This article about local food in San Francisco illustrates the problems with subsidizing food production on pricey urban real estate. When people could more profitably use land for other purposes than growing fruits and vegetables, it takes huge subsidies to keep it cultivated. It’s not enough for people to prefer local food — they have to be willing to pay so much for it that no other use of the land would be more profitable:

“It’s really a conundrum,” says Sibella Kraus, president of nonprofit Sustainable Agriculture Education, or SAGE, which encourages sustainable local farming. “There is this demand for local, but we’re not really investing in local.” Ms. Kraus, known for her work planning the San Francisco Ferry Building market, says that while development is at a lull now due to the real-estate downturn, government at the state and local level hasn’t created enough incentives to prevent farmland loss when economic activity rebounds.

It’s worth noting that the advocates quoted here are not fighting for environmentally sound agriculture, or for forging relationships with farmers, or for supporting small farms. All those things could be done at a distance. They want the farming to take place at close geographical proximity; they think minimizing the physical space between grower and consumer is what matters. If what they really cared about were the environment or small farms, they would drop their demands for farmland in San Francisco — where it makes no sense economically — and instead support those practices where farmland is affordable. Which is more sustainable: farming in a rural area where land values are stable and crops pay for themselves, or farming next to a big city where the high price of land means the enterprise would fail without subsidies?

So, advocates should abandon the idea that “local” is a code word for “sustainable” or “better.” It isn’t. It just means “close by.” If you look around and see that the farmers near you are environmentally responsible, you can’t conclude that farmers everywhere are equally responsible. And those other farmers are local from the point of view of their neighbors. Every destructive, unsound farming practice is local to the people who live near it.

When cities or state grant subsidies to local agriculture — and in every policy I’ve seen proposed, “local” is defined in terms of a geographical area — they can’t be sure that those subsidies will go to the good local farmers and not the bad local farmers. Even if all the farmers who currently work in that region are all virtuous, there’s no guarantee that an unscrupulous farmer from somewhere else won’t move in to become local and claim the subsidy.

To those who argue for such subsidies in Missouri, I say: Not in my backyard!