PHILIP MILLNER
Organic growers consider their first duty is to nurture the soil in which they grow their produce and to do so without the use of artificial fertilisers, herbicides or insecticides. Organic growers believe in using natural fertilisers and letting nature do the work for them. This article is about compost which many assert is nature’s supreme fertiliser. A small amount of compost will go a long way in promoting the biological activity that is so essential in organic growing.
Anatomy of a good compost heap
Good compost is made of many different ingredients. Perhaps the most important is some sort of manure: cow, horse or poultry. Other ingredients include grass clippings, leaves, weeds, sawdust, wood shavings, kitchen scraps, seaweed, straw and hay. The addition of rock phosphate or dolomite will enrich the finished compost. Water is also an essential ingredient. If the heap is not moist enough it will not work. Nine times out of ten a compost heap flops because the ingredients used were too dry. The key to good composting is using ingredients that will give you the right carbon to nitrogen (C/N) ratio. Nitrogen is best introduced into compost heaps in the form of animal waste or well-chopped, soft, green materials such as young weeds or lawn clippings. Carbon is mainly derived from aged, stalky material such as straw, old vegetable plants, dried grass, dried leaves and from wood waste such as shavings or sawdust.
The micro-biological process that occurs during composting is a form of rotting and decay but without the unpleasant smell often associated with that process. A well-made compost heap is one in which the C/N ratio is anywhere between 25:1 and 30:1 as it creates an environment in which the decay-causing bacteria (micro-organisms) can live and reproduce at the highest rate of activity (see table). As a result of this activity the ingredients are converted into a dark, cool, sweet-smelling matter called humus. Humus is a non-living, finely divided organic matter derived from microbial decomposition of plant and animal substances. It consists of about 60% carbon, 6% nitrogen and smaller amounts of other substances such as phosphorous and sulphur.
Humus is valued by farmers and gardeners because it provides nutrients essential for plant growth, increases soil water absorption and improves soil workability. To speed the decay process you can add leaves of yarrow or comfrey. Three or four chopped leaves of each herb to two cubic metres is sufficient to activate and accelerate the chemical reactions in the compost heap. It’s a good idea to have three compost
heaps: one being built, another decomposing and a third one in use.
There are a number of different types of compost-making boxes, tumblers and barrels on the market. Most of the time there is really no need to erect any structure to contain your compost heap; all you need is bare earth on which the heap will sit. If, however, the compost heap is liable to be subject to the effects of exposure to extreme winds, rain or sun, some sort of holding structure is advisable.
How long will it take before the compost is ready to use? That depends on a number of factors including how often it is turned, the climate and the ingredients. In our temperate climate zone a heap should normally be ready for use in twelve weeks. The process can be speeded up by turning the heap more often and by adding comfrey, yarrow or both.
Making your compost step by step
- Choose your ingredients
- work out your carbon to nitrogen proportions aiming for 25–30:1
- Choose your site
- close to ingredients and water
- not too much sun
- Collect your ingredients
- have enough materials to build at least a 1.5 cubic metre heap
- Wet your ingredients
- until they feel like a well wrung-out sponge
- Lay out a base
- place sticks in a row or upend the books of a bale of hay
- Build the heap
- alternate different ingredients in layers keeping them thin, say 150–200mm for plant material and 75–100mm for manure
- sprinkle soil, rock dust (dolomite), seaweed
- add yarrow and/or comfrey to both
- strive to give it a dome shape
- Keep wetting the heap
- it is essential to maintain the moisture level
- Make the heap high enough, about 1.5m
- use stakes to make air holes when the heap is complete
- Protect the heap from the sun
- use a thick layer of hay so that it doesn’t dry out
Troubleshooting
If the heap fails to generate heat with 24 hours pull it apart and build it again. If it hasn’t started heating up by then it never will. Failure to heat up means that the necessary micro-organisms have scorned the living arrangements you provided for them and failed to take up residence.
To find out why, ask yourself these questions:
- Is the pile too wet or has it matted down so much that there is not enough air in it?
- Are there too many bulky ingredients or has the weather been hot and dry so the pile has dried out and there is not enough moisture?
- Is the pile too small so it has too much surface area in relation to its internal area and is shedding heat too fast?
- Are there too many nitrogen-rich ingredients so that micro-organisms are being offered a diet that is like bread without butter?
- Are the layers too thick so that the microbes are being offered a diet of carbon or nitrogen in different places but not both at once? Neither the bread nor the butter is very enticing on its own.
The most likely cures for these problems are the following:
- Turn the heap more frequently to mix up the ingredients and incorporate more air.
- Sprinkle the heap with water if necessary.
- Add more green mulch and/or manure to boost the nitrogen content.
If your heap is very dry when you turn it, it probably means that it has too much surface area and so is losing moisture and heat too rapidly. The larger your pile the smaller the ratio of surface area to internal volume. Another possibility is that the heap is the wrong shape. Sprinkle it with water and rebuild, aiming for the dome shape.