What’s the Difference Between Soil and Aquaponics Farming?

by Lexi Harder

4/23/18

There is a battle unfolding around the USDA’s organic certification and whether it should include foods that are grown via aquaponics or hydroponics. Here, Lexi Harder — an aquaponics farmer — explores the similarities and differences between soil and aquaponics farming.

As an aquaponics farmer at Oko Farms, I raise fish and plants in the same body of water. On farms like ours, fish — together in a symbiotic relationship with beneficial bacteria — provide nutrients to plants via their waste, and plants clean the water by taking up these nutrients/waste.

As with any “new” technology, claims about the potential applications for aquaponics vary wildly. Proponents say that aquaponics saves 90 percent more water at the same time as being 10 times more productive than conventional soil farming. Critics say growing plants in water, whether by aquaponics or hydroponics, is a threat to those who want to “save the soil” and some say it is unnatural. In my two years of working at Oko Farms, I’ve encountered more people than I can count who view aquaponics with suspicion.

Common between these two groups is a lack of understanding about the actual difference between aquaponics and soil farming. Most of the literature comparing soil and aquaponics — and by extension, hydroponics — focuses on the divergence in method and outputs between the systems. But what I’ve noticed, is that regenerative and sustainable soil production is more similar to aquaponics than the current literature lets on.

So, what really is the difference between plants grown in soil, and plants grown aquaponically? We already know that plants don’t necessarily need soil to grow, but what do they really need?

Factors Necessary for Plant Growth

On a base level, all plants need access to water, nutrients, air, light, temperature and support in order to grow. In soil as in aquaponics systems, plants may or may not access these factors in different ways.

Water + Nutrients

All plants need to take up nutrients in order to thrive.  Plants primarily absorb essential nutrients through their roots. In order for this to happen, nutrients like nitrogen, phosphorus, and potassium need to be dissolved in water at the correct pH level.

In soil, plants are able to take up nutrients after water from rain or a watering system has dissolved nutrients “locked” in the soil. In aquaponics, nutrients suspended in water are always readily available for absorption.

Air

Just like people, availability of oxygen and carbon dioxide are necessary for plant health. Plant roots, which we don’t often think about, need oxygen, or else they will rot and the plant will die. This is why instructions for houseplants often come with strict warnings not to overwater. In between rain or watering sessions, soil will drain and plant roots gain access to the oxygen they need. In aquaponics, there is always a base level of oxygen dissolved in the water (as water in motion accumulates oxygen), but many farmers add oxygen to their systems from an oxygen pump, like we do at Oko Farms.

Light

Plants need light in order to perform photosynthesis, which allows them to grow. In outdoor soil farms as in outdoor aquaponics farms, the sun provides all the light needed for plants to grow.

Temperature

Different plants have different temperature requirements. Gardeners familiar with soil temperature zones will know that it’s impossible to grow a mango tree outside in Canada, for example. All outdoor farmers, soil or aquaponics, have their own methods for dealing with temperature, from building greenhouses to using shade cloth.

Structural Support

Soil provides the structural support that plants need to stay upright, but in water farming, the farmer has to provide the structure. This can come in the form of a soil substitute like gravel, or by holding plants in a floating raft.

Microbial Communities

One crucial thing that links aquaponics and soil farming is the community of tiny organisms that live in both systems. In my post about the science of aquaponics farming, I describe the nitrogen cycle, of which a key component are the bacteria nitrosonomas and nitrobacter. These very same bacteria play an important role in soil as well, attaching to the roots of some plants and creating organic nitrogen fertilizer.

In fact, in soil, we know about the existence of thousands of different microorganisms that are essential for not only breaking down waste, but hundreds of services necessary for plant health. For example, a thriving bacterial colony in soil contributes to robust plant growth, resistance to disease and water retention in soil.

In aquaponics, bacterial colonies have only been studied in depth as far as their role in the nitrogen cycle. Who knows what we would discover if we paid attention to the microbial life that exists in an aquaponics system? At Oko Farms, we know that since we don’t replace our water, we have nurtured our microbial community for around five years.

Those who argue that soil is the more “natural” system seem to overlook the fact that aquaponics also plays host to beneficial bacteria. The process of building good soil, then, closely resembles the process of building good water in an aquaponics system. They both rely on being careful about inputs like pesticides and fertilizers, and both rely on prioritizing the health of the whole system, not just production.

What’s the True Difference Between Farming in Soil or Water?

To me, aquaponics differs from soil in that plants get their requirements for growth a little differently, and aquaponics also requires raising fish. I think that each system is suited towards different places. In a city, aquaponics makes sense because farmers don’t need to worry about starting plants in contaminated soil, and they can save water. In areas with more land and access to water, farming in the soil can make more sense. To tell the truth, my dream is to have both soil and aquaponics on my future farm, so that I can reap the benefits of both ways of farming.

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