The mantra
Air Water Food Warmth Air Water Food Warmth Air Water Food Warmth
These four basic requirements
are the same for us humans and pretty much every life form on the planet – so
it shouldn’t be hard to remember. What you are doing when you construct an
aerobic (with air) compost heap, is creating the environment for the billions
of microorganisms that make the compost happen -their food are the materials
which you put on the heap.
Air
It is most important to think
in terms of whether the material you put on your heap will add air or water. A
happy heap will have a balance of the two just like a squeezed out sponge; the
whole surface area is coated in water but there are airspaces in between. If
the pile is too dense, squeezing out all the air, or the air spaces get filled
up with water then all the beneficial life forms in the compost heap are not
going to survive and will be replaced by the ‘bad’ microbes – the anaerobic
(without air) ones which are responsible for all the bad odours you get from
putrefying substances. This is bad news for your compost and can be toxic to
your plants if you put this material on them.
So, creating air spaces in
the compost is vital – twigs, dry plant stems, wood chips, all serve to open up
denser, wetter materials and allow air through. People often want to keep this
material out because of the subsequent ‘nitrogen robbery’ that occurs when
woody material is incorporated into the soil, but it is absolutely vital to
maintain airflow throughout the heap and the problem of uncomposted woody
chunks is a minor concern really – see the chapter on using compost, pp xx.
Cardboard and paper are very
absorbent and so will dry out the compost rather than adding air spaces,
although it can be a good idea to add both because twiggy woody materials allow
liquid to drain but are not absorbent.
Water
The microorganisms that we want to dominate the compost heap are all aerobic (with air) organisms that nevertheless also need water to live. If we create a dense, wet anaerobic (without air) heap then only anaerobic organisms (predominately anaerobic bacteria) will survive, and create unpleasant smells; If it smells bad then it isn’t really composting – it’s fermenting or anaerobically digesting (however this can be controlled in a positive way, (see Bokashi - under food waste)
You add water to the heap in the form of fresh vegetable peelings, green leaves, grass cuttings, tea leaves, coffee grounds and so on. Beware of adding too much - with small domestic heaps there is often a surfeit of over wet material without anything to absorb it or let the air through. You can add cardboard and paper which will absorb any liquid but they can also dry out the compost rather than adding air spaces. It helps to scrunch up paper and card but will only provide short term air spaces, as soon as the paper or cardboard gets wet it loses its structure. On the other hand only adding dry plant material leads to a heap that can get too dry; if you feel your heap is too dry, then water it and don’t forget that urine is not only wet but fantastic compost activator!
Grass
cuttings
It is important to balance out the air: water ratio; fresh grass cuttings are great way to add water (and heat) to a heap; they are very ‘green’, full of water and high in nitrogen, so they start to compost very readily; however they can very quickly become a smelly, sludgy mess. One way of avoiding this is to alternate them with a layer of cardboard. You MUST make the layers of grass cuttings very thin, especially if your grass is lush and well chopped up, or you will create slimy layers. Use a 1”/24 mm layer of grass cuttings and then a layer of flattened out corrugated cardboard box. Alternatively mix grass cuttings with more structural material, such as twigs, hedge prunings or woodchips.
Food
– the materials you are composting.
We know we should eat a diet comprised of unprocessed whole foods to be healthy and that we need to eat a balanced diet with proteins and carbohydrates and plenty of fibre; the same principle is true of the compost heap.
The microbes in the compost
heap comprise of two main groups: the bacteria and fungi. Very simply put, the
bacteria consume the nitrogen and the fungi the carbon. The relationships
between not only these two groups of organisms, but also all the other
microscopic and macroscopic life forms in the compost, is incredibly complex
and there is still much that we do not fully understand.
As long as you add a good mixture of materials balancing wet and dry, some which will add structure, some which will be absorbent, and some fresh green material, and as long as they are mixed well together (so you don’t get big air spaces or have denser materials compacted down), then your compost should be fine.
Warmth
The right balance of materials provides the perfect environment for an explosion of microbial populations. Colonies of bacteria can double every hour as long as there is enough warmth.
The most common problem with domestic compost heaps is that, because they are small, they lose heat; without the heat to drive off moisture they tend to get over wet, particularly in the colder months of the year.
But to maintain warmth you must either build larger heaps in which the outer layer is forming an insulating ‘jacket’ around the central core, or insulate smaller heaps, eg, by putting a compost duvet around your bin – indeed some compost manufacturers will sell you a fitted duvet for their bin. (Green Johanna will sell you one as an extra and you can get special insulated covers for the ‘Recycle Works’ New Zealand bins) Or you can construct your own bin incorporating insulating materials, even old carpet is good.
Full details in Nicky
Scott's book pub Sept 24th 2009 'How to make and use compost - in spaces big
and small'
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http://www.greenbooks.co.uk//store/product_info.php?products_id=322
