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Sunday, March 8, 2009

Barley loaves and small fishes?

I gave a talk to the Sustainable Clonakilty on Thursday. I think what they expected was a 'A How to grow Vegetables' sort of thing but I started thinking about how a community of 5,000 could feed itself. On the basis that adults need 2,700 calories a day (or 1 million a year) and that no chemical fertilisers would be used, I searched the internet for yields of various food groups, averaged them out, decided that a diet of 40% cereals, 5% pulses, 30% fruit and veg, 20% nuts meat and dairy, and 5% oil would be a reasonable mix and came out with a figure of 3880sqm or about 80% of a football pitch would be needed per person.  This adds up to nearly 2,000 hectares for the whole population of the town. The grey part of the map is 2000 hectares. I was surprised that it was so large.  I made some whopping assumptions but if the figures are close to reality it means that a total of 400 cultivated hectares would be needed. Using fossil fuels to do this is unsustainable, so I looked at the biofuel option and found from the internet (and again I don't know if these figures are at all accurate) that a yield of 1200 litres of biodiesel  per hectare was possible and that about 900 litres of diesel, and I don't know how much chemical fertiliser, was needed to produce that 1200l. So it would take the surplus biodiesel from at least 3 hectares to grow one hectare of food. Not a runner, I thought. Horses would be more efficient, but they would need a good ha each and I would guess (not very scientific) that it would take a good horse for every 10 hectares or a total of 20 horses. Maybe we could use battery powered tractors charged with wind? If we were to do it by hand, could four gardeners manage a hectare? That would be 1600 gardeners and quite a market for osteopaths. 


  1. Hi Madeline - brilliant stuff to get working this out. Simon Fairlie has a range of scenarios in his 'Can Britain Feed Itself' article - (downloadable here:


    He generally comes to a figure of about 2 hectares to feed 7.5 people for an organic system. So that'd be about 1500 hectares for Clon. This includes land for pasture but not for biofuels.

    I think this is a fascinating topic - although as Ireland has a low population density it should be pretty easy for us to feed ourselves. The UK is a different matter though!

  2. Thanks for that link Tom. I have compared his figures for the organic with livestock scenario with mine. Ireland has a total of 4.4 million hectares of agricultural land with 400,000 hectares of it arable as opposed to Britain's 18.5 million agricultural with 8.1 arable. Interestingly we both allow 2,700 calories per person. He allows 2666sqm per person, half of it arable whereas I came to a figure of 880sqm per person cultivated and 3,000sqm of grazing/nut trees. This seems to reflect the different proportions of arable land, to total agricultural land, on the two islands. He allowed for grain for the animals, which I didn't. I used a figure of 3.3t/ha yield of cereals whereas he used 4.3T/ha. My yield of beef was 1/3 of his. His fossil fuel use would be high due to the higher proportion of cultivated land. However overall his figures are not that different

  3. Hey Mads, just found your blog from our favourite Irish grower. Will read it avidly. Love from London, allan on Observer Organic Allotment blog


  4. Sorry was posted as anonymouas, could not understand the naming protocol, and site rejected me

  5. Hi Alan, It was your blog that got me started on the addiction, now I'm following lots of them.