Saturday 13 April 2024

How to grow climate friendly food at home

Were you as shocked as I was to hear that growing your own food at home has six times the GHG emissions as conventional agriculture? Surely it does not have to be like this? I looked at the study behind the news [1]. (It came out in January and I do not understand why it has hit the news again now.) The good news is, the main sources of emissions from home grown produce are easily avoided, with a bit of care. 

Compost heaps were one of the main sources of GHG emissions from urban gardens - but good practice can minimise this.

The study was fairly small and overweight on Poland.
Was it a good study? Well, the sample size is a little small and biased towards Poland, though I am not sure how that might affect the results. There were a total of 73 locations, including 9 community gardens, 7 urban farms and 55 individual gardens. Nearly half of the locations were in Poland and only six in the UK (mostly community gardens); others were from France, Germany and the USA. The urban agriculture projects were compared to emissions from conventional farming as reported in other studies. The emissions included were from infrastructure (e.g sheds and raised beds), crop inputs (such as fertiliser and weed suppressing membranes) plus, in the case of conventional farms, transport from the farm to the city.

Community gardens may have been unfairly assessed.
There was a huge amount of variation in emissions from one location to another. The chart shows that the best urban farms were about the same as conventional while community gardens were the worst. However, many community gardens do a lot more than produce food and in my view some of the emissions were allocated unfairly.
Chart from [1]

The main sources of emissions were infrastructure and compost heaps.
The main sources of GHG emissions discussed in the text were:
  • Infrastructure materials – sheds, raised beds, tools, greenhouses, paving for paths etc. These accounted for more than half the emissions at most urban locations.
  • Poorly managed compost heaps generating methane and nitrous oxide emissions – the main source for 22 out of the 73 sites.
  • Synthetic fertilisers - these were the main source for only 5 sites. None of the community gardens used these at all, relying on composting alone. The urban farms used them but much less than on conventional farms.

Use second hand/waste materials to keep infrastructure emissions low.
Infrastructure was the main source (more than 50% of emissions) from two thirds of community gardens and three quarters of individual garden but only a quarter of the urban farms. There are two ways to reduce emissions from infrastructure: use second-hand materials so there are no primary emissions and/or keep them going for a long time so they are amortised over many years of food production. The latter approach can be difficult for community gardens as they may not own their land and it may be requisitioned for other purposes after only a few years.
Compost heaps emit powerful greenhouse gases if they are not well aired.
Compost heaps always generate some CO2, because the organisms that break down the material into compost have to breathe. The problem, though, is the methane and nitrous oxide, both of which are far more potent as greenhouse gases. These gases are produced by anaerobic organisms and to minimise them you just (!) need to make sure your compost heap is well aired. 

Which makes we wonder how the emissions from composting were calculated. There is no mention of actually measuring it, which would be expensive so it must have been estimated, but there are few published measurements to go on. I found one study [2] that measured four community composting systems in Spain (all handling kitchen waste). The better ones used a rotation of slatted bins and the worst used a single module (in at the top, out at the bottom). This is not surprising because slats are good for letting air in and rotating material from one module to another is extremely effective. But it does take effort. This study also compared its results with some other studies on home composting and commercial scale composting. The community composting units came out best, which surprised me but perhaps the commercial systems were optimised for speed rather than consistency.

How do I know if my compost heap is well aired and what do I do if not?
A healthy compost heap has no nasty smells (which usually means it is too wet) and has plenty of worms in it - which do not survive well without oxygen. There is some good guidance on how to fix composting problems here from 7 Signs Your Compost is Struggling (and What You Can Do About It)

In summary then, there is no reason for your home grown produce to be high in GHG emissions. The important strategies are to:
  1. Use home composting to minimise synthetic fertilisers, but keep your heaps well aired. 
  2. Avoid buying new tools and equipment – use second hand and keep stuff going for longer; use waste lumber for your raised beds. 
As for paths, I am not convinced their emissions can sensibly be allocated to the food you grow. They are more like garden furniture than growing infrastructure. Or you could use grass instead of hard materials and (ideally) use an electric strimmer when necessary.
[1] Jason Hawes, Benjamin Goldstein et al, Comparing the carbon footprints of urban and conventional agriculture (Nature) 2024
[2] Daniel González, Raquel Barrens et al, Addressing the gaseous and odour emissions gap in decentralised biowaste community composting (Waste Management) 2024

1 comment:

  1. I've found that recycled plastic raised bed planks last many times as long as timber ones (especially s/h timber ones), and would recommend them to anyone. Round here planks come from Filcris in Bourn. I think that's a better suggestion than using recycled timber in general. It's superior technology, encourages a plastic recycling market, and (I presume) is low-carbon - if only because it will last at least 30 years, and there is no risk of timber preservatives getting into your soil.


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