A Cultural Benefit to Using Soil as a Growing Medium in Aquaponics

By Kailey Burke, Sustainable Food and Farming Student

Spinach and lettuce at Pettengill in Salisbury, MA
Spinach and lettuce at Pettengill in Salisbury, MA

Aquaculture + Hydroponics = Aquaponics

In this system, fish excrete ammonium (NH4+) waste which passes through media that contains bacteria, which convert

Pettengill Aquaponics System with Koi
Pettengill Aquaponics System with Koi

ammonium to nitrate (NO3-, which is the most accessible form of N that plants are able to take up), plants uptake nutrient filled water, and the rest of the water then returns back to the fish, purified.

Sounds simple enough, but there are many ways to build this system, ranging from an intricate technical system to a low cost setup. One of these variables includes the growing media which provide plants with support, moisture retention, and access to nutrients. It is this variable that could play an important role in connecting traditional farmers to a progressive growing system.

Peas growing in 5 inch pots on a growing table
Peas growing in 5 inch pots on a growing table

The growing media is responsible for housing bacteria and purification, could either be soil or a soil-less material. Though soil is the standard growing medium in most other production streams soil-less substrates can also supply plants with the essential elements through materials such as clay pellets, fiber mats, or bare root systems. Though there have been countless trials showing benefits of soil-less materials in an aquaponics system, and there are certainly systems in which soil-less substrates are the appropriate material to use, there is something accessible and fundamental about using soil… maybe it’s that plants have been evolving for 425 million years to be growing in soil? And, we are only beginning to understand and value the billions of relationships that are interconnected between soil biota, nutrients, and plant ecology.

While it is true that not all fish are able to withstand the water quality fluctuations that soil systems can carry with them, there are a large variety of fish that are able to thrive in these systems – such as tilapia, koi, and catfish. The images below, from Pettengill Farm in Salisbury, MA and Growing Power in Milwaukee, MI show a low-tech, low-cost aquaponics system that uses soil and common greenhouse pots and flats.

Will Allen at Growing Power in Milwaukee, WI with 7000 Lake Perch
Will Allen at Growing Power in Milwaukee, WI with 7000 Lake Perch

In this system tilapia are contained in a tank made of wood and pond lining that sits 4 feet into the ground. This water is then pumped from the fish tank to two 30 foot long growing tables that are pitched at a mere 2 inches. The tables are wood lined with pond liners and have rocks on the tabletops to allow the soil filled pots or trays to sit a bit higher out of the water. The water flows down the table through the rocks, pots, and roots back into the fish tank.

So, why highlight soil as a viable aquaponics media? Well, using soil as a growing medium not only plays an important biological role, but it also plays a cultural role in integrating the idea of an aquaponics systems into appropriate modern day small farms. Using a soil based system, farmers are able to easily integrate and take advantage of the many functions that aquaponics can play, such as; providing thermal mass and temperature stabilization in a greenhouse, water conservation, reduced reliance on outside fertilization, and educational attraction. In conclusion, soil is a language that farmers and gardeners speak, and having this material as the basis for plant growth bridges a gap between the variations of production in an aquaponics system to that of traditional methods.

Nasturtium in the Growing Power greenhouse
Nasturtium in the Growing Power greenhouse
Nutrient solution leaving the aquaponics table returning to fish tank below
Nutrient solution leaving the aquaponics table returning to fish tank below
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