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They sure don’t now. According to Jeff Vail, they just might:
How much of its own food can suburbia produce? In America, the average suburban lot size is approximately 12,000 square feet. That’s about a quarter-acre. At an average of 2.56 people per household, and a rough average of 10,000 feet per lot not covered by structures, that’s just under 4,000 square feet of yard per person. Of course, this ignores the potential for parks and other open spaces in suburbia to be converted to food-production. It is also an average figure–some neighborhoods will have far less space, others far more. Despite these sources of variability, it is a good jumping-off point. Is 4,000 square feet enough to provide for a person? There are three requirements: calories, nutrition, and the variety and selection necessary to support culture and quality of life. In addition, there are four limiting factors to food production in a given area: sunlight, water, labor, and soil/nutrients. In the interest of space, I’ll only address three of these: calories, nutrition, and soil/nutrients–please feel free to discuss the other requirements and constraints in comments.
Can 4,000 square feet produce enough calories to feed one person? At 26 calories per ounce and roughly 8,000 pounds of potatoes harvested from 4,000 square feet (based on intermediate yields from John Jeavons “How to Grow More Vegetables,” p. 92), that’s 3.3 million calories, or 9,000 calories per day. This is, of course, completely unsustainable, insufficiently nutritious, etc. But it does answer the question–it is possible to grow enough calories on 4,000 square feet per person. The real limiting factors are nutrition and soil, discussed below:
Can 4,000 square feet produce enough nutrients to feed one person while simultaneously sustaining and improving the soil? One issue is that topsoil has been scraped away from more recent suburban developments. How effectively can we re-build soil, and how long does it take? John Jeavons has addressed this question in depth (summarized at p. 28-29 of “Grow More Vegetables”). He concludes that 4,000 square feet is roughly enough to feed one person a complete, nutritious diet, while simultaneously improving soil quality. His method involves 60% (by area) focus on growing soil-improving crops (high carbon content food crops for eventual compost), 30% mixed high-calorie root crops, and 10% mixed vegetables.