Balancing air and water

The decomposers in your heap breathe in oxygen and exhale carbon dioxide, so it is vital to keep air flowing through it. They also need moisture, and some microbes live in a thin film of water. To make compost successfully, create a balance of air and water by limiting the volume of moisture-rich materials, such as vegetables and grass, which can quickly lose structure and create inhospitable, airless conditions if added in excess.

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Turning the contents of your compost will introduce air but can be hard work.

Why is air important?

The composting creatures and microbes in your heap all need oxygen to work as you want them to. When all the air in a heap is used up, the invertebrates die or move out and the microorganisms are replaced by bacteria that respire anaerobically (without oxygen). These will continue to break down the waste material, albeit more slowly, but produce unwanted by-products, including methane, carbon dioxide, ammonia, and hydrogen sulphide. Methane and carbon dioxide are two of the greenhouse gases responsible for global warming, while ammonia and hydrogen sulphide are toxic and will produce bad odors that no one wants in their garden.

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Well-aerated heaps will draw large populations of earthworms.

How to keep oxygen flowing

A good way to keep your compost sweet and filled with air is to turn over the contents of your bin to introduce more oxygen. If you have the space and strength to do this, it will invigorate the various decomposers and help to produce compost quickly and efficiently. However, turning the materials can be hard work and it may also be difficult to achieve in a small garden; you will need to empty the bin every week or two, mix up all of the contents and then put them back. Alternatively, you could try just stirring the contents with a fork, but this too requires quite a lot of muscle-power.

If you have a smaller garden and not a lot of spare time or energy, a more practical option is to create air pockets in the heap by adding a range of woody or paper-based materials as you go along. Compost piles often become anaerobic when an excess of green waste, such as grass clippings or kitchen scraps, rots down quickly, forming an airless, soupy sludge. To prevent this, start off your heap with a layer of twigs or small branches on the base to allow air to filter in from the bottom. When you are filling up the bin, include more twigs, plus scrunched-up newspaper, cardboard boxes, the inner tubes of toilet tissue, and eggboxes, along with your green waste. These wood and paper materials inject oxygen into the mix, keeping the composters working well and preventing odors.

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Lining the base of a compost heap with twigs and small branches allows ventilation.

Water needs in the compost heap

Compost invertebrates and microbes require water to live (see Feeding the Decomposers), but how do you know whether your bin has the right amount of moisture for their needs? In a perfect compost heap, water will account for 50–60 percent of the total weight, which means that for every piece of dry cardboard you add, you need to include roughly the same quantity of water. This is obviously quite difficult to measure in reality, but there is a simple method of maintaining a good moisture content.

The first step is to ensure that your heap is well insulated so that water does not evaporate and dry out the contents too quickly. Then limit the volume of kitchen scraps, soft green garden waste, and grass clippings that you add at any one time, as they will introduce too much moisture and drive out the air. These materials are made up of about 80 percent water or more, which will be released into the heap as the decomposers get to work on them. Supplement your green stuff with porous brown materials such as paper and cardboard products—the same ingredients, in fact, that also help to aerate the bin.

You can test the moisture content of the heap by taking a handful of compost from the middle and squeezing it in your hand—a few drops of water should come out if it has the right amount. If the compost feels too wet, add more browns to it; if it is too dry, include more green materials (see also Recipes for Success).

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Apples and other fruits consist of more than 80 percent water, which is released when they are added to a compost heap.

Composting Know-How | BALANCING AIR AND WATER

EXCESS GREEN WASTE

If you have too much green waste, consider creating two bins where you have space and enough woody material to mix with it in the second one. Alternatively, leave it out for curbbside collection, if your town provides a green waste bin, or take it to a municipal recycling center. It will then be used to make compost or added to an industrial anaerobic digester, the gases from which are siphoned off for methane-rich biogas. This is used in combined heat and power (CHP) plants to produce electricity and heat, and no methane is released into the atmosphere.

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