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Composting, or the production of decayed plant material which can be used in the garden, is probably one of the most talked about activities in gardening. Everybody has an opinion and many books have been written on the subject, there are even Web-sites devoted only to its discussion. The truth is that there is no 'right' way just a way which suits your own purposes. If left as a pile in the corner for long enough, plant material will break down with the aid of invertebrates, bacteria and fungi, to a crumbly compost suitable for digging into the garden. However there are a number of ways to reach the final product more quickly.
Most planting media are called 'compost' and although they may contain the product being discussed here, they are prepared with other additives to suit the job in hand. John Innes Composts have varying proportions of loam, peat, grit and nutrients determined by formulae developed at the John Innes Institute in the mid 1930s. Multipurpose Compost is mostly peat or peat substitute with added nutrients to last for about six months.
There is a movement which follows a method of cultivation known as Biodynamics. Followers of this method use lots of organic matter and make special Preparations to enhance it. Other manures are used and receive treatments with some of the Preparations as they are matured.
Another method to follow is Permaculture which is a more relaxed method, but uses lots of organic matter spread on the surface to develop better growing conditions.
During the composting process green matter is broken down by bacteria which use nitrogen - and if aerobic, oxygen as well (hence the need for good ventilation when building a compost holder) - decomposing the plant to obtain nourishment. The aerobic process releases heat and this is a useful by-product as it will kill many of the pathogens (disease-bearing organisms) and weed seeds present in the material. There are also anaerobic bacteria which do not need oxygen and do not produce heat, this process is much slower than the aerobic method and releases smelly gases - most commonly experienced with a heap of grass alone which settles into a smelly, wet mass as the oxygen becomes used up and there is no way for fresh air to enter. The way to encourage the quicker aerobic process is by mixing different types of material, turning it to add air, and insulating the heap to keep in the heat.
The woody material is broken down by fungi and takes the longest time. They also break down fallen leaves, from which the tree has extracted most of the nutrients - the fungi are obtaining energy captured during photosynthesis which they cannot carry out themselves. This process does not require heat, so producing leaf-mould is usually done separately and can be done in an open wire frame.
Moisture is also an essential for microbial activity. A 50:50 ratio of moisture to material is best - most is contained within the material. As an indicator a handful squeezed very hard should produce a drop or two of water. Every particle should have a thin film of moisture to allow the microbes to live and move around. Adding too much dry matter will slow down decomposition or even halt it, as can occur at the edges so turning helps to overcome this and some water can be added if needed.
The invertebrates involved include Worms, Woodlice, Millipedes and some species of Slugs. These detritovores are particularly good at breaking down the larger particles. Although they look similar, the Worms involved are not the same species which live out in the soil. The Brandling or Red Worms (Eisenia fetida) are smaller and can live in higher densities than their outdoor cousins; they are also more sensitive to temperature and can be killed if the heap becomes too cold.
A separate wormery can be used to process kitchen waste. This is a specialised method carried out in a bin designed to produce a liquid manure that is drawn off from the bottom and the upper layers slowly mature into a fine compost which is sterile and ideal for potting.
The size of material is a factor - the smaller, the faster decomposition occurs. So a shredder is a useful tool, but domestic models are rather tedious to use for large quantities. Running the lawnmower over the material works quite well, but care must be taken not to damage the machine. Picking up leaves with the mower yields a ready-made mixture with grass (the grass content should be about 10%).
The ratio of carbon to nitrogen (C:N ratio) in the heap is a controlling factor to decomposition as the microbes doing the work need nitrogen. Material high in carbon but low in nitrogen, e.g. straw or sawdust, will take longer to break down. Green matter is higher in nitrogen, but the addition of too much at once can result in a slimy compact goo, so it should be blended with drier material. Young nettle tops or comfrey leaves are high in nitrogen and other nutrients which their deep roots bring up from the soil. Nitrogen can also be added as ammonium sulphate, blood and bone meal, livestock manure (herbivores), a high nitrogen artificial fertilizer or urine (collect in a container and bring to the heap if the neighbours might be offended! An ideal solution is water and urine in a ratio of 10:1), also not when taking antibiotics as they will kill off the micro-organisms in the compost. Lime is not usually needed unless there is a high content of pine needles or vegetables and fruit, which increase the acidity. However the addition of lime with nitrogen will cause ammonia to be produced so losing the nitrogen - add separately after decomposition if needed.
