Dear Mr. Rawles,
I've been an organic farmer in Ontario Canada for 12 years, and thought Chris S made some excellent points regarding underestimating the amount of fresh food required to sustain life. I've 'done the math' myself, and found that a subsistence garden consisting of potatoes, corn, winter squash and beans would require a minimum of 5,000 square feet per person to provide 2,000 calories per day over the course of a year.
These crops were chosen with consideration for yield, nutrition, and storability. Another consideration is repeatability i.e. the ability to replant from your own stores.
Assuming you wanted 25% of calories from each crop, expected yield from this garden would need to be 90 lbs beans (dried), 100 lbs dried corn, 400 lbs potatoes, and 700 lbs squash. This is achievable on good land and with good farming practice, but you are only one crop failure away from starvation. Even experienced growers can have crop failures. For example, this summer very few of my winter squash set fruit because of excessive hot/dry conditions. If I was depending on this crop for survival, I would be in trouble. So I would want to increase the planting of all crops by at least half, and have some 'cushion'.
By the way, the same tendency to underestimate applies to livestock as well. My in-laws were recently visiting, and were surprised to find I was raising 75 roaster chickens just for the two of us. I explained this was only 3 chickens every two weeks over the course of a year, certainly not excessive even if I get two meals from each bird (I usually get more than that because I make soup and stock as well).
By the way, I raise my meat birds in outdoor portable pens, and rotate them and my pigs through the garden area. This seems to benefit both vegetable and livestock crops.
Chris S is providing a valuable 'reality check' in his article, to which I hope I've contributed. Not to say a smaller plot of land can't contribute, but self-sufficiency in food takes a lot of space and skill.
Scott K. - Ontario, Canada