If it seems that compost occupies my thoughts to a greater extent than say . . . yours – it’s because I’m looking after 10 bins at present. Five are our own, three are in our new shared kitchen-garden at Sue’s place, and the last two belong to the holiday house whose pool and garden I tend. Each one gets fed different stuff, cooks at different temperatures, takes different times to finish. And has different inhabitants . . .
These are cicada larvae (we think – I’ve put a few in a clear plastic container, and we’ll see what emerges . . . ). There are scores of them in our slow, cool, old compost bins. And when I mused aloud about their nutritional potential and how some cultures might drool over such a handfull, Mary (the unseen, unsung and Indespensible Gardener) suggested I entitle the photo : ‘Grub’ . . .
There are few creatures that can survive a freshly-fuelled, well-mixed compost pile. For it to do its job of killing off mauvaises herbes and herbes persistantes, and wild seeds and pathogens – then temperatures of between 130 and 170 F. ( 57 – 77 C.) are needed. Higher than this and our friends the microbes take a vacation to cooler climes, or die in their trillions. Some concerned gardeners would rather not be part of such wholesale massacres, and only do long slow cool compost cooking. ‘A chacun, son goût’ – which actually means : I think they’re unscientific wimps. High combustion will indeed wipe out vast civilisations of microbes, as will washing your hands before eating, or boiling your kettle for a cup of tea.
Fortunately, this high-burn period of compost alchemy doesn’t last for ever, and new civilisations are soon happy to make the epic trek back to the centre of your personal Chernobyl, from the outer edges of your bin whence they fled when their own particular global warming got unbearable. Microbes, better than humans, can cope with these temporary discomforts. They are, after all, us. Just in a smaller, more adaptable form.
Size matters. The size that humans are, and the discovery/necessity of fire are intimately linked. If humans had evolved smaller – say mouse size – their need for fire would have resulted in tiny, precarious bonfires of straw and twigs – easily blown away or blown out by the the slightest wind. Humans upped to the size of mastodons would have had enough bulk to keep warm – without fire. The need/invention of fire was a result of our finely-balanced/pure-chance size : not big enough to provide our own heat, but smart enough to make heat for our skinny little bodies . . .
Well, so it is with compost heaps. There’s an optimum size, and the only sure way of cooling your heap is to reduce its bulk. Three foot cubic will get it cooking – two foot will cool it. The previous post showed me turning a cooling pile, and adding water (and of course air). The temperature dropped through the floor. For a day.
I went back there today with our room thermometer, which was showing a comfortable 27 C. as I walked across the village. Flip open the pile, and just a few inches under, the mercury shot off the scale : heading towards 77 C. and total genocide. There are months to go – and several turnings – before this lot ever gets near a plant. Going by my previous piles, each bin should be a writhing mass of slithery creatures, partying in a sweet-scented, crumbly black heap.
When the war is over, and the thermo-nuclear event has passed, in will come the creepies, and the crawlies, and our friends – the worms.
Now for the academic cavalry, courtesy of Google, and Washington State University :
“In aerobic composting proper temperature is important. Heat is released in the process. Since composting material has relatively good insulation properties, a composting mass large enough (3’ x 3’ or 3 x 3 metres ) will retain the heat of the exthermo-biological reaction and high temperatures will develop.
High temperatures are essential for destruction of pathogenic organisms and undesirable weed seeds. Also, decomposition is more rapid in the thermophilic temperature range. The optimum temperature range is 135° -160° Fahrenheit. Since few thermophilic organisms actively carry on decomposition above 160° F, it is undesirable to have temperatures above this for extended periods.
Eggs of parasites, cysts and flies have survived in compost stacks for days when the temperature in the interior of the stack is around 135° F. Since a higher temperature can be readily maintained during a large part of the active composting period, all the material should be subjected to a temperature of at least 150° F for safety.
Sometimes compost operators avoid prolonged high temperatures because the nitrogen loss is greater at high temperatures because ammonia vaporizes, which takes place when the C:N ratio is low. But there are other ways of minimizing nitrogen loss than operating at a lower temperature. The advantages of destroying pathogenic organisms and weed seeds, controlling flies, and providing better decomposition outweigh any small nitrogen loss due to high temperatures.
A drop in temperature in the compost pile before material is stabilized can mean that the pile is becoming anaerobic and should be aerated. High temperatures do not persist when the pile becomes anaerobic. The temperature curve for different parts of the pile varies somewhat with the size of the pile, the ambient (surrounding) temperature, the moisture content, the degree of aeration, and the character of the composting material. To maintain high temperatures during decomposition, compost must be aerobic. The size of the compost pile or windrow may be increased to provide higher temperatures in cold weather or decreased to keep the temperatures from becoming too high in warm weather. Experience shows that turning to release the heat of compost piles, which have become so hot (170°-180° F.) that bacterial activity is inhibited, is not very effective. When the material is actively decomposing, the temperature, which falls slightly during turning, will return to the previous level in two or three hours. Also, it is impossible to bring about any significant drop in temperature by watering the material without water logging the mass.
Variations in moisture content between 30% and 75% have little effect on the maximum temperature in the interior of the pile. The initial temperature rises a little more rapidly when the moisture content is 30% to 50% than when it is 70%. Studies show an important and significant correlation between the moisture content and the temperature distribution within the pile. When moisture content is high, temperatures near the surface will be higher, and the high temperature zone will extend nearer to the surface than when the moisture content is low. For example, in experiments at University of California during mild weather when the air temperature fluctuated between 50° and 80° Fahrenheit, the zone of maximum temperature in a pile with a moisture content of 61% extended to within about one inch of the surface while the maximum temperature zone in a pile containing 40% moisture began 6 inches below the surface.
Deeper piles caused higher temperatures and better temperature distribution, and subject more material to a high temperature at any one time. Hence, the actual mass of the material evolving heat is important in providing adequately high temperatures.”
There. Science is so satisfying. Not as much fun as faeries at the bottom of your garden – but in the end, more fascinating.