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.