost – Garden Organic’s list of what you can compost is a good place to start. If you’re new to composting then find out whethere there any local volunteer Master Composters who can give you advice.
Now that they’re well fed, we need to think about life’s other essentials for the organisms working in our compost heap. They need air, and they need water. A heap that gets too dry will compost very slowly, or not at all. The solution is to add some water – or pee! – to speed things up. What’s too dry? Well, you’re probably not going to want to stick your hand in there to find out, but if you’ve got a plastic compost bin you can gauge whether there’s any water in there by looking for condensation on the sides. And if you’ve got wood lice and ants crawling over the surface then it’s on the dry side.
A very wet compost bin is easy to spot – it’s likely to be smelly and slimy! Not nice at all. The cause (beyond leaving the lid off in the rain) is adding too many ‘greens’ – adding too many grass clippings in one go is the usual culprit. A lot of grass, or fresh weeds, or kitchen waste, will need to be balanced out with some extra browns – scrunched up newspaper or shredded paper is good for that job.
As well as adding too much water, adding a lot of ‘greens’ in one go – particularly grass cuttings – can squeeze out all the air. Proper composting is a process that takes place with oxygen; without oxygen you still get decay, but the end product is nasty and slimy and smelly. Scrunched up paper, corrugated cardboard and cardboard tubes can all add pockets of air (and ‘browns’) to help out.
The ‘black gold’ standard
If you want your compost to be all that it can be, then the hot composting method is the way to go. It works best in large gardens, which produce large volumes of waste to be composted, and which can find room (in a sunny spot) for 3 large composting bays. And it works best for people who have the time and energy to actively manage their composting process. You have to collect materials until you have enough to fill a bin at least 1 cubic metre in volume, with the right ratio of greens to browns. Once you’ve filled your bin you can sit back for a day or so – composting bacteria will go nuts in their new home and create heat, which will encourage decomposition. After a couple of days, though, they will run out of air and the heap will cool down. This is where you come in, turning the compost over into the next bay along, where it will heat up once again. Another couple of days and you turn the (much reduced volume) into the third bay and leave it for several months to finish composting down. The end result is lovely, weed-free compost your garden will love.
Hot composting is a batch process, and out of reach for most amateur gardeners these days, but there is an more accessible option. Cold composting involves adding new materials to the bin as and when they become available. Unless you’ve got a big batch of grass clippings, it turns out that kitchen and garden waste, mixed with waste paper and cardboard, does a pretty good job of providing a balanced diet. Adding it in small amounts doesn’t create the heat, but there are plenty of compost organisms that work at lower temperatures – they just take longer. You can treat composting this way as a batch process – filling one bin, then moving on to fill another whilst leaving the first to rot down – but in smaller spaces you can think of it more like a conveyor belt. Whilst you’re adding new material to the top, finished compost is appearing at the bottom, and can be harvested every few weeks.
Cold composting doesn’t kill weed seeds or disease spores, so avoid adding seeding weeds and diseased plant material to the heap. The finished product is still good for use anywhere in the garden. Although you haven’t invested as much time and energy in it, as soil improvers go it’s still worth its weight in gold!