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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. The Multipurpose Compost wirely available in shops and garden centres 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. In the 'no-dig' method of garden cultivation the compost is spread on the surface and becomes incorporated in the soil during planting and by worms - though not so much where the dreaded flatworms have become established.
With 'no-dig' the compost is spread over the soil initially in a thick layer over thick paper or cardboard to suppress weeds, but in subsequent seasons in relatively thin layers. It is a common misconception that nutrients are leached from garden compost, but this is not the case, its main function is to improve the soil structure and feeding the microorganisms, so applying in the autumn or winter will not be a problem.

During the composting process green matter is broken down by invertebrates and microorganisms which use nitrogen - and if aerobic, oxygen as well - decomposing the material 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 is 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 and tend to clog up with softer green material. Running the lawnmower over it works quite well, but care must be taken not to damage the machine. Picking up leaves with the mower yeilds 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, eg. 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, 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.

Seaweed is an excellent material for the compost heap. It can be collected from the beach provided the owner has given permission, but only unattached material can be taken, anything growing on the rocks must be left alone. It has most of the essential nutrients and trace elements and because it is supported by the water, it does not have the tougher support structures of land-based plants so it breaks down quickly. The alginates from the decomposed seaweed bind with soil particles helping sandy soils to hold moisture and improve drainage in clay soils. It can even be put directly on the soil and in that case there is no need to wash off any salt residue - this may be necessary if adding large quantities to a compost heap. A liquid plant food can be prepared by decomposing it in water and the resultant concentrate can be diluted to make a foliar feed (about 1:10).

Suitable Material

    Leaves, grass, straw, non-woody trimmings, vegetable haulms (not blight-affected potato or tomato), kitchen waste, non-flowering annual weeds.
    Grass should be mixed with other material, eg. newspaper torn into 2.5cm strips (broad-sheets, downwards and tabloids, across), but keep the content below 10% to balance the C:N ratio.
    Junk mail can be used even with colour printing. There has been concern about heavy metals in inks, but modern ones are largely free of them and tests have found negligible traces. It is best to be shredded first.
    Current concerns about security make it essential to keep any letters or receipts out of the wrong hands so the product of a paper shredder will never be recovered if composted.
    Branches and twigs greater than 5mm should be shredded.
    Fresh wood ash can be added if kept below 1%, it adds potash, but it should be white and powdery as any charcoal will use up nitrogen.

Unsuitable Material

    Plant material which has recently been treated with a herbicide. There may still be residues which will be carried through to the finished product. (Recently a selective herbicide called Aminopyralid used on grazing fields has been found in manure and has been affecting plants grown in ground enriched with it.) Usually the herbicide denatures during the composting process, but this can take a year or more. Similar problems have been occurring with so-called peat-free or reduced peat potting composts where processed green waste material has been used as a substitute and the herbicide residue is still present.
    Diseased plants and weed seeds, as the temperature may not rise enough to destroy them, especially on the outside of the heap. This includes onion skins, even from shop-bought onions as they may have dormant white rot sclerotia and will contaminate any ground where the compost is used.
    Roots of perennial weeds, eg. dandelion, dock, couch grass, also some cultivated plants, eg. Crocosmia corms. Potato tubers can carry disease and will grow to produce lots of small ones which survive and become a nuisance. (You can leave such material for a few weeks to dry out in the sun, when they will be completely dead and compostable, for making anerobic a liquid fertilizer or use the plastic bag process described below).
    The rinds of citric fruit are very acidic and excessive amounts should be avoided.
      Coal ash is high in iron and sulphur, which may be toxic to plants. Also ash from bonfires may contain residues such as dioxins and heavy metals depending on what has been burnt.
      Glossy magazines - possibly toxic colourings (see above) and the coatings resist breakdown.
      Neat cooking grease, meat, dairy products and whole eggs will go rancid and attract vermin.(This material can be put in the green waste bin)
      Non-herbivore pet faeces (dog and cat), apart from the smell, could contain serious health hazards, eg. Toxoplasma.
      Human excrement (Humanure) can be composted in a special latrine, but it is a separate process for the dedicated and should not be included with garden compost.
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picture of a compost bin

The most practical construction for a domestic composting system is a fixed, E-shape structure of two 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 can be roofed over to give better control of the moisture content - too much rain will slow the process down. If it's too dry, water or urine can be added, the latter contains Nitrogen to feed the micro-organisms, and Phospherous. 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.
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 worms 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. If the bin is in an area with trees and shrubs it may be necessary to put some weed barrier fabric down to prevent the roots from growing into the compost, this is usually required for leaf mould heaps which stay around for a few years.
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 treated wood are likely to be very low and not toxic to plants - it doesn't last long when in contact with moist soil as seen with modern so-called treated fence posts.
Deciding when the compost is ready poses a question to which there is no definitive 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.

Compost Tea is not the same as the fluid from a wormery or liquid decomposition. Instead completed compost can be used to make the Tea by soaking it in water with regular stirring to add air. Other ingredients such as kelp extract and molasses can be added for a more complex 'brew', and a fish tank or pond aerator used to keep the mixture stirring. A recent study comparing compost tea, regular fertilizer and plain water, found that the compost tea produced sturdier stems and root development in grass. The control with just water had the weakest growth. The soil where the compost tea was used was found to retain water better than the other two treated areas. The greenkeepers at the Rael Madrid football ground use compost tea to feed the pitch.
The compost tea contains micro-organisms which help the plants to take up nutrients, and may protect against pathogens.

If all this seems too much but you still want to make compost there is an anerobic method which uses a stout black plastic bag. A 150 litre capacity bin liner / rubble sack, or a multiprupose compost or bark chip bag inside-out will do. 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. This method can be used to break down the perennial roots which cannot be added to the ordinary heap.

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 - 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 lorryloads. 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.

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