Finance / 

09 Nov 2018

How many acres do you eat?

Reading a Metafilter post on “The End of Big Ag” (spoiler alert: probably not the end of Big Ag) got me curious about the feasibility of cities growing their own food, and exactly how much land it takes to keep a person fed. So I did a little napkin math.

tl;dr: It’s theoretically possible, I suppose, for a city to grow its own food, but it would be a huge challenge. Current cities can’t do it.

Dent corn has the highest calorie yield per acre of any row crop, at least of those commonly farmed today. (Potatoes are close, and the highest-performing root crop. I think the highest-yielding perennials are bananas/plantains, but not sure.) The average yield for field corn is 171 bushels/acre (although that’s always going up, and probably with effort it could go higher—but we’ll stick with typical US farming practices), at 56 pounds per bushel of edible corn, and ~1500 nutritional calories per pound, that’s about 14 million calories per acre under cultivation. An active person needs about 1 million calories per year (though at 2k calories/day it’s actually only 730k, but we’ll assume city-dwellers walk more than us lazy suburban schlubs).

So just as a low bound, absolute minimum, you’d need 1 acre under intensive row-crop-style cultivation for each 15 people, and that assumes zero waste during food preparation and service, etc. (Also, hope people figure out how to make hominy, or pellagra is going to be the cool new look next season.)

There are supposedly 1200 acres of flat-roofed commercial buildings in New York City suitable for rooftop farming. So without changing existing land-use patterns, you’d only be able to feed about 18,000 people.

This report on land use and tree cover in NYC (PDF) suggests that there are about 30,000 acres of land currently covered with “Grass or shrub area that is theoretically available for the establishment of tree canopy.” Or, in a pinch, food crops. And that’s without cutting down any trees. That’s another 900k people. If you’re willing to bust out the chainsaws and jackhammers and start using all the land currently covered by trees and arguably-useless impervious surfaces (not buildings or roads, but parking lots, plazas and the like), the sort of thing you might do out of desperation, you can get 120,000 acres. At two harvests per year — which is probably doable with the right type of corn — that’s 3.6M people fed. In a city of 8.5 million I’d still be very suspicious of what the hotdog vendors were selling.

Basically, there’s no way to do it given how cities are currently structured. If the transportation system that brings food into a major city like NYC stopped working, at best you’d be able to sustain a fraction — not an insignificant fraction, but under half, best case — of the current population. This shouldn’t be a surprise: modern cities are shaped, by transportation, so we’d expect that they wouldn’t exist without it.

It’s probably possible to design a city that isn’t as dependent on long-distance transportation as current US cities are, perhaps by using greenbelts, but there’s no real current-day economic incentive to do so. Long-haul transportation, particularly of bulk foodstuffs like grain, is very cheap — even a significant uptick in the price of oil probably wouldn’t do it. Only some other incentive, or top-down restrictions on development, would keep farming close to cities.