If I may butcher Dickens briefly, it was the best of heaps, it was the worst of heaps, it was the fair-to-middlin’ of heaps, it was the age of wisdom, it was the age of foolishness, it was the epoch of compost.

For the last few months I have been obsessing over rapid composting, a technique which promises finished compost in 3 weeks or so. To achieve this requires a number of conditions to be met. I’ve now built and managed three heaps following this technique with varying degrees of success. I thought it would be interesting, if a little geeky, to compare and contrast the three heaps.

Heap 1

Built at the end of October, this heap’s life was diarised on this blog some weeks back. It was made mostly of horse manure (G), spent hops (G) and cardboard (B), in roughly a ratio of 1:1 G:B.

[G = Green, B = Brown)

Heap 2

Built in mid November, this heap was made of horse manure (G), spent hops (G), cardboard (B) and fallen leaves (B), in a measured ratio of 1:2 G:B. Both Heap 1 and Heap 2 were built in a bay measuring 15 cubic feet. As noted in a recent post, this is below the recommended minimum for this technique to work well.

Heap 3

Built just a few weeks ago in mid December, Heap 3 was made of horse manure (G), spent hops (G), spent grain (G), hay (G), cardboard (B), wood shavings (B) and straw (B), in a measured ratio of 1:1 G:B. This heap was built in the enlarged compost bays, 35 cubic feet, more than double the original volume. I stuck rigidly to the turn schedule recommended by the late don of rapid composting. I also did a better job of mixing the heap. The technique requires the heap not just to be turned, but turned inside out. The centre of the heap, insulated by the outer layers, retains the heat best and is typically 10-20 degrees hotter than the outer layers. To ensure that all the material in the heap spends time in the hottest part of the heap, the outer parts of the old heap need turning into the middle of the new heap, the inner to the outer. This makes the turn a slightly more fiddly process.

Steamy! I was digging out the core of the heap to fling into the corners of the next.

The fastest composting happens between roughly 55°c and 70°c. This is the ‘hot zone’ in which fast breeding thermophyllic bacteria work best. Aside from the self-sustaining heat they generate, another sign these guys are at work is a characteristic white residue left on the compost heap material.

White fungal residue

It’s instructive to compare temperature profiles for the three heaps together. This chart plots temperature over time. The temperature was taken in the centre of the heap, daily where possible. Time is plotted along the x-axis, temperature in Celsius along the y-axis.

compost chart

Heap 1 got quite hot quite quickly, reaching the hot-zone by day 5 and staying there, just, for 11 days before cooling rapidly and tailing off to around 30°c by the end of the three weeks. The high 50s is hot enough to kill pathogens and a good portion of many types of weed seeds, so this isn’t too bad. I don’t think it stayed hot enough for long enough for the composting process to properly finish, but the results were sufficiently good to use as a mulch.

Heap 2 was the same size as Heap 1, but a different ingredient ratio, more browns than greens. It took twice as long as Heap 1 to reach the hot zone, and stayed for a too-brief 3 days then cooled off to the still useful mid-40s before settling at about 30°c, same as Heap 1. I shan’t be building a heap with a 1:2 G:B ratio again.

Heap 3 was the most compliant of the three heaps, by which I mean I stuck more closely to the prescribed technique, providing as well as I could the optimum conditions. The bay was above the required volume; the ingredients were more carefully measured, in the correct 1:1 ratio and in small pieces; moisture was provided; turned frequently; turned inside out and finally it was covered for the duration, partly to keep heat in, but mostly to keep rain out.

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This heap got hotter faster, reaching a maximum of 72°c, then stayed in the upper reaches of the hot zone for 12 days, longer than the other two heaps. Importantly, it stayed above 62°c for 8 days. This is the temperature above which pathogens and nearly all weed seeds/roots do not survive, a good thing as the manure probably contained a lot of weed seeds and potentially gut pathogens I might prefer not to sprinkle around my edibles. While the heap temperature has dropped in the last few days of the three week period, the core of the heap is still in the warm zone, so there is still useful decomposition going on. I will leave it be until the temperature drops to 30ish.

I conclude that it was well worth me extending the compost bays to cope with larger volumes, it does seem to have been the decisive factor in maintaining high temperatures. From Heap 3, I estimate I will have 600 litres of finished peat-free compost, in just a few weeks, all from zero cost and freely available ingredients that might otherwise have been wasted.

The technique is very effective, but it is quite an overhead to manage properly and does generate large amounts of finished compost, good uses for which I am rapidly struggling to find. Much as I remain fascinated by this process, I don’t think I need to run another hot heap until later this year, the autumn perhaps. In the meantime I will revert to the slowly accumulating heap process I have always followed.

I’ll be back soon with more of this sort of stuff.