by Rachel Tinker-Kulberg, PhD
The USDA estimates that 23 million people have inadequate access to healthy and affordable food and as a result food-related illnesses such as Type 2 diabetes, cancer, heart disease, and obesity are currently on the rise in this country.
Organically soil grown produce is gaining popularity because consumers are carefully scrutinizing what they are putting into their bodies and want to buy food that is not treated with hazardous chemicals. Organic certification of produce grown hydroponically, a form of agriculture where plants are grown in water and a fertilizer solution (i.e. soil-less), has been mostly prohibited due to the chemically-synthesized nature of the nutrient solutions (inorganic mineral salts) and because the growing substrates are usually not sustainable (i.e. like rockwool). Frustrated because they believe this has no bearing on the quality or safety of their produce, hydroponic growers are now experimenting with “organic” nutrient mix solutions to seek certification but many are running into problems since these substances don’t dissolve easily and must be broken down through microbial action which is not possible in a standard “sterile” hydroponic set-up.
Putting aside the bureaucratic definitions of the “organic” labeling, another growing concern among consumers is whether hydroponic produce is as healthy as produce grown organically in the soil.
By providing a complete diet of minerals required for plant growth via chemical fertilizers, plants grown hydroponically have been shown to achieve higher growth rates and yields. Some hydroponic proponents assume these ideal growing conditions must produce healthier plants, after all a complete diet of plant minerals should translate into higher “nutritional” plant content – right? The truth is that plant physiology is not that simple and scientists and educators in the field of agriculture need to address critical questions related to nutritional value of plants since mass food production trends using soil-less hydroponic systems are on the rise in order to meet the demand of rising populations and limited availability of arable land.
The experimental question of whether hydroponic crops raised on a perfect mix of macro- and micro-nutrients result in a more nutritional rich product compared to soil-grown produce is hard to address experimentally since many hydroponic growers use different nutrient formulas depending on the crop being grown and their environmental parameters can also vary.
Soil farmers experience these same type of variations with respect to soil health and fluctuations in environmental conditions. For example, water quality and variations in temperature and humidity can place stress on crops potentially changing their biochemical make-up regardless of the growing method being used. Because of these variations, studies to date comparing the nutritional content of produce grown hydroponically to soil-grown have had mixed results- with some studies showing no difference between the two methods, while others showing that soil-less systems fared either better or worse than soil grown controls in the nutrients levels being tested. As you can imagine experimental design and conditions varied widely between these studies and depending on how they were designed affected the outcome and the significance of the findings.
For example, one recent study by Treftz, 2015 compared the difference in nutritional quality in strawberries and raspberries grown in soil and soil-less conditions. The hydroponic nutrient solution used was a general commercial hydroponics standard (Flora series) and the soil used was a mixture of Nevada topsoil and Miracle-Gro potting soil (1:1 mixture) that was re-fertilized with Miracle Grow fertilizer every six weeks. Miracle grow is a synthetic fertilizer and therefore cannot serve as an organic soil amendment control, in fact some growers believe that it has the potential to kill beneficial microorganisms in the soil due to its high ammonia and excess mineral salt content. Healthy soil containing beneficial soil microorganisms allow plants to maintain optimal nutrient content in the face of environmental stresses through their interactions with root-based transport systems. Therefore the proper experimental design where the health of soil microbes is considered is crucial to evaluate differences between soil-less and soil-based growing systems.
Nevertheless, the results from this study showed that the “healthy” anti-oxidant compounds (e.g. Vitamin C, tocopherol and total polyphenolic compounds) were significantly higher in hydroponically grown strawberries compared to the soil-grown but the opposite was true for raspberries! Interestingly, earlier studies by Premuzic 1998 showed that tomato fruit grown in healthy organic soil (100% or 50% vermicompost) contained more Vitamin C than the same fruit grown hydroponically while other studies by Buchanan, 2013 showed that hydroponically grown lettuce contained more Vitamin C than soil-grown varieties. In this latter study, no reference was made in the exact type of soil that was used other than remarking that it was a “noncommercial soil enriched by natural composed material”. Interestingly, in many of these peer-reviewed comparison studies, the levels of sugar (fructose & glucose) were higher in the soil grown fruit which might explain why hydroponic produce is sometimes characterized as “less tasty” than soil-grown.
It seems that no real “broad” conclusions can be drawn from the experimental results from these and other past comparison studies.
For these experiments to be truly valuable, future researchers need to be more careful with their “soil grown” controls so that they actually mimic standard organic farming practices which maintain the integrity and health of the soil. This is no easy task, since the standard practice of adding organic matter to the soil and the conversion of this matter into essential nutrients by microbes and worms is usually liberated gradually over time into the soil. Perhaps by collaborating with well-established growers in both these farming industries and taking random product samplings from them and various marketplaces, researchers can help refine their scientific studies.
Lastly, there is a growing body of evidence that plants have evolved to up-regulate healthy anti-oxidant compounds during stressful conditions to help them survive environmental insults.
It stands to reason that if hydroponics operations provide “perfect” conditions for plants by giving them everything they need to grow, perhaps these anti-oxidant compounds, and others we have yet to characterized, will be induced at a lower levels (or not at all) compared to plants grown in “less than ideal” conditions of soil farming where stressful environmental conditions are inevitable? One classic example of this phenomenon is when tomatoes are put under osmotic or salt stress, the beneficial phytochemical called lycopene was shown to increase significantly. In fact, closed-loop water recirculation systems like hydroponics and aquaponics (the coupling of fish production with vegetable production) may in fact offer an advantage over soil-based growing since the water chemistry of both these systems can be manipulated to boost natural plant bioactives for health benefits. Growing these types of “superfoods” are becoming the focus of hydroponic researchers since they not only offer a food product with high nutritional value, but also have the potential of demanding a higher market price for the grower.
As we move toward the high-tech world of food production, we need to make sure we have performed the proper scientific research to ensure that these advancements are also protecting the health and nutritional content of our food as well as our environment.
Future nutritional research evaluating eco-friendly sustainable growing methods like aquaponics and Integrated agri-aquaculture systems (IAAS) is also critical in order to optimize production of nutrient-rich field crops as well as aquatic species (fish & shrimp) while also protecting our environment since the health and access of food – as an outcome of food security – is after all also vulnerable to environmental degradation.
Treftz, C. and Omaye, S.T. (2015) Nutrient Analysis of Soil and Soilless Strawberries and Raspberries Grown in a Greenhouse. Food and Nutrition Sciences 6: 805-815.
Premuzic,Z., Bargiela, M., Garcia, A., Rendina, A. and Iorio, A. (1998) Calcium, Iron, Potassium, Phosphorus, and Vitamin C Content of Organic and Hydroponic Tomatoes. HortScience 33(2): 255-257
Drew N. Buchanan, Stanley T. Omaye (2013) Comparative Study of Ascorbic Acid and Tocopherol Concentrations in Hydroponic- and Soil-Grown Lettuces. Food and Nutrition Sciences 4: 1047-1053