Wednesday, 16 March 2011

How to build a cold compost heap

Garden composting is an important topic; there are basically three types of compost heap:

•    Hot compost heaps
•    Cold compost heaps
•    Slimy compost heaps.

In this garden, we tend to prefer the cold compost heap. There are lots of articles on the internet about how to do it properly. The basic process is to try to get layers of different materials, together, such as grass, leaves, household waste, and anything other than perennial weed roots or seed heads that comes out of the garden. You are of course supposed to turn them.

If you do everything properly, you get a compost heat that starts to warm up enough so that it generates enough heat to kill of any seeds. This is a hot compost heap. In fact, some large compost heaps are so hot you can see steam coming off them in the winter.

There are a lot of instructions on how to make a hot compost heap on the web. So I won’t go into any detail.

There are far fewer to how to make a cold compost heap, because they are not common in back gardens.

A cold compost heap is similar to the hot compost heap, in that it will slowly start to rot down. Compost will eventually be produced. It just isn’t hot enough to kill seeds. A hot compost heap can compost waste within weeks. A cold one may take six months to two years. Now, most people say not to use cold compost heaps. And it is true that if you have waste matter with seeds in, or a small garden, a cold compost heap is a waste of space. When you have as much room as we do, a cold compost heap can be great. You leave it for a couple of years, slowly build it up, and go on to the next one.

It is much less hassle as you are less dependent on getting the mix right, and if you don’t mind it taking a year or two to rot down, you don’t have to turn it either.

A slimy compost heap is one where you have got the balance completely wrong. I would say most grass piles come into this category. If you pile grass up, it forms very thick layers that will bead down, and will rot very slowly. The resulting mess turns into a slimy pile. It is disgusting. It does still rot down eventually, but is not at all pleasant in the short term.

Trying to get the balance right so you don’t go from a cold compost heap to a slimy one involves two things:

•    Making sure the mix is right

•    Keeping it from getting too wet

How do you make sure the mix is right? Use only thin layers of grass, and for each layer of grass, make sure you put another layer of leaves, twigs or stems. Intersperse with vegetable waste from your garden. If you have roughly equal amounts of grass and other garden waste, each in a thin layer, you are much less likely to get a slimy compost heap.

The second requirement is that the compost heap should be a little bit damp, but not too damp. The traditional allotment solution is to cover most of the heap with an old carpet. But you can use cardboard boxes or even plastic bags as long as you make sure there are some holes so the heap can breathe. You don’t want to cover it entirely. It does need some water and air.

Cold compost heaps are largely rotted down by a mixture of some bacteria, but especially worms. While a hot compost heap works well even if it is not touching the soil, you really want to have a cold compost heap directly on the ground.

Now... there are also some extreme old fashioned things you can do to this heap. If you are a bloke, saving your pee is a traditional way to add nutrients. Just dilute it a bit, and chuck it on the heap. Make sure the heap is far away from neighbours. In the UK this is not legal, and is just a historical piece of advice.
People interested in permaculture often invest in a composting toilet too.

If you are not so environmentally friendly you can use just what the garden produces, plus household vegetable waste.  That’s what we do. It is easy, and we get great compost after a few years.

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