Call them warehouse farms, literally. We're not talking about high-efficiency confinement buildings used by some livestock and poultry farmers. We're talking about a farm built in an old warehouse.

Using hydroponic and aeroponic agriculture and other variations on the theme, warehouse farms can grow salad greens, herbs and select other crops quickly, if not efficiently.

One of the advantages they have over "regular" farms is they can switch crops as demand changes. If a customer needs an emergency shipment of arugula, a warehouse farm can change its production and have a crop in 14 to 28 days, according to Jolanta Hardej, CEO of FarmedHere, which operates a warehouse cum farm outside Chicago.

Hardej calls it "on-demand farming."

Of course, the customer could also pick up the phone and call a produce distributor and have the arugula the next day, but that wouldn't be quite as innovative.

Warehouse farms are the latest iteration of the urban farm, which began with the backyard gardens that nearly every house once had. Then they took root on rooftops -- the grocery chain Whole Foods even has a rooftop garden in New York. And now farmers are taking over warehouses and vacant urban lots.

Visionaries even predict that glass-enclosed skyscrapers one day will house high-rise urban farms. Designs have been proposed for places like London, Singapore and even Saudi Arabia -- at least there's plenty of sun there.

So what, exactly, does the future hold for farming? Here's our prediction: In the future, the vast majority of farms will be -- a drum roll please -- in rural areas, where they have always been. Rural areas have what it takes to farm: sun, soil and water. Oh, and they also have millions of farmers, who know how to coax a crop from the ground and harvest it at a price that won't make customers choke.

Most "urban" variants on farms rely on a single factor -- that food prices will skyrocket. If you're a Saudi prince, a bunch of high-priced arugula from your 20-story glass-enclosed farm won't be a problem.

But if you're a working mother with four kids, you'll be watching the prices as you roll through the local grocery store.

Indoor farms are based on urban real estate, which is generally expensive. An urban farm proposed for London will grow lots of food, but it's on some of the world's most expensive real estate. In England, 90 percent of the population lives on only 8 percent of the land, so why would a person buy "farmland" downtown?

Add to expensive real estate the cost of lighting, heat, fertilizer and labor, and hydroponic and aeroponic warehouse agriculture has plenty of challenges. We're not saying it won't succeed, it's just that we have another idea.

It's been around for millennia, and it can be found almost anywhere.

It's a farm.

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