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 (which can produce biogas).
The composting process involves 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. Insects, worms and other visible creatures break down organic material into a form suitable for microscopic organisms 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)
- composting using other critters
A heap (loose or contained) – is the most widely used and least laborious process. 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.
- garden waste / grass cuttings
- veg peel and waste veg
- tea bags and coffee grounds
- 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)
Compostable 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
- coal ash – won’t break down
- disposable nappies – contain inorganic materials
- plastic, glass, metal, polythene bags – come on, you didn’t really think they were, did you?
Vegetarian pet waste – fine; but most advice about dog or cat faeces will be to bin it rather than compost it, due to the risk of disease. However, landfilling dog or cat waste could lead to higher risks than home composting, as it may leach into groundwater, and it won’t break down very quickly in landfill because of the lack of oxygen. This is for experienced composters only however – and the key is allowing the compost to break down for longer, and using the compost on trees or bushes rather than veg. See here for more information and advice.
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’s a wonderful soil improver, rich in nutrients, organic material and essential microbes to help your garden flourish.
Root systems of plants ‘grab’ the nutrients they need from composted materials in the soil, and from the air (nitrogen, with leguminous plants). Chemical fertilisers, on the other hand, are salts that are entirely water soluble, and their use causes several problems:
- plants have to take them up when they take up water, so they become big and tasteless
- plants don’t have to ‘work’ for their nutrients, so their root systems become weak and unhealthy
- because they are soluble, they leach from the land and cause pollution problems in watercourses
- they don’t add to soil structure, and so begin a downward spiral that requires more and more fertiliser
- they only contain nitrogen, phosphorus and potassium (N,P,K), not the large range of nutrients that compost does – like calcium, boron, magnesium etc, so plants suffer in the long-run
- the salts can kill some of the bacteria and fungi responsible for transferring nutrients to the plants, requiring ever more chemical fertiliser.
Other benefits of composting are:
- saves money on buying compost
- don’t need peat composts (which 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
- 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 think about 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’s 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’s too dry, water it. If it’s 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, you could look into community composting, which may involve training for larger-scale composting and waste collection, and/or using the finished compost in public areas.
For more complex composting strategies, there are specialist books and websites. Finally, remember that there’s no such thing as ‘good compost’ – if it’s not good, it’s not compost.
Thanks to Scarlett Penn of WWOOF UK for information.
The specialist(s) below will respond to queries on this topic. Please comment in the box at the bottom of the page.
Sarah is a Master Composter and spent 10 years as volunteer Community Compost adviser with Garden Organic and her local Council. She lives on a 4 acre smallholding in rural Herefordshire, runs loads of interesting courses and is an enthusiastic blogger.
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