Aquaponics is a method of raising fish and plants (usually vegetables) together.

The fish waste nourishes the plants, and the plants clean the water, which is then returned to the fish.

The Aztecs practiced a form of early aquaponics by raising fish alongside crops. They built artificial islands known as chinampas in swamps and shallow lakes and planted them with maize, squash and other plants. Canals navigated by canoe surrounded the islands and were used to raise fish. Waste from the fish fell to the bottom of the canals and was collected to fertilize plants.

As my grandmother used to say, there’s nothing new under the sun. The ancient food-growing techniques of the Aztecs are being revamped for modern sustainable vegetable growing.

The name “aquaponics” derives from the combination of “aquaculture” (raising fish or other aquatic organisms) and “hydroponics” (growing  plants in nutrient-rich water).

In most aquaponic systems, the water in the fish tank is piped to growing beds where vegetables are planted in a special medium. The plants remove most of the fish waste from the water, which trickles through the medium and back into the fish tank, dropping some distance to aerate it.

An aquaponic system can be as simple as a small tray of lettuce or herbs sitting above a 10-gallon tank of guppies, or as complicated as raising trout or tilapia in 1,000-gallon tanks with “vertical tubes” holding strawberry plants.  

The only inputs are fish food, seedling plants, fingerling fish, occasional additions of water to replace that lost by evaporation, and a little electricity to pump and aerate the water.

In return, the aquaponic gardener gets tasty home-grown vegetables and fish. Many people raise food fish from fingerling stage to “dinner-plate” size in their aquaponic system.

It is expensive to set up a commercial aquaponics system, but backyard gardeners can use recycled materials to build a simple setup. Tanks and growing beds can be made from recycled 55-gallon water barrels. However, the barrels must be food grade and have previously contained only food products as fish are very sensitive to chemicals and certain metals.

The medium used in the growing beds is typically the same as that used in hydroponics: either clay balls, gravel, perlite or peat moss. The goal is a coarse medium that drains well and has a lot of surface area for the bacteria that break down fish waste.

Aquaponics offers many benefits to the vegetable gardener. The system requires much less water than a conventional garden, as only a small amount is lost through evaporation. The harvest is inevitably organic as any chemicals would kill the fish. Aquaponic vegetables appear to grow much faster than in a conventional garden. Although I don’t know of any formal studies comparing growth rates, one grower in Australia reported harvesting cucumbers 25 days after transplanting seedlings.

The best  plants to grow in an aquaponic setup are leafy greens and herbs. Because the main component of fish waste is nitrogen, and high-nitrogen fertilizer promotes lush foliage, leafy edibles such as chard and basil do well.  

Fruiting plants such as tomatoes and cucumbers also seem to respond well. Root crops are not good candidates. Because the growing medium is typically coarse and heavy, potatoes and carrots would be misshapen and difficult to harvest. They are happier in garden soil where they can develop without having to push gravel out of the way.

Australians have pioneered aquaponics using their native fish species. In the U.S., tilapia are often raised by this method.

Designing aquaponic systems for backyard gardens provide a lot of opportunity for experimentation and innovation. To learn more, download the aquaponics handbook from the National Sustainable Agriculture Information Service (www.attra.ncat.org/attra  pub/PDF/aquaponic.pdf). Also, the University of California has an aquaculture site at www.aqua.ucdavis.edu/.

(Napa County Master Gardeners are available to answer questions in person, by phone or on their Web site. Call 253-4421 or visit www.mastergardeners.org for information.)