composting

factsheet

composting

what is it?

It is the breakdown (or decomposition) of organic material (anything that was once alive) in the presence of oxygen (i.e. aerobic decomposition). Organic material can also decompose without oxygen, but this is slower and smellier, and tends to be called anaerobic decomposition or digestion. The composting process involves many tiny organisms, including bacteria, fungi, insects and worms. These organisms utilise the two main components of organic waste – carbon and nitrogen – and work in a series of stages. Each group breaks down organic material a little more and converts it into a form suitable for the next group in the chain to act upon. The end result is a beautiful crumbly compost that contains a mix of minerals that plants can absorb as nutrients. There are many composting methods:

basic heap - a pile of material
enclosed containers (e.g. tumblers) - stop weed seeds blowing in
digesters - e.g. the Green Cone system
fermentation - e.g. the Bokashi system, suitable for flats
leaf mould - keep leaves in a bin liner or in a separate pile; they take longer to break down (up to two years) but contain minerals the tree obtained from deep in the ground, that may not be found at the surface
wormeries (also suitable for flats).

A heap (loose or contained) – is the most widely used and least laborious process, so we will focus on that. There are plenty of purpose-built composters on the market, many available at subsidised rates from your local council. It is also very easy to build your own from waste wood (e.g. pallets) and chicken wire.

You can compost:

garden waste / grass clippings
veg peelings and waste veg
tea bags and coffee grounds
eggshells
fruit waste, including citrus peel
paper (best if scrunched up)
cardboard, middles of toilet rolls, corks, matchsticks
human & pet hair
wood ash - contains potash (don't add too much though)

You can only compost with difficulty:

cooked food, meat, grease, bones, dairy produce - will attract vermin, but OK if you use the Bokashi method, Green Cone, wormery (only small amounts of animal products in a wormery), or if you make it vermin-proof with a strong container, car tyres, bricks etc.
perennial or pernicious weeds - OK with Bokashi, Green Cone or wormery, or keep them in a bin liner until they are sludge, then add them to the heap
diseased plants - again, OK with the above systems
contents of vaccuum cleaner - usually inorganic, but OK if you have a natural-fibre carpet
sanitary products - only if made from organic materials

You shouldn't compost:

dog / cat litter - can be pathogenic
coal ash - inorganic, won't break down
disposable nappies - contain inorganic materials
plastic, glass, metal, polythene bags - come on, you didn't really think you could, did you?

what are the benefits?

In nature, plants die, break down and return to the soil, but when we grow food, we remove a crop which is not allowed to return to the soil. So we have to add something else if we want the soil to remain fertile, and the best thing is compost. It is a wonderful soil improver, rich in nutrients, organic material and essential microbes to help your garden flourish. Other benefits are:

saves money on buying compost
don't need peat composts (destroy peat bog habitats)
reduces waste sent to landfill, so reduces harmful leachates (liquids) and methane (a potent greenhouse gas), and fuel required for trucks to transport it
reduces the need for garden bonfires (CO₂ emissions, plus it's anti-social)
increases biodiversity in your garden
improves the structure of soil, so that it holds more moisture without becoming waterlogged
don't need to buy energy-intensive chemical fertilizers
makes your food taste better – chemical fertilizers are water-soluble, so when the plants take in water, they also take in fertilizer, and grow large and watery; compost is not water-soluble
you can compost your confidential papers instead of shredding them!

what can I do?
Actually, you can't stop dead organic materials composting, so you don't have to do much really - you just have to organise your process so that it produces compost relatively quickly and easily without any odours. If you have even the smallest of gardens or back yards, it really is something you should be doing.
When choosing a site for composting, bear in mind that the process will be quicker in a sunny area, and directly on to soil. Composters can be placed in the shade or even on concrete providing there is drainage (add a few spades of earth at the bottom to introduce necessary micro-organisms), but the process won't be as fast.
It is important to include a roughly even mixture of ‘greens’ and ‘browns’. Greens are high in nitrogen and include vegetable matter and grass cuttings. Browns provide the carbon content - examples are dead leaves, small twigs, scrunched-up paper and cardboard. These browns are very important, as they also provide the structure of the heap. Without them the heap would be too compact, oxygen could become depleted, and the heap could start to degrade anaerobically (resulting in a slimy, smelly end-product, giving off methane, a greenhouse gas). So remember, when adding material, do so in alternating green / brown layers very roughly 10cm thick.
Other forms of aeration can help. Traditionally this involves ‘turning’ the heap, but that can be hard work or impossible if it is contained. Try pushing a broom handle through the centre of the heap and ‘stirring’ instead. Specialised tools for this task are available but not necessary.
Your heap should have approximately 50% moisture content and the consistency of a wrung-out sponge. If it is too dry, water it. If it is too wet, add more ‘browns’. By following these guidelines, you should have compost formed at the bottom of your heap after approximately 6-9 months. If you’re in a rush add nettles, comfrey leaves, chicken manure or urine, all of which are compost accelerators. Pre-shredding your material also speeds up the process (but uses energy).
If you want to help others to compost more effectively, or set up a community site, look out for Master Composter training courses. These are often run by your local authority and are free of charge. All they ask in return is that you spend 30 hours of your time promoting home composting.
For more complex composting strategies, see some of the books and websites in resources (below).

resources

more - information, books, links etc.

Thanks to Scarlett Penn for information.

printable version
of this factsheet (pdf)

you can make your own compost containers from waste wood such as pallets, or you can buy slatted wood containers like these (they look prettier). Note that there are two bins - one is added to until full, then it is allowed to break down while you fill the other one; when that one is full you empty the first one, and continue the cycle. The slats make it easy to empty the bins when the compost is ready

fresh organic matter recently added to the top of a compost bin - you can see fruit and vegetable peelings, eggshells, weeds, straw, paper and cardboard

if you don't make your own compost, but buy bag compost from a garden centre, make sure you don't contribute to the destruction of peat bogs by buying peat compost - buy peat-free compost instead (like this one)

factsheet