TWO FARMS IN ONE: The aquaponic greenhouse at Cabbage Hill Farm in Mount Kisco, N.Y.
John Matson / © Scientific American

Confounded by the inexhaustible array of choices available when you stroll through a supermarket today? Well, here's another one to add to the list: How would you like your environmental degradation? By land or by sea? Whether it's pesticides and fertilizers leaching out of croplands or marine fish stocks vanishing by the boatful, every food purchase carries increasingly visible ecological costs.

Against this backdrop, a growing cadre of academics, farmers and aquaculturists is working to refine and popularize a technique that could slash those costs for both fish and vegetable products. The technique, dubbed "aquaponics," integrates fish farming and hydroponic agriculture in a sort of closed, symbiotic loop—the fish serve as fertilizer factories, the plants as water purifiers. The idea is to maximize food production while minimizing environmentally taxing inputs and potentially polluting outputs—a sustainable approach to growing healthy food.

At Cabbage Hill Farm in Mount Kisco, N.Y., a sizable aquaponic greenhouse offers an established proof of concept.

Slide Show: Take a Guided Tour of the Cabbage Hill Aquaponic System.

Six cylindrical tanks ranging in size from 1,000 to 3,200 gallons (3,785 to 12,100 liters) line the north side of the building, each home to hundreds of edible fish such as bass or tilapia. As the fish respire, their gills excrete ammonia into the water. Ammonia is a normal metabolic by-product but is toxic in sufficient concentrations, so it must be removed to keep fish healthy.

The key to aquaponics is that this waste is put to good use: In a churning bioreactor resembling a giant blue cauldron, ammonia-laden water drawn from the nearby tanks is processed by colonies of two helpful bacteria, Nitrosomonas and Nitrobacter. The former turns ammonia into nitrite, which the latter then converts to nitrate, a potent fertilizer. By using fish excretions to feed plants, "you don't have to buy all these fertilizers that are made from petroleum products and that take energy" to produce and transport, says agricultural scientist James Rakocy of the University of the Virgin Islands (UVI), a longtime proponent of aquaponics who has a demonstration facility at his institution. "Our fertilizer would be a waste product; instead we're using it to grow plants."

Growing food crops at Cabbage Hill takes place in long, shallow tubs on the south side of the greenhouse, which are filled with newly nutritious water from the bioreactor. On so-called rafts (repurposed polystyrene insulation panels) floating in the tubs, basil, bok choy and lettuce plants grow hydroponically—that is, without soil—their bare roots dangling through holes in the rafts to draw nutrients directly from the water below.

Stripped of its nitrate, the water is ready for return to the fish tanks, having essentially been filtered by the roots of fast-growing, edible, high-value plants.

Cabbage Hill's resident aquaponic guru, Kevin Ferry, views the process as an extension of natural nutrient cycles. "All we're doing is speeding up nature—the bioreactor is just composting," he says, comparing the churning aeration inside the cauldron with the turning of a compost pile.

As with any attempt to accelerate or enhance nature, aquaponics has costs and complications. During a tour of Cabbage Hill's facility, Ferry stops frequently to tinker with the systems—tossing handfuls of feed into a tilapia tank, adding lime to the bioreactor to bring up the water's pH (reduce its acidity), dipping his fingers into a bass tank and tasting the water to gauge its salt content (about eight parts per thousand, his taste buds register). "A place like this can't be left alone for very long," he acknowledges.