Compost
My husband has just made me a new run of three wooden compost bins and it’s prompted me to write about a subject that is dear to my heart. I actually wrote a book on compost, a long time ago. It’s simply called Compost, and you can still just about get it on Amazon. At the time of writing it, I had an allotment in London and the publishers had seen an article I’d written for Gardens Illustrated where I’d interviewed some fellow allotmenteers about their composting secrets. Out of the blue the commissioning editor rang me and asked whether I’d consider writing a book on the subject. I think I laughed – an entire book on compost? Me? How on earth would I find enough to fill a whole book. But having thought about it for all of five minutes, of course I said yes, and started researching the subject in detail – which meant talking to more of the wise old men on my allotment as well as ordering some worms through the post for a new worm bin. I started learning the basics of soil science, and discovering how much life there was literally underneath our feet.
In making compost you are in effect speeding up what is a naturally-occurring process. A compost heap is very much a living entity, with billions of soil bacteria, fungi and other organisms rotting everything down, and the main aim when making compost is to mix up your ingredients to create the optimum conditions for these bacteria to perform their job as quickly as possible. The four elements they need are carbon, nitrogen, air and water, and getting the right balance is the key to success. Looking at the carbon/nitrogen ratio is the easiest and most visual way to approach the way you make your compost, and it helps to make sure you have a good balance (roughly half in half) of both carbon- and nitrogen-rich material. Carbon-rich materials (brown) include straw, woody stems, cardboard and newspaper, while nitrogen-rich materials (green) include vegetable scraps, grass clippings and animal manure. Layering the greens and browns, and then further mixing them up by turning the heap to add more oxygen, will help to speed up the process. If the ratio is skewed too far either way, you will have problems. Materials high in nitrogen such as grass clippings tend to be wet and sludgy. Too much of this type of material can result in anaerobic decomposition which releases nitrogen into the air as ammonia, giving off a bad smell. Mixing the sludgy stuff with plenty of carbon-rich straw, dry leaves or shredded woody stems will help. On the other hand, too much dry, carbon-heavy material will take an age to rot down, needing a nitrogen kick (from a load of nettles or comfrey leaves for example) to get it going again.
A lot of people stress about getting their compost heap super-hot, but what causes that heat and why is it such a good thing? There are various groups of micro-organisms in the decomposition process that thrive at different temperatures: the psychrophiles are low-temperature bacteria that oxidize or ‘burn’ the carbon in the heap to create energy that will eventually raise the temperature enough for the next group, the mesophiles, which, provided the conditions are right, will raise the heat further, paving the way for the thermophiles, which need temperatures of 40-70C to survive – hot enough to boil an egg. The advantage of a heap reaching these temperatures is that decomposition is much faster – and more importantly, weed seeds are killed. But because you need large volumes of material to generate this heat (and time to keep turning the heap), most gardeners won’t be able to achieve these temperatures. My compost heaps never get that hot – but over time they rot down in the same way. You just have to be a bit patient, and there are ways to speed the process up if you have time to devote to it. Turning your compost heaps regularly can speed things up as this will introduce more oxygen which the microbes need to thrive. The final ingredient is water, which is easy to manage with old-fashioned common sense: if the weather is dry, water your compost heap; if it’s rainy, cover the top so that it doesn’t get too soggy. After six months, a year, two years, or however long your special mix of organic matter takes to rot down, you’ll have compost that you can put back into your garden. Use it as a mulch, spreading it over your beds to give back the nutrients that your plants have taken out of the soil; dig it into your vegetable beds; sieve it and use it to enrich a potting compost mix; or use it as a soil-conditioner around trees or shrubs. Making compost is the ultimate in recycling and we should all be doing it.