The most practical construction for a domestic composting system is a fixed, E-shape structure of two (or more) containers side-by-side with removable front sections. This allows more rapid compost making, one is filled with layers of suitable material - it's best to make these layers at least 30cm deep with a mixture of material types. The second bin is to turn the first into, to keep it aerated and to adjust the moisture content if necessary. A third bin can be added to store the finished product or it can be bagged up. All three are roofed over to give better control of the moisture content - too much rain will slow the process down and leach nutrients from the mature compost. If it's too dry, water or 'used orange juice' can be added. A few layers of plastic on top of each heap keep in the moisture and heat. If the heap gets too wet and smelly try turning and adding some dry material, like straw or shredded paper.
(Above - an actual photograph of the web master's own compost bins made from recycled wood & palettes)
The contents of the conical plastic bins provided by some local councils should be turned as well. Lift the bin off, place beside the heap and refill. Add drier material or water depending on its condition. The compost at the bottom may be ready for use.
A way to allow air to the centre of large heaps is to add a chimney using a cylinder of chicken wire 15 to 20 cm in diameter, and tall enough so that it protrudes from the top. Overlap a few layers of wire to make it more rigid and build the material around it. In a really hot heap you should see plumes of hot air rising from it.
A mixture of grass clippings and shredded paper to control the moisture.
Composting is quicker in the summer due to the higher temperature and will probably cease in the winter. Any material which has not broken down enough first time around can be thrown back for another cycle. You can use a one or two bin system and let the earthworms do the turning for you, but this is a cool method and will take longer. With a one bin system the useful material will be at the bottom, so you will have to turn out the fresher material on top and use, or bag up the mature stuff, then refill.
Whatever system you use it's best to have the bins on soil and not a paved surface to allow excess moisture to drain away and give easy access to worms and soil bacteria. The bins can be built with concrete blocks or from timber, wooden pallets make ready-made walls, their slatted structure allows plenty of air through, but also lets the heat out. Some people have reservations about using treated timber due to leaching of the preservative, but the concentrations in modern wood are likely to be very low and not toxic to plants. Deciding when the compost is ready poses a question to which there is no definite answer, it can be used with some recognizable particles in it for soil conditioning and mulching. If you want potting compost then fully broken down, dark-brown, crumbly material is best, but this may take up to two years to form. Sieving less mature compost can yield quite a fine product and the retained material can be returned to be broken down further.
If all this seems too much but you still want to make compost, a stout black plastic bag can be used (about 150 litre capacity bin liner / rubble sack, or a multipurpose compost or bark chip bag inside-out). Fill with a suitable mix of material, add a tablespoonful of balanced fertilizer (Growmore or Fish Blood and Bone), 1 litre of water and a cupful of lime, to counteract the extra acidity of the anaerobic process. Fill in situ for ease of handling. Close tightly to exclude air and leave for about six months to a year. No attention is required and the bag can be concealed behind shrubs.
A similar plastic bag can be used to produce leaf-mould, but it needs to be perforated all over with a garden fork to admit air. The addition of a little nitrogen, some water to coat the leaves if they are dry, and the occasional shake, helps the process along. Grass clippings can be mixed in to add the nitrogen, this occurs by default if you use the mower to lift them, but leaves must be the majority component (about 90% leaves). Left for a year or two this should result in a useable product.
The types of leaves used can determine the length of the process, those from evergreen plants take longer to break down - as do beech leaves. They should be shredded first. Walnut leaves contain a small amount of a toxin called juglone which has allelopathic properties and suppresses growth of other plants, this should break down during the composting process, but it may be prudent not to add large quantities of them. Tip - fresh walnut leaves are a good Midge deterrent.
Another 'lazy' method is to dig a 60cm deep pit and fill with the green waste in two 15cm layers interlaced with soil. This will rot down over the next few months and makes a good planting spot for moisture-loving plants. It is an old method for preparing ground to plant runner beans, starting the previous autumn.
Quite often a compost heap is included in the design of a garden or starts as a pile of waste in a corner and just becomes an eyesore. With local authorities now having to recycle garden waste it is probably more suitable to let them do the work and buy the resulting material when required. Some use all of the compost in public parks and gardens, but excess material can be purchased. Others contract out the process and these companies sell it in bags or in lorry loads. This is not the greenest of policies, as carting such material and processing it uses fossil fuels, so if you are concerned about your Carbon Footprint making compost at home is the best option.
Larger scale compost heaps of the type for commercial production are known as windrows. Here the material is ground up and mixed to the correct proportions then laid out in rows which are turned by machinery which work along the heap. This can be a bucket type loader or there are special machines which straddle the row lifting and turning the material as they move slowly along.
In a large garden there can be enough green waste to have a small windrow-type setup as seen below.
Three heaps of green waste at different stages. The nearest pile has a covering of grass clippings over some drier dead stems from the perennial borders. The middle pile has been turned once. The furthest one is fully decomposed and ready for use.