Best Thermo-Compost Bins - Why Do Some Like It Hot?

Of the reasons for achieving high temperatures -- and they can get up to a whopping 170 C (no typo, Centigrade!), making compost quickly is the most common. In nature as it gets hotter, chemical reactions happen more quickly and everything speeding up of course means compost is made more quickly -- some claim in as little as two weeks. But no compost can really be made or 'finished' in two weeks because the second part of the composting process, called 'curing', does take time.

Curing happens once the heap has begun to cool and different decomposers start to get to work. Now the fungi and small insects begin their part of the alchemy and these guys, working at lower temperatures, take longer. If you put the two week's old compost straight on the garden soil the same decomposers will appear because there's a job to be done, not ideal to have among some of your precious plants. Some plants will be fine with this -- think woodland floor where this sort of decomposition goes on all the time but tender stems typical of a summer flower border might not be so robust.

Most people who want compost made quickly do so because they want more of it. No gardener can have enough compost but those really keen for it tend to be growing vegetables. Brassicas, for example, are known as 'heavy feeders'.

Other reasons for wanting to get the pile steaming are to be certain it destroys any diseased material. That can be diseased plant matter, human germs such as from tissues and pathogens for instance from animal faeces. If temperatures of around 55 C (130 F) are achieved for three days or more not much will survive. Lower temperatures over a much longer period have the same effect.

Getting your compost to heat up isn't the only factor to consider if you want to ensure germs of one kind or another are destroyed safely. A secure or 'closed' system is important so that the pile isn't raided by vermin or pets.

There is a great sense of achievement in inserting a compost thermometer (not essential but unless you're touchy, feely...) and seeing the dial race around to temperatures more typical of your cooker than your garden. Don't get too good at it though. If temperatures climb above 60°C the heat-loving bacteria start to die and the whole thing grinds to a halt until it cools down enough for them to thrive again.

So how can it be done? Compost heaps will naturally reach high temperatures given the right ingredients and assembly of them, enough space and appropriate ambient temperature. Generally 1m³ is considered the minimum size of a free standing heap for the middle to get hot. In smaller masses there is too much 'outside' surface where heat can escape. Ambient temperature can't be controlled, ie in mid-winter Russia not many microbes are active and decomposition will grind to a halt, whereas by the equator decomposition happens all year round at a rapid rate. Bins with jackets or thermally insulated walls, the Thermo-bins', are increasingly coming onto the market in an attempt to get composts hot in cooler climates.

The 'ingredients' of the compost bin determine the air and moisture content, both of which have an impact on the internal temperatures that can be achieved. Compost bin inventers try and simulate the ideal conditions in terms of temperature, air and size but can't control, only advise, on what to put into it.

The Aerobin concentrates on introducing air as well as insulating the sides. This new Australian-designed bin achieved excellent results in tests carried out for WRAP (UK). It outperformed other bins in maintaining steadily higher temperatures and yet not driying out. Other themo-bins in the trial did dry out and thus the composting process slowed down. Microbes need moisture as well as air to perform at their peak and adding insulation to the sides of the bin appears not to be sufficient to increase the chemical and biological reactions occurring within.

Although temperatures of 55 C were achieved the trials were carried out in the Australian summer and even then 55 C would not pass the British Standards set for green waste compost. BSI PAS 100 insists that green waste compost reaches 65 C for a minimum of 7 days. This results in a 'sanitized' product that is tested for pathogens such as Escherichia coli and Salmonella as well as heavy metals. According to Joseph Jenkins in 'The Humanure Handbook' it just takes longer at lower temperatures and at 55 C the Aerobin isn't far behind.

Green Johanna has no patented lung for introducing air but is another closed system with insulation that makes claims for getting hot and has a winter jacket to keep it composting through the winter. In the author's experience Green Johanna gets hotter than the ambient temperature but has yet to creep out of the low twenties (in a warmer than average British summer). Rather than drying out, this cone-shaped plastic bin seems to retain moisture almost too well (easy enough to adjust for when you know).

NatureMill grinds the input material, supplies heat and turns it all for you AND it can be used indoors. Aside from the initial cost and the (minimal) running costs if you want to see compost made quickly this is a seriously good contender. If you have a cat and kitty litter to dispose of it may be the best thing since sliced tuna. The NatureMill requires sawdust or some carbon-rich material to keep it functioning. Buy a biodegradable litter and you cat could be doing more good to your garden than you!

Want to learn more about the alchemy of composting and how to choose the right system for you? Go to www.CompostEverything.com and sign up for a FREE 10 part mini-course now!
Sarah Cowell Dip. Hort. is a gardener and writer on horticulture matters