Exploring the gospel principles of earth stewardship

How to compost

My last post ended with a bit of a teaser – that I would be giving away the secret to making your own free miracle fertiliser/soil amendment!  I hope I shall not disappoint.

I’ll try to keep this post entirely centred around theory, with a follow-up on specific techniques and technologies.

The science behind composting is probably fascinating to a biologist, but as I’m an engineer, I classify it as just bog-standard miraculous.  Simply defined, composting is the biological breakdown of any organic matter in the presence of oxygen.  A wide array of life forms participate in this process, everything from bacteria and single-celled organisms up to fungi, dust-mite sized creatures and worms.

Harnessing the power of this giant organic army is simple – provide them with the right conditions and they will take over.  All of these organisms are present in the environment and on the material you wish to compost, so don’t waste money on ‘innoculant’ or ‘compost starter’.  If you are worried about it being too sterile, find something that is already rotting and toss it in!

Once you have your material gathered containing the naturally occurring organisms they need four things:

1. Air
2. Water
3. The proper Carbon to Nitrogen ratio
4. An appropriate temperature range

The first two on the list are the most important and easiest to arrange.  Exactly how you get air into your compost depends on your technique.  At home this is most often done by turning or ‘fluffing up’ your compost using a garden fork to prevent complete consolidation.  Commercial compost technologies apply air via a fan and can achieve complete breakdown in less than 36 hours!  As I’m usually not in a rush, my heap gets turned no more than once a month.

Water is the other magic ingredient and it takes some experience to be comfortable with how much you need.  An excellent rule of thumb, taught to me by commercial composter, is that a handful of compost, when picked up, should feel damp and drip water if squeezed lightly.  If it drips water without squeezing, it’s too wet, if water can’t be squeezed out, it’s too dry.  Most home compost heaps tend to be too dry.

These two elements are managed in concert – the more air a heap gets, the hotter the compost will be creating the need for more water.  Similarly, a heap that is too wet will create a ‘rotten egg’ smell, which can be remedied by turning or the addition of a dry bulky material.  A properly running compost heap will have a damp ‘earthy’ smell and will be typically quite warm just below the surface.

The last two requirements are much more loosely defined for home composting unlike in industrial settings, where time is at a premium, these are highly controlled.  The ideal carbon to nitrogen ratio is 30:1, but a home composter isn’t going to be weighing inputs or taking samples to lab.  It is best to remember that you need to add some ‘green coloured’ waste for all the ‘brown coloured’ waste – things like grass clippings are a good source of nitrogen, as is chicken manure (if that is available).  If the C:N ratio isn’t bang on, it won’t mean disaster, just a slower rate of breakdown.

Lastly, the temperature needs to be within reason – not too hot and not too cold.  There is little you can do about this in a home setting, other than turn the pile if it appears to be getting too hot (you’ll see steam).  If it is winter, don’t fret, the compost will heat itself, but it may slow the process down a bit.

That’s all the theory  you need to start home composting!  As there is no cost involved and, strictly speaking, no equipment needed, anyone with the place for a pile can start at any time.

Anything that was once alive can be composted – food waste, garden waste, natural fabrics such as cotton, wood chips, paper, etc…  Although, if you live in the city, it is best to avoid composting meat as it can attract rats.

Next time, I’ll go over a few things about composter technology and some ideas for DIY composting projects.

  • Peter Ashcroft says:

    One important difference between aerobic and anaerobic digestion is the chemical composition of the end products.  A well aerated composter releases its carbon as CO2.  That’s a greenhouse gas, but no more so than before you grew the vegetables in the first place.  Anaerobic digestion, on the other hand, releases some of the carbon as methane.  That methane will float around in the atmosphere as a particularly potent greenhouse gas for about a decade before it eventually reverts to CO2.  So a healthy composter is not just an odor-free source of rich garden soil, but also an easy way to reduce our own greenhouse emissions.

    I’ll defer to any biochemists out there, but that’s my understanding.

    • Steve says:

      This is correct – composting is a aerobic digestion, which will produce CO2.  However, this is ‘natural’ CO2, in the sense that it comes from a contemporary biological source unlike fossil fuels, which has essentially been sequestered for a few million years.  See http://en.wikipedia.org/wiki/Carbon_cycle for a nice infographic!

      Methane (CH4) is about twelve times more ‘potent’ than CO2 in greenhouse terms, so the savings are always multiplied when we avoid releasing methane into the atmosphere.  Interestingly, if you can capture and burn the methane that’s being released, you’re getting yourself an excellent ecological bargain.  In a previous job I worked for a company pioneering a technology to trap methane emissions from agricultural manure storages.  We simply flared (burnt) the gas and reduced it’s impact by factor of 12, although this could have easily been used as a fuel, which would have essentially reduced consumption of fossil-fuel methane.