Making good garden compost
Creating a healthy soil is fundamental to wildlife gardening. The soil is the base of the food chain and the vehicle through which nutrients are recycled. If it is healthy then plants will grow more robustly and resist pests and diseases more easily. Invertebrates too will flourish, in turn encouraging higher life forms like mammals and birds to use the garden.
Organic matter such as garden compost is the key to a healthy soil. By composting garden and kitchen waste and applying it to the soil you achieve double benefits: less waste to go to landfill and a garden that is more welcoming to wildlife.
In a natural ecosystem organic material recycles itself, and in the wilder parts of a garden you can allow nature to do the job for you. But in the more managed parts it is essential to remove organic material, either because it forms a crop, such as vegetables, or because this is the best way to keep the garden looking good and to stay in control.
This is where the compost heap enters the picture – composting is the best and quickest way to convert the organic material that you remove into a form that the soil can take up and make available to plants.
Building your own container
The cheapest way to make a sizeable compost container is to build your own New Zealand box – see diagram below. The timber for the sides should be at least 25 mm thick for a one-metre square box, which is about the minimum worthwhile size, thicker if you make them larger. The posts should be 75 mm square - treated fencing posts are ideal. You can use slotted concrete fencing posts instead, but you must set them very firmly in the ground so that they do not bow outwards with the weight of the compost.
A cheap and cheerful alternative is to make bins from old pallets nailed together. Nail five together in an E shape, then strap two across the front using galvanised wire wrapped round nails. Line the inside with cardboard to make up for the thinness of the wood.
You can make the bins as big as you like, provided you produce enough material to keep them busy. At several Trust nature reserves we have bins two metres square that work very well. How many you need depends on how much of the garden you manage actively - borders, vegetable plot, mown grassland and so on - but two one-metre-square boxes are a minimum for all but small town gardens.
It is best to have two bins side-by-side, so you can be filling one while the other is cooking. It also saves labour because, if you do have to turn the compost, you can turn from one bin into the next rather than having to take it all out and put it back in again. Three together are even better.
Direct sun in summer provides too much heat and rain in winter too much moisture, hence the container(s) should be in a sheltered spot and, ideally, covered by a roof. A piece of corrugated iron does the job, if not very prettily. Boarding covered with green roofing felt is more sightly.
There are various proprietary compost bins on the market and some local authorities supply these at good prices. They sometimes have labour-saving features, for example can be swivelled to turn the compost and/or to empty, and may be more convenient in smaller gardens. However, beware of bins that do not follow the principles that underlie the New Zealand box design: they must be capable of keeping the compost warm and supplied with oxygen while it is decomposing, something that a standard plastic container will not do.
Making the compost
The term 'compost heap' has done composting a bad turn, because it implies that all you need to do is to throw material in a heap and wait. True, you can make compost this way – eventually – but it is a hit-and-miss business and, because a loose heap will not get as hot as compost made properly in a container, the process will take much longer and the compost will be full of viable seeds that will spring up all over the place as soon as you use it.
Here are some basic rules, applying specifically to the New Zealand box but mostly relevant for other types as well. Follow these and you will soon discover how to get good results reliably from your particular container.
Include anything green, not cooked.
All the uncooked vegetable waste from garden and kitchen can go on the heap, with the exceptions given below. Invasive bulbs like grape hyacinths or alliums seem to survive composting unless crushed. It is also worth damaging tough material like cabbage stalks by smashing them with a brick or the back of an axe.
Don't include ...
- leaves or twigs, because they slow the process down - put them in a separate leaf bin or spread them under a hedge;
- roots and underground stems of perennial weeds like dandelions and couch grass, until you are confident that your heap will heat up enough to kill them;
- the same applies for plants that are diseased or may spread disease, such as old strawberry plants.
Provide a supply of oxygen
Without adequate oxygen you get anaerobic decomposition, producing a slimy mess. Oxygen can be introduced by leaving gaps in the sides of the container, but this makes it more difficult to keep the heap warm (see next point). An air channel under the heap works well, made with double rows of bricks or perforated plastic drainage pipe. Turning the heap has the same effect, but is much harder work unless you have a proprietary bin designed to tumble the material.
Try to put in a mixture of coarse and fine material, so that the material does not compact down so much as to exclude air. If necessary stack fine grass cuttings beside the box for a while until you have some coarse material to mix with them.
Keep the heap warm and moist
When the heap is cooking, damp heat escapes rapidly from the top. This is why it is important to seal the top, with a lid if your bin has one, and otherwise with a layer of soil, a square of old carpet or, failing either, plastic bags. To help keep the heat in, you can also line the bin with old cardboard boxes as you build up the heap.
Compact the heap down occasionally, especially round the sides, to avoid large air spaces and keep the process going at full speed.
Encourage the recyclers (bacteria & micro-organisms)
If the heap stands on the soil, recyclers will find their own way in eventually, but you can add some soil or part-rotted compost to the top of a freshly made heap to 'inoculate' it and start the process more quickly.
It is worth using an accelerator, especially in the colder months. Several good organic products are available, such as Biotal, Fertosan or QR. A sprinkling of nitrogen-rich fertiliser such as dried blood will do almost as well. Urea is good also, so pee – discreetly of course – on the heap and save water by not flushing the loo. But remember that an accelerator does what it says: it will make a properly made-up heap go faster, but it will not help otherwise.