Whether the cream or the jam comes on top, the big gap (I’m tempted to say the abyss) is between those who think compost heaps need to be turned over and those who don’t.
Admonitions to turn around are everywhere. To give an example, here is a well-known gardener who recently wrote in a well-known gardening magazine: “Rotate the mixture regularly. I turn mine monthly to add air and balance compost materials with moisture levels. “
I have now been gardening for over 40 years, making compost in a variety of gardens and containers; My first compost bin was assembled from cannibalized parts from an old Anderson bomb shelter. I’ve made a lot of compost in all that time, and I can honestly say that I haven’t made a compost heap or even felt the slightest need to do so.
My guide has always been an experiment carried out a few years ago by Paul Alexander, then the RHS compost master. Paul was interested in comparing the behavior of compost bins of different sizes and materials.
He used standard slatted buckets (volume around ¾ cubic meters), typical municipal “Daleks” made of plastic and an open pile, the last two around ⅓ cubic meters each.
All were filled with the same mixture of typical garden waste in mid-November; Large woody material was chopped and smaller material was shredded. Some piles were turned once a month, others were left alone. To see how small piles are compared to a really big pile, he also made one around 70 cubic feet.
Despite air temperatures below 10 ° C, the large pile quickly rose to around 70 ° C. After two weeks it was turned over and the cold air let in briefly reduced its temperature to below 50 ° C. But it was soon above 60 ° C again and stayed above 50 ° C for several months (with a monthly turnaround). For a whole year it never fell below 40 ° C.
The smaller piles never reached more than a few degrees above air temperature, with or without turning. Probably because of its larger size and better insulation, the wooden container was the best of a bad pile, and the plastic container was slightly warmer than the open pile, but none came anywhere near a temperature that would kill pathogens or weed seeds.
The reason the typical size heaps fail to get hot is simple: their surface area to volume ratio is too great to prevent the heat from escaping faster than it is being produced. If you want a hot bunch, it has to be bigger or heavily insulated.
The effects of spinning were very minor. Turned piles sometimes got a degree or two warmer than un-turned piles immediately after turning, especially in winter, but this did not affect their performance. After a year, twisted and unturned heaps produced similar amounts of more or less indistinguishable compost.
Where is that for us? Making garden compost is the easiest, most rewarding recycling you will ever do, and everyone should be encouraged to compost garden and kitchen waste. Given that the oft-repeated instruction to turn compost heaps regularly is enough to put many gardeners off the whole idea, I think it’s time to stop.