Back in late August I described the process of putting together an 18-day compost heap. That heap was made on August 26. When I measured the temperature on August 30 it was just over 50 degC. I suspected at the time that the temperature had been higher during the four days in between, because of the amount of green vegetable matter used, but I couldn’t be sure of this.
On September 5 I turned the heap for the first time, but before turning it I checked the internal temperature – 52 degC – still within the thermophilic phase of composting(1) where composting is at its most rapid and is bacterially dominated. The thermophilic phase occurs around 40-65 degC and at these temperatures pathogens and weed seeds will be killed, but “heat loving” organisms will thrive. Above 70 degC it is necessary to start thinking about reducing the temperature so as to avoid death of the beneficial composting organisms. This heat is being produced by the metabolic processes of the organisms doing the composting, so it is good to see that they have been hard at work for a week or more. The fact that it is progressing so well indicates that, during the time I wasn’t taking measurements, the temperature didn’t get into the 70+ degC, or at least not for any appreciable time.
The first surprise was the extent to which the original heap had subsided – it was right up to the top of the frames in the background. I should have expected this, for two reasons: first, the green material was pretty coarse, so there would have been a lot of air spaces in the heap to compact down under the weight of the overlying material; and second, that green material made up a large proportion of the heap, and once it started to break down there would have been a lot of water released, thus reducing its volume. It is still well above once cubic metre, which is roughly the size required to reach and maintain hot composting.
You can see that a lot of the drier/harder materials have only started to break down, but what isn’t so obvious is that the vast majority of the original green material is difficult to identify – this is after 10 days of composting (in true 18-day compost procedure the first turn would have been after four days, but I did warn in the earlier post that this would be “more or less” 18-day compost).
What is important, but not obvious from the photo is that the heap generally had a good water content, though of course the outer layers (and in this case the bottom layer too) were on the dry side. I had a hose handy to water both the working face of the old heap and the new heap whenever I came across material that was too dry. A heap at this stage is too wet if you can squeeze water out of a handful of material, and too dry if a hard squeeze cannot produce any “cohesion” of the material (it doesn’t have to produce wet clumps, just some cohesion resulting from its wetness). If in doubt, a bit too dry is way better than too wet, because too wet leads to anaerobic processes.
The first thing I noticed was that there is quite a bit of woody and dry leaf material. The woody stuff is expected because of the use of coarse mulched tree material from under the power lines in the area. The bigger bits of this won’t break down fully in an 18-day cycle, but they will during the later “maturing” phase of fungal-dominated composting. Among these larger components was a matrix of fine material well on the way to becoming compost; this would have derived from the green vegetable waste and the matured broiler manure.
The second thing, and this was a bit of a surprise, was that the 20cm of chip mulch base which had been in the bin for a couple of weeks before I made the heap had become fungus-matted, rather like tempeh (the Indonesian fungus-impregnated soya bean delicacy). You can see where I’ve thrown this material onto the top of the new heap in the photo above. Clearly the temperature at the bottom of the heap had not gone high enough to kill off the fungi but was within the mesophilic range (25-45 degC) where fungi are encouraged. Here’s a close-up of that material.
This material provides a lovely base of fungal inoculation of the heap, so I made an effort to include it in as many layers of the new heap as possible. I did worry a bit about the possibility of “matting” of these clumps into impervious layers, rather like paper tends to do in a compost heap if you add too much in one layer and make it too wet. All the more reason to spread it through the new heap as much as possible.
That reminds me – it is very important when you are making or turning a hot compost heap to keep it “fluffy”, i.e. to incorporate as much air as possible, because without air the (aerobic) organisms you want to do your composting can’t survive. They will be replaced by anaerobic(2) organisms, and where these dominate they produce intermediate compounds including methane, organic acids, hydrogen sulphide and other substances. In the absence of oxygen, these compounds accumulate and are not metabolized further – many of them have strong odours and some are phytotoxic (poisonous to plants(3)). One of the things I like about my long-handled manure fork is that it is very easy with a flick of the wrist to turn a forkful upside down as I throw it onto the heap, thus loosening it up as it falls.
So that’s it. With the heap covered with a thick layer of loose straw it can sit and do it’s thing until the next turning.
The open area in the back of the above photo is where I can turn the car when I’ve got a trailer behind. In the background are the sections cut out of an old water tank that had been through a bushfire. These now hold maturing broiler manure and horse manure.
(1) There are many good sources of information on the stages of composting and the factors affecting the process. For a quick overview of the main points you can go to the Cornell Composting web page.
(2) Misra, R.V., R.N. Roy and H. Hiraoka (2003). On-farm composting methods. Land and Water Discussion Paper 2. FAO, Rome. [This publication has details on many composting techniques, including a number of anaerobic methods. You can download the whole document here].
(3) Brinton, W., & Trankner, A. (1999). Compost maturity as expressed by phytotoxicity and volatile organic acids. In Orbit-99 Conf Proceedings, University Bauhaus Weimar. Retrieved from http://www.solvita.com/pdf-files/voa_eu2.pdf [Gives some idea of the potential for composts to become phytotoxic and the compounds involved].