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22 December 2020

How to make compost

Discover the best materials to put in a compost bin and improve the structure, health and vitality of your soil
Tom Morgan

There are many benefits to making your own compost. Adding the finished product to your flower and veg beds is the most natural way to improve the structure, health and vitality of your soil, which in turn improves the health and vitality of your plants.

Compost helps the environment by providing food for all the organisms involved in digesting the ingredients and then it provides food for the soil-dwelling organisms that continue the decomposition process.

It also helps the environment by allowing you to recycle what might otherwise go into council recycling bins. Kitchen scraps, garden prunings, leaves, grass clippings, and a lot of household paper and cardboard rubbish are all perfect ingredients for making compost.

What to put in a compost bin

The composting process requires a mix of soft, nitrogen-rich material and woodier, carbon-rich material to work properly. These ingredients need to be in roughly equal proportions, although it’s best to err on the side of slightly more brown than green material, rather than the other way around.

If the heap becomes very wet and slimy or starts to smell, you should try adding more brown material. Conversely, if it looks dry and isn’t breaking down, add some more green waste.

In general, reducing the size of the ingredients produces compost more quickly. So large kitchen scraps or deadheaded flower stems should be chopped up, pruned material should be shredded, cardboard should be torn up and paper should be scrunched. Toilet roll tubes and egg boxes can be added as they are. Don’t add eggshells, perennial weed roots, weed seed heads, dog faeces or cat litter, plants suffering from viral diseases, coal ash, bread or cooked food.

The compost is ready when it’s brown and looks like soil, even though there may still be some twiggy bits or lumps in it.

What is 'green material'?

This includes kitchen scraps (such as citrus peel, onions and rhubarb leaves), coffee grounds and tea leaves, weed foliage, deadheaded flowers and grass clippings.

What is 'brown material'?

Shredded prunings and hedge clippings, wood chips, wood ash, sawdust, thin card, cardboard and paper. If you have large amounts of anything, such as hedge clippings or ash, it’s best to add them gradually, making sure you mix them with enough green material each time.

How to start a compost bin

Composting works if you simply throw the right sort of things into a pile when you have them to hand and leave them to rot down, although it will take at least a year and the quality will be variable.

In one test, we showed that even by just turning the compost once every two months you can halve the time it takes to rot down. But other tests have revealed that a more systematic approach is what really pays dividends.

  1. Start by placing your compost bin or heap on the ground so worms and other organisms can get access.
  2. Collect and mix roughly equal quantities of green and brown material. You will need enough to make a 30cm-deep layer.
  3. Add a spadeful of compost from an existing heap to the first layer if you can, to help the composting microbes to quickly build up.
  4. After that, add 30cm layers of a 50:50 mix of green and brown material every week to 10 days.
  5. Water the compost to keep it moist if it starts to dry out and turn it or just mix it every month using a fork or a spiral aerating tool. This method should achieve rich, brown compost within six months.

If pre-mixing the green and brown material is difficult, we found that a more unconventional ‘lasagne method’ works surprisingly well. Start the heap or compost bin in the same way as described for the previous method, but add the green and brown material in alternate layers, each one only 10-15cm deep. Repeat this every week to 10 days. We didn’t mix or turn the heap, but we did keep it moist and lifted the layers with a garden fork to keep air circulating. After six months, the top couple of layers were still in their unrotten state, but everything under that had rotted down and produced good, crumbly compost.

How to compost grass clippings

If you have a large lawn and like to collect the clippings after mowing, it can amount to an awful lot of grass through the year. Grass becomes slimy as it rots if you add too much to your heap, and finding enough brown material to prevent that happening can be difficult.

Our tests have shown that adding dry autumn leaves, or a few spadefuls of topsoil in a 1:5 ratio, will create good compost in a few months. Adding a product called Complete Rot, made by Wiggly Wigglers, also provided enough carbon to help the grass clippings rot successfully.

How to compost leaves

Leaves descend on gardens in large quantities in autumn and some are slow to break down, which is why they are often composted on their own. We’ve found that shredding them more than doubles the speed of the process and can be done by running a lawnmower over them or collecting them in a leaf-shredder vacuum.

When we tested this, we put the shredded leaves into black plastic bags, moistened them and made drainage holes in the bags before storing them. We got perfect, crumbly leaf mould in less than a year, rather than the 18 months to two years it took for the unshredded leaves to break down. You can also make leaf mould in composting bins made from stakes and chicken wire. Air circulation would be better with this type of bin, but it’s harder to keep them damp if the weather is dry.

Do you need to turn compost?

The organisms involved in decomposition in a compost heap need air, so if the heap becomes compacted and airless, the process will slow down or stop. Keeping the ingredients mixed and aerated can involve emptying out your whole bin and refilling it, or turning the contents into another bin – both of which are hard work and difficult if you lack space. We found that mixing the composting material, lifting and turning it with a border fork to introduce air was just as effective as getting it all out.

We also tried a spiral compost mixing tool, which you screw down through the compost to introduce air. If you’ve got a closed bin with a relatively small opening at the top, this is even easier than using a fork and was effective.

We’ve also tried out tumbling compost bins in which compost is mixed by turning the bin with a handle, or sometimes by rolling the whole bin. It sounds simple, but our experience shows that once the compost starts to break down, these bins can become heavy and difficult to turn. Most of the ones we tried were slower at producing useable compost than much simpler (and cheaper) compost bins that were mixed manually.

What's the difference between 'hot' and 'cold' composting?

The difference comes from the construction of ‘hot composting’ bins, which are normally fully enclosed and made of insulating materials, the technique used for adding the ingredients and the amount of time it takes to rot down.

Creating conditions where the material heats up and then breaks down quickly at a high temperature requires the addition of a lot of material in one go, which should be well mixed with the right proportions of soft, green kitchen scraps, etc, and carbon-containing brown waste.

A hot compost bin can also be used to rot down a much wider range of waste, including cooked food and bones, which should be left out of a colder bin.

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