Compost is decomposed organic matter, which has been prepared for use in the garden. It contains humus, nutrients and beneficial micro-organisms.
Making compost is a sensible way to recycle organic household wastes while improving garden fertility.
When preparing compost, one is working with living forces, which transform the raw materials into a useful finished product. The organisms, which create compost, need to be tended by the gardener, ensuring they have enough water, air and balanced organic nutrients to do their job of decomposing.
Benefits of compost in the garden
Nutrients for soil
Beneficial soil micro-organisms
Organic nutrients for plants
Holding soil minerals (chelating) for plant nutrition.
Improve soil structure
Improve water holding capacity of soil
Buffer pH extremes
Materials to compost
Any organic matter may be used to make compost, provided a range of different materials are used which create a balanced combination of carbon and nitrogen.
Carbon, which forms the bulk of a compost heap, is present in all organic matter. Dry fibrous plant materials contain the highest percentage of carbon.
Nitrogen is the fuel, which cooks the compost. It is present in animal products and manures, with the manure of smaller creatures having the highest amounts of nitrogen. Young green leafy matter also has a moderately high proportion of nitrogen.
If there is insufficient nitrogen in a compost heap, it will be cool and will take a long time to break down. Too much nitrogen will result in a heap, which becomes slimy and offensive smelling. Compost should not smell bad while it is being made. A faint ammonia smell is the sign of a well balanced carbon-nitrogen ratio.
Commonly used materials for making compost are food scraps, weeds, lawn clippings, manures, wool waste, feathers, lawn clippings, manures, straw, hay, paper, leaves, cotton rags, sawdust, wood shavings and garden refuse such as light prunings, old or dead plants, etc.
Bulky or woody materials are not suitable unless they are chopped up. A shredder is the most useful tool for this job.
Do not add plastic or metal materials. They do not break down and will only pollute the garden. Do not add toxic materials such as permapine, paint or pesticides. A good compost may break down some poisons down into harmless substances but this should not be relied upon.
Compost accelerators may be used to speed up decomposition. Adding a culture of micro-organisms to the compost can help it to break down quickly. Plant accelerators include comfrey, nettle, yarrow, tansy, valerian and chamomile. Biodynamic growers add special herbal preparations to the compost heap to improve the quality and rate of decomposition. An elder tree growing beside the compost heap will improve the break-down of the compost.
The most convenient method of making small quantities of compost is to gradually add materials to a suitable container. This suits small gardens and allows for the recycling of household and garden wastes.
A large dark coloured plastic bin is the most common and convenient container for gradual composting. The dark colour of the bin will hold the heat of the sun, speeding up the activity of the micro-organisms, which produce compost. Closed solid containers keep vermin out of the compost. Plastic containers last longer than metal or wooden ones, which quickly break down when in contact with decomposing organic matter. The bin should be open at the base to ensure good drainage and contact with soil organisms.
It is best to have two containers. While one is being topped up, the other is breaking down its load. Gradual composting is a slow process and it may take more than six months for the compost to be ready.
A high proportion of food scraps gradually added to a compost bin will result in acidic conditions. This is characterised by a sour smell and the presence of small ‘vinegar flies’. Counteract this effect by adding occasional handfuls of horticultural dolomite to the bin. A layer of soil over the contents of the bin should also be added to improve the texture of the compost and balance any tendency towards sourness.
The disadvantages of using the gradual method of composting are:
the length of time it takes for the compost to be ready
low composting temperatures means that weed seeds will not be destroyed
lower composting temperatures means that disease causing organisms will not be as effectively eliminated.
A well made compost heap will quickly provide a garden with high quality clean compost. It entails more effort and a bit of organising.
Collect sufficient materials to build a heap, which is at least one cubic metre in size. Prepare an area for making the compost. The heap may be built in a bin or be free standing. Free standing heaps need to be 1.5m wide and at least 1.5m in length. Compost bins may be wooden, made from galvanised iron or built from stacked straw bales. The bin should be 1m to 1.5m wide, at least 1m deep and allow for a heap to be built 1m to 1.5m high. It is useful to build two bins side by side to enable the compost to be easily turned.
A small heap will not have the mass necessary to generate peak temperature. A large heap will not get adequate air and moisture into the centre. Bulk compost is produced in long windrows, 1.5m wide and high.
Build up the compost heap with interleafed layers of the different materials, ensuring an even mix of materials through the heap. Start with a layer of straw or some other stalky plant material then build it up with other types of material. Layers of plant material can be up to 10cm thick. Materials high in nitrogen should be in layers no thicker than 1cm. Layers of soil may be added to the heap as it is being built to add minerals and give the compost more body. An occasional layer of lime or dolomite may also be added to the heap to provide minerals and reduce any possible loss of surplus nitrogen. Build the heap between 1m and 1.5m high. A free standing heap should be 1.5m high. Cover the heap with clean underfelt, carpet, plastic or iron sheets to prevent the loss of heat and moisture.
Warning - carpet and underfelt are often contaminated with insecticides as the result of termite treatment of houses. Only use them if you know their history and can guarantee that they are clean.
Water the heap as it is being built so the materials are damp without being soggy. Check the moisture content of the heap every day for the first few days and water it as needed. The heap will tend to dry out as the peak temperature inside reaches 65 degrees Celsius.
Turning the heap is done to further mix the materials and keep the compost aerated. It is also a good time to check the moisture content. The heap may be first turned from 2 to 14 days after it has been built. The more often it is turned, the more quickly the heap will break down. Finely chopped materials in a compost heap will decompose much more quickly than anything, which is chunky and solid. Compost made from finely shredded materials and turned every two days will be ready in two weeks. Compost made from coarse materials and turned only once will take up to six months to be ready. Compost is ready for use when the most of materials are transformed into dark crumbly matter.
Check that the compost reaches peak temperature during the first few days. Push a length of metal such as a water pipe or a crowbar into the centre of the heap. Leave it for two minutes then remove it. Peak temperature is reached if the metal is painful to grasp tightly.
Failure of the heap to heat up could be due to:
lack of nitrogen
lack of water
small size of heap
A poor compost heap may be improved by rebuilding. Turn the heap, adding (depending upon the problem) bird manure/water/dry straw as you go.
Seasonal difficulties are encountered when making compost. Cold temperatures in winter slow down the process after the compost has reached peak temperature. Dry hot weather in summer means paying special attention to the moisture content of the heap.