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Soil-less or soul-less plants?

We are rapidly moving towards a high-tech environment for food production. Whether synthetically produced products or artificially fortified processed foods with vitamins and minerals, supermarket shelves are full of new food choices. But do we have enough evidence that these foods are nutritionally rich, healthy and safe for us and our environment?

Hydroponic cultivation is one example of fast-growing and popular technique for plant production, without soil, in recirculating and re-used nutrient-rich water and supporting bucket systems.

There are few types of hydroponics, depending on different characteristics. Some hydroponic systems use inert mediums like stone wool, clay pebble or perlite to provide support for the plant’s root. Usually plant roots are suspended into nutrient-fortified solutions where they grow quicker with higher yield than in traditional methods.

This nutrient solution may be recirculated (closed system) or introduced on every irrigation cycle (open system). In the closed system, the nutrients concentration is in constant recycling, adjusted and monitored state. While in the open system, it’s discharged after each nutrition cycle. Oxygen, nutrient content, salinity and pH have to be closely monitored because roots, constantly immersed in water, are highly susceptible to fungal infection. Mold and algae can multiply very rapidly in the reservoir, especially in complex commercial systems, depriving plants from oxygen and shifting pH water. To protect the plants, hydroponic growers often use chemicals, UV light and other cleaning techniques. Moreover, humidity, water quality and temperature variations may impact biochemical content of plants. Some growers also use electric lights instead of the sun, artificially extending day-length and manipulating light wavelengths to produce year-round high yield fast-grown pants.

The use of herbicides and pesticides may not exist in hydroponic systems, but organic certification has been mostly prohibited for hydroponic products because of chemically-synthesized nature of nutrient solutions used in this form of agriculture. Hydroponic proponents believe that by providing ample nutrients and oxygen via chemical fertilizers, plants grown hydroponically are nutritionally “healthier”, and the use of synthetic fertilizers does not impact the quality and safety of their products. That’s why many farmers are experimenting with “organic” cocktails of nutrients to seek certification. But such organic fertilizers are generally insoluble in water, requiring microbial action to be converted into soluble, which might not be possible in standard “sterile” hydroponic environments.

Some hydroponic growers openly promote their produce as healthy as organic products. Unfortunately, research comparing nutrient content of organically soil-grown plants and plants produced hydroponically is scarce. Some studies show no difference between two methods, while others report better or worse nutrients levels for soil-based plants. But they typically compare non-organically produced products to hydroponics. For example, soil-produced raspberries were reported to have much higher survival rate, lower yield and statistically significant higher vitamin C content. However, tocopherol, total polyphenolic compounds and catechin content were similar to hydroponically grown-raspberries. Interestingly, higher polyphenolic content for soil-grown raspberries was shown in the harvest of the previous year. Another study found no difference in the sensory evaluation between lettuce types, irrespectively from how they were grown, but it did not analyse their nutritional quality. Studies on tomatoes showed no significant differences in the levels of lycopene content (carotenoid responsible for the red colour in tomatoes), but more optimum levels of sugars, pH and organic acids in tomatoes produced under highly controlled hydroponic conditions. The content of lutein, beta-carotene and some photosynthetic pigments were lower in hydroponic lettuce compared to soil-based, due to less exposure to sunlight and temperature.

Unfortunately, it’s very difficult to compare plants, as their nutritional composition depends on the particular variety, ripeness, storage methods after harvesting, climate, altitude and weather.

Hydroponic system relies on the simple principle that plants only need three fundamental things: water, oxygen and nutrients to thrive. But the relationship between soil, microbes, bacteria, light and plants is much more complex. It’s an entire ecosystem. Healthy soil with an abundance of beneficial micro-organisms allows plants to better survive in a stress environment through their interaction with root-based transport systems. Health and biodiversity of soil is the core principle of organic growing, but hydroponic proponents argue that their systems are aligned with environmental principles, although their production methods require more energy.

It’s true that the use of hydroponic methods avoids negative impacts of conventional agriculture such as inefficient and high use of water, large land requirements, soil-erosion, high use of herbicides and pesticides, large waste; but regular irrigation and fertilization may result in possible contamination of surface and groundwater. In addition, to control infestation and disease, disinfection systems are mandatory. In terms of bioavailable compounds, we know that naturally-grown plants offer much more than just vitamins and minerals. Plants contain hundreds, if not thousands, of different constituents that interact in a complex way. We are still discovering more and more phenolic acids and other phytochemical compounds, exercising all sorts of health benefits. By artificially providing “perfect conditions” to plants, won’t they lose natural bioactive elements, anti-oxidants and flavonoids, produced to protect themselves from UV radiation, insects and other natural stressors? We need more research to understand the real nutritional value of hydroponic plants and not just measuring whether hydroponic tomatoes smell better.

Hydroponic may be a solution in places where traditional farming is not an option. It’s also quickly gaining popularity in large cities with limited space where vertical-farms are designed in building spaces, allowing growing plants indoors. Even large supermarkets start installing hydroponic systems offering customers the experience of harvesting their own plants.

In future, restaurants will also offer similar vertically grown plants to customers.

Surprisingly, our traditional Europe seems to be the leading growing market, followed by the USA and Asia Pacific, reaching USD 16bn by 2025. The adoption of hydroponics has been the highest in the Netherlands, followed by Spain and Turkey. According to the industry experts, almost 90% of the greenhouses in the Netherlands are converted to hydroponic structures. Hydroponic market is also expanding in Russia, Germany and France.

Commonly produced products include lettuce, other leafy greens, tomatoes, herbs, cucumbers and strawberries based on hydroponic methods.

Knowing this I probably need to spend more time checking the origin and production methods before buying my fruits and vegetables. Of course, it’s a personal choice whether you want to consume products low in man-made chemicals but also potentially deprived of phytonutrients. Another potential trade-off is whether to prioritise your beliefs in the symbiotic relationship between soil, sun, water and living organisms but potentially at the cost of being exposed to a certain level of natural chemicals used even in organically grown plants.

Currently, organic certification is prohibited in European Union (from 2021), Canada and Mexico for hydroponic products, but not in the USA. Appropriate labelling should be required to help us navigating in this tech-growing food industry.

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