Further to my earlier message - I just happened to be looking up a reference for one of our members in: Agriculture and the Citizen by Colin R W Spedding/Chapman & Hall. ISBN 0-412-71520-1 and I found a table listing how many people whose protein requirement could be met per hectare of land, planted to various crops.
Cabbage 34
Field Beans 26
Peas 24
Potatoes 22
Wheat 20
Sugar beet 17
Maize 16
Rice 16
Barley 15
If energy requirements rather than protein are considered, sugar beet comes out top with 33 people per hectare, potatoes second with 22, the best cereal is rice at 19.
Bronwen Humphreys
The Vegetarian Society (UK)