How Much Land to Feed a Person?

I've just been reading this post on Naked Capitalism, which raised the question of how much land you need to feed a person. In the past I've seen numbers in the range of 1 - 2 acres per person, but do you really need that much?

I think working out the answer is a bit complicated, but I thought I'd start with something simple. What yield would a 1 acre apple orchard give in the UK? Apple trees on MM106 are supposed to yield 30 - 60 kg a year on average when fully grown, and be about 4m wide. That means that one tree covers 12.57 m2.

Now, an orchard doesn't have a closed canopy, so lets say that the canopy area is 50% of the ground area. That means that in an acre you can fit 159 trees, with a yield between 4.8 and 9.5 metric tonnes. That is 13 - 26 kg of apples a day, and each kg contains about 10 apples. Of course, you can't eat the apples evenly through the year without some method of preservation, but let's pretend that you can.

Next, an apple contains about 50 calories. 13 kg of apples is therefore 13 * 10 * 50 = 6500 calories. The average intake for a man should be around 2500, so this is far more than required. If we only care about energy, and ignore all the other nutrients people need, then our hypothetical man in fact needs at most 0.4 acres, and perhaps as little as 0.2 acres.

Of course, this is a massive simplification, for a few reasons. These are:

1. No-one would try to survive by eating 5 kg of apples a day
2. This assumes that the yield can be consumed evenly throughout the year
3. It assumes that there is no spoilage in storage
4. It assumes that there is no need to worry about variations in yield due to weather etc.
5. It assumes that all that matters is energy, and not other nutrients
6. It assumes that there is no need for additional land to grow fertility / mulch materials

But allowing for some pessimism, land to grow a bit of food for chickens (meat/eggs) etc., 1 acre per person for a good diet does seem like it might be about the right answer for the UK.

Another way to approach the problem is to divide the total amount of agricultural land in the world by the number of people (it doesn't work for individual countries due to food imports, but luckily the world as a whole doesn't import any food) . This gives an answer of 1.8 acres per person, based on numbers from Wikipedia. But again, this answer is imperfect for any particular place because of wide variations in climate across the globe, and because the world's land isn't used to produce a healthy, sustainable diet in the most efficient way possible. This is obvious since on the one hand we have overconsumption of luxury foods like meat in much of the world, and hunger in much of the rest. In addition, the current agricultural system relies on massive injections of mined / artificial nutrients, so a sustainable system could actually require more than 1.8 acres per person at a global level.

So what's the right number? A minimum is probably at least 0.5 acres per person. A realistic number with a bit of meat/eggs is probably more than 1 acre per person. How much more I'm not sure.