Barfield Allotments

Whetstone, London N20

Composting on allotments

Why compost?

  • Recycling saves resources and money.
  • Replaces nutrients contained in crops when taken away for food.
  • Adding chemical fertilizer is no substitute. It doesn’t improve soil.
  • An old gardener’s adage is “Feed the soil not the plants”.
  • Composting improves soil structure and composition.

What is Soil?

Soil consists of about 90% mineral in the form of ground rock and 10% organic matter called Humus.

 Much of the organic matter is composed of partially decomposed organic matter and micro-organisms which are essential to plant growth and health, and also take an active part in the composting process.
Soil may contain from 100 million to 3 billion organisms per gram. So there are more microorganisms in a teaspoon of soil than there are people on earth!
Also present in soil is water and gaseous air.
Soil also contains many insects, worms, arthropods, molluscs and crawling creatures, many of which assist and indeed are essential to the composting process.
You could say that soil is just a gigantic but slow compost heap. What we want to do is speed it up to return the nutrients so that they can be re-used quickly.

 Composting Methods

Depending on how much material you have to be composted, there are several ways to produce compost.

A simple heap.
If you have a small plot and produce little waste material you may find that a simple heap is all you need.
A container is not absolutely necessary, you can make a simple pile in a unused corner of your plot. However a cover will exclude rain and prevent material drying out and blowing away. It will also exclude light and prevent weeds growing out of the heap. However foxes do so love to dig in them.
With a heap you can add small amounts of material as it becomes available.
A small heap will not heat up, so it will take a long time, maybe over a year, to rot down to useable compost.

A container.
You can make a container out of anything you like. It will retain the compost and provide some insulation.

  • Pallets or scrap wood
  • Bricks or concrete blocks
  • Corrugated iron sheets – though they tend to rust and often have sharp edges, so not very safe.  
  • Wire mesh on a supporting frame with a lining such as weed control fabric.
  • An old dustbin with a few holes drilled into the bottom and sides for aeration.
  • Commercial composters on the market vary from simple plastic cones, rotating drums, insulated boxes, stackable interlocking boards. Some cheap, some not so cheap. And some just plain extortionate.

Making compost.
There are a few basic requirements for producing decent, useable compost.


For the process to work at all, there needs to be a good mix of “Green” and “Brown” materials. It also needs air and water.
So what is meant by “Green”? These are materials which are rich in Nitrogen.

Green materials comprise:-

  • Plant material from the plot such as discarded vegetable plants and leaves, potato and bean haulm, annual weeds (though not seeded), perennial weed tops (not the roots and not the seeds.) But perennial roots dried out thoroughly in the sun can be added.
  • Lawn or grass cuttings – well mixed with Brown material.
  • Green leaves from shrubs, fruit canes or trees. But not woody material which takes a long time to decompose.
  • Urine, animal or human, but NOT faeces from dogs or cats (or humans either!)
  • Kitchen scraps, vegetable and fruit peelings, egg shells, stale bread and cake.
    But NOT meat, fish or dairy products. These can attract rodents. Also the way that animal products break down is not the same as vegetables and it will smell unpleasant.
  • Comfrey and nettles are particularly good to add to the mix as they are high in potassium (K) which is necessary for flowering and fruiting.

What is meant by “Brown“? These materials are high in Carbon.

Brown materials comprise:-

  • Straw and pet bedding
  • Paper and cardboard, either shredded or crumpled up.
  • Dry leaves
  • Dead and dry plant material – for example – dry grass
  • Sawdust is small quantities, well mixed in.
  • Wood chips are best left to rot separately as they take a long time to decompose.

For a good balance in the compost you need a 50:50 mix by volume of green and brown. Either mixed together or in alternate layers of about 3″ (7-8cm) thick.
 It also needs to be chopped up where possible, as this increases the surface area available for the composting organisms to act upon.

Air. The process of decomposition is Aerobic, so it requires air. Turning the compost regularly will help add air to the heap. Ventilation gaps or holes in your container are useful. Anaerobic decomposition smells bad, often sulphurous, and will not produce good compost.

Water is also required. It should be at about 50%. If the heap is too dry it will not decompose properly and the same applies if it is too wet.
The commonly used description is that the heap should feel like a well wrung out sponge. With experience you will soon be able to judge the moisture content.
If it does become dry, add a bucket or two, or more, of water and mix it well. If too wet then add some straw, shredded newspaper or dry plant material and mix it in.

It’s easy to ignore the compost heap and just add material as you go along, but a regular mixing will make a big difference to the end result.

Stages in Composting.

So what is going on in your compost?
This is the science bit, if you are interested.

Green materials wither and the constituent  cells break down. Called autolysis, it releases the cells contents and makes them available for fungi and micro-organisms to use as food.
Generally there are three types of micro-organisms in the process. Called “psychrophilic”, “mesophilic” and “thermophilic”.

Psychrophilic organisms function at low temperatures around 13°C.
Mesophilic organisms function best between 20°C and 30°C.
Thermophilic organisms function from 40°C and 70°C.

Depending on the time of year, Psychrophilic organisms will begin the process. As they break down the material they will generate heat, which allows the mesophilic organisms to work.

Most of the decomposition is carried out by Mesophiles. At this temperature larger organisms are working in tandem with the mesophiles. So worms – particularly  Brandling worms (Eisenia foetida, also called Tiger worms), insects, slugs, snails, woodlice, centipedes and others – all work together to reduce the material to something you recognise as compost. Along the way you may notice the presence of white cobwebby  and stringy material on top and in the heap. These are fungi (actinomycetes and streptomycetes) which also breakdown the material and at the same time produce natural antibiotics which keep harmful anaerobic bacteria at bay.

You can get higher temperatures, up to 70°C with the right conditions. For this you  require a good starting bulk of mixed material – around half to one cubic metres. This can then heat up to 60-70°C and remain there for about 3 to 5 days or so before it starts to cool again. At this stage you can mix the heap again and add more green and brown material.
You can do this up to three or four times, before the thermophiles die off or their food is exhausted. As the heap cools the mesophiles again take over and decomposition continues at a lower temperature.

Most compost heaps are in the mid temperature range and work very well. You will get good compost in 6 to 12 months depending on the volume and time of year.

High temperatures help kill off diseased material and weed seeds, but if your heap doesn’t get hot don’t worry about it. Our friends, the bugs and small creatures are still there working on your behalf, producing excellent nourishing compost for you to dig back into your plot.

Carbon to nitrogen ratios in compost.
All  plant materials contain a mixture of carbon(C) and nitrogen(N). The main component is carbon with just enough nitrogen to aid decomposition. Bacteria use the carbon for energy and nitrogen to grow and reproduce. Heat is a result of the oxidisation process. Getting the right balance is important else the organisms die or work very slowly. Ideally the ratio of compost should be 30 parts C to 1 part N.
Grass cuttings for example are about 20:1, whereas straw is about 40:1. If mixed together in roughly equal quantities you get 30:1.(C:N)
So as a good working rule mix 50% Green (N) with 50% Brown (C).