Are Soilless Crops Less Healthy?
Every few months, I find myself sitting across the proverbial table from someone concerned with the relative nutritional values and overall ‘naturalness’ of hydroponic and aquaponic produce, compared to soil-grown produce. Though on rare occasions these conversations stem from a genuine interest in managing micronutrients across different growing techniques, more frequently, these questions come from defensive soil-farmers or misguided health enthusiasts.
The first time I faced such an interrogation was while I was conducting crop growth-rate research at Messiah College, Mechanicsburg, Pennsylvania. An elderly, local gardener, who came by the greenhouse under the guise of discussing soilless growing methods, berated me about how aquaponic and hydroponic methods directly circumvent God’s will for the earth, and how the efficiencies of Controlled Environment Agriculture (CEA) undermine the punishments of mankind laid out in Genesis 3, related to the toils of tilling and growing. More recently, arguments against the healthiness of soilless crops typically center around the apparent lack of micronutrients in hydroponic and aquaponics fertilizer solutions.
In addition to carbon, hydrogen, and oxygen, which are absorbed from air and water, there are fourteen elements needed to cultivate healthy plants. These elements are divided into two classes: Macronutrients (Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, and Sulfur), which are needed in larger quantities, and Micronutrients (Iron, Chlorine, Boron, Manganese, Copper, Zinc, Molybdenum, and Nickel), which are required in lower quantities.
When discussing nutrient levels and healthiness, it is important to remember that plants and humans are very different physiologically, and require different concentrations of different elements in different forms to function properly. I want to touch on this point, because misconceptions often arise when tracing various elements from fertilizers, through plants, to consumers.
I recently read a blog post by a soil-farmer who challenged a hydroponic grower about maintaining proper cobalt levels in their fertilizers. After outlining the critical roles that cobalt, in the form of vitamin B12, plays in DNA replication, oxygen transportation, cellular functions, and immune responses, the writer went on to argue that hydroponic produce is less healthy than soil-cultivated, because it doesn’t provide consumers with such critical vitamins.
Before diving into questions related to other micronutrients, I want to take a short moment to address a common misconception related to cobalt that I have come across a couple of other times, in addition to the aforementioned blog post. Though Vitamin B12 is a cobalamin molecule, containing cobalt, increasing concentration of cobalt in fertilizers, in order to increase the concentration of cobalt in the harvested crops, and ultimately the levels of cobalt being consumed, does not equate to an increase in available B12 to the consumer.
Unlike certain Vitamins, such as Vitamin D or Vitamin K, which are synthesized within the human body from outside inputs, the human body cannot produce B12 from consumed cobalt from plants, nor do plants or fungi actually contain B12. B12 is synthesized by algae, and by bacteria that are found in the guts of ruminant animals, and therefore enters our diets through the consumption of certain meats, eggs, poultry, and milk products. For this reason, it is not uncommon for strict vegetarians and vegans to be at risk of pernicious anemia.
When discussing the concentrations of micronutrients in plant fertilizers, and in assessing the health benefits of crops cultivated in different environments, it is critical not to extrapolate health benefits, or the lack thereof, from the mere concentrations of elements in grow media or fertilizer solutions.
With regard to other nutrients, knowing the concentration of particular elements within a fertilizer solution does not perfectly inform the relative healthiness of the crops being cultivated. Looking at iron as an example, roughly a year ago, I got a call from a grower who was experiencing significant iron deficiencies across her crops. Regardless of the high levels of iron that she was dosing into her system, her plants continued to exhibit iron chlorosis, a yellowing of the plant leaves that leads to decreased plant health. Upon a site-visit to the facility, I found that she was dosing her system with Iron (III) Oxide, not chelated iron.
Though the fertilizer in her water contained high concentrations of iron, the iron in her fertilizer was insoluble, and unusable by her crops. Chelated iron, which should have been dosed into the system is water soluble, and readily available for plant uptake and utilization. Whenever discussing concentrations of various elements in growing fertilizer, it is critical that attention be paid to the form of the element; this is true for macronutrients as well, such as nitrogen, which preferably will be held in nitrate nitrogen, and phosphorus, in the form of phosphates.
Let’s circle back to the question of the relative healthiness of hydroponic and aquaponic crops relative to soil-grown crops. In recent years, soilless growing methods have demonstrated an ability to produce crops with comparable nutritional values to conventionally grown crops. Contrary to what some might argue, U.S. and European hydroponic facilities do, in fact, monitor and maintain proper levels of micronutrients in their fertilizer solutions.
In my personal research, related to developing natural hydroponic fertilizers and sludge-handling aquaponic systems, my team has been able to demonstrate that soilless growing methods can produce a variety of crops with higher nutritional values than the United States Department of Agriculture (USDA) reported averages from conventional soil growers. Over 5 years ago, to demonstrate the ability for soilless operations to compete nutritionally with organic soil cultivators, I constructed a commercial, low-discharge, aquaponic facility, with the ability to process all of the sludge waste into usable plant fertilizer. In this facility, rotations of lettuces, kales, collards, tomatoes, and a wide range of other crops, were grown.
Tissue testing, by third-party labs, showed that this facility consistently cultivated lettuce crops with 24% more protein, 88% more calcium, 37% more magnesium, 18% more phosphorus, 41% more potassium, and 109% more zinc than reported averages for lettuces cultivated in soil. Similar testing of kale and collards showed increases in protein of 48% and 241%, 341% and 63% more calcium, 134% and 162% more magnesium, 41% and 167% more phosphorus, 33% and 84% more potassium, and 243% and 353% more zinc, respectively.
In addition to increasing the nutritional value of certain crops, soilless growing methods carry a number of inherent advantages over traditional farming practices. Given the high level of control over inputs, hydroponic and aquaponic crops are significantly less susceptible to heavy-metal uptake and contamination from pathogens and pests than outdoor, soil-cultivated crops.
Additionally, as has been demonstrated by the European hydroponic tomato industry, safeguarding crops from soil-borne pathogens and other stresses of outdoor production, strains of tomatoes can be developed that are selected for taste, texture, and nutritional value. This is opposed to many out-door, soil-oriented varieties that have been historically selected for harvest-weight, durability, and pathogen or pest resistance, a process which led to the proliferation of ‘water-bomb’ tomatoes, as noted by Sanwen Huang, Agricultural Genome Institute.
As a final note regarding the naturalness or healthiness of plants cultivated with various growing techniques, often the greatest factor that impacts the nutritional value of crops is the time from harvest to consumption. One of the leading drivers for the boom in the soilless growing industry is the ability to achieve high-volume crop production near consumer-bases, within cities, or along major distribution routes; this allows for short distribution times and decreased nutritional losses.
