Aquaponics in a Natural Pond

Take advantage of the nutrients in your fish pond by floating a vegetable garden on top.

| June/July 2018

Imagine growing vegetables on rafts in a backyard pond. You won’t need to water, weed, or fertilize plants when water is your growing medium. Because a pond will stay cooler than soil, you can harvest cool-weather crops, such as lettuce, for a much longer period. Ponds that contain fish provide a natural source of nutrients that help support an aquaponic vegetable garden. Not only will your floating vegetable plants produce food, but they’ll also keep algae levels in the pond low by consuming the nutrients algae need to thrive.

Historical Aquaponics Systems

Combining plant and fish cultivation is far from new; societies have been growing food on lakes and rivers for a long time. The Aztecs created chinampas, large artificial islands and peninsulas on which they grew trees as well as chile peppers, squash, corn, tomatoes, and beans. In modern-day Iraq, the Marsh Arabs still grow food on rafts that are large enough to hold their homes and even large meeting halls.

The idea of growing food on water makes sense. An aquatic ecosystem can provide all the moisture and nutrients plants require and eliminates soilborne pests, while the plants keep algae blooms at bay and provide cover for fish. Despite all these benefits and the long history of growing vegetables in natural water systems, the concept is rarely applied to backyard ponds.

Pond Nutrients

Nutrients naturally accumulate in a pond when fish waste and dead organic material decompose. Nitrogen, the nutrient that’s most critical for fish health and plant growth, cycles through the pond with the help of a variety of decomposers. See the diagram in the slideshow for an example of how this works.

Organic matter, such as dead animal and plant material, accumulates in the pond. Bacteria then start to decompose the organic material. The initial decomposition produces ammonia, a chemical that’s very toxic to fish. Fortunately, bacteria will also convert ammonia to nitrite and then to nitrate, neither of which are as toxic to fish. Some nitrogen will escape into the air, but most of it will remain in the water as nitrate until plants use it again as they grow.

Algae requires higher levels of nitrate to grow than most other plants, so the key to controlling algae growth is controlling the level of nitrate in the water. In a natural pond, this is often done by including a lot of decorative plants in the water and at the edge. Instead of decorative plants, try growing vegetables right in the pond water. They will not only control algae, but will also provide food for your table while they work.

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