A question from Yahoo! Answers:
How much land does it take to support a human?
The question is meaningless as asked. Land and capital are substitute goods, so the amount of land necessary to support a human being depends on the amount of physical capital available; the more capital you have, the less land you need.
Modern agriculture (machines, fertilizers, and pesticides) yields ten to twenty times more grain per acre than primitive agriculture did. Hydroponic farming offers even greater yields, but requires a lot of capital, so it’s not widely used.