Discussions about whether we can feed the burgeoning population expected in 2050 or 2100 usually use average crop yields per acre in their calculations. This is misleading, said Jonathan Foley, professor of ecology at the University of Minnesota, speaking at the Sustainable Foods Institute 2012 in Monterey, Calif. He noted that crops such as corn and soybeans that are fed to livestock or cars are an inefficient use of land.

At the same meeting, journalist Charles Mann noted that potatoes produce more food calories per acre than wheat and corn. (“Is American agriculture really efficient?” by Samuel Fromartz, May 22, 2012; www.chewswise.com/chews.)

John Jeavons of the California-based environmental research and education organization Ecology Action was thinking of calorie farming back in the ‘70s when he wrote “How to Grow More Vegetables (and fruits, nuts, berries, grains, and other crops) than you ever thought possible on less land than you can imagine” (Ten Speed Press).

So, rather than consider yield of garden crops — as in pounds per acre or pounds per 100 square feet, why not consider calories grown per unit area and how many calories might come from the garden?

According to the USDA (at www.cnpp.usda.gov/Publications/USDAFoodPatterns/EstimatedCalorieNeedsPerDayTable.pdf), the average moderately active 40-year-old male needs about 2,600 calories per day. The average moderately active 40-year-old female needs about 2,000. A moderately active 10-year-old boy or girl needs about 1,800 calories. So caloric requirements vary with age, sex and one’s level of activity.

To find out how many calories various crops might produce, I multiplied the number of calories present in a pound of fresh produce by the average U.S. yield per 100 square feet, as given in Jeavons’ book, to come up with average calories per 100 square feet. In the few cases where data were missing in “How to Grow…,” I used the Johnny’s Selected Seeds catalog and one or two Internet sources. Here are the results, all based on fresh weights:

Crop — Average calories produced per 100 square feet

Potatoes, Irish — 21,957
Garlic — 18,215
Rutabaga — 16,253
Squash, Acorn — 15,702
Onions (bulbing) — 14,303
Potatoes, sweet (Jewel) — 12,750
Burdock — 11,938
Carrots — 11,887
Squash, Hubbard — 10,744
Celery — 9,424
Beets, Cylindra — 9,316
Parsnips — 8,475
Cabbage, regular — 7,066
Brussels sprouts — 6,900
Mustard greens 6,524
Squash, Patty Pan — 6,405
Corn, sweet — 6,120
Eggplant — 5,069
Tomatoes — 5,043
Squash, Zucchini — 5,028
Peppers, green — 4,838
Cabbage, Chinese — 4,470
Cauliflower — 4,368
Rhubarb — 4,216
Melon, honeydew — 4,145
Lettuce, head — 4,116
Broccoli — 3,345
Kohlrabi — 3,196
Cucumbers — 2,938
Watermelon — 2,700
Lettuce, leaf — 2,662
Spinach — 2,533
Beets, regular — 2,074
Kale — 2,048
Salsify — 2,037
Beans (bush, pole) — 1,651
Radish (not Daikon) — 1,188
Peas, bush — 986
Asparagus — 409
Artichokes — 331

So Charles Mann is right: Potatoes are a winner, calorie-wise. A 40-year-old woman of average activity could meet all her annual caloric needs (2,000 calories x 365 days = 730,000 calories) from potatoes (which would leave her deficient in some nutrients), which could grow in 3,325 square feet — a plot 56 by 56 feet in size. Or she could grow 4,008 square feet of garlic — a 63-by-63-foot plot.

If we’re lucky, no one will get all his or her calories from garlic, but you get the points. When we think of yield, we should think not just of pounds per acre but of calories per acre for human consumption (and, in my opinion, some for animals). When we think of yield, we should realize that the average yields above will be low compared with a well-tended intensively planted garden. And when we want to add diversity to our diets, we can consider not just additional areas of less-caloric vegetables, but edible landscape plants as well — blueberries, apples (maybe not this year), strawberries, raspberries, nut crops. And we can eat weeds — check out burdock’s 11,938 calories per 100 square feet!

The main point, I think, is that we will be able to produce enough calories for 10 billion or so people — and home gardens can contribute significantly to that. Right now world agriculture produces enough to supply about 2,700 calories per person per day, and that’s without much focus on calorie farming, on minimizing waste or on intensive growing methods. But 9 or 10 billion people will probably have other detrimental effects (on water usage, CO2 contributions, etc.); and distributing food equitably to all people is already problematic for political reasons.