"While it is difficult to challenge the assertion that soil is important in terms of delivering nutrients to plants, there is no evidence that shows that soilless production systems have any less of an impact on plant health and delivery of nutrients. On the contrary, the control, precision, transparency, and other key factors associated with soilless growing, especially in fully-controlled environments, confronts many of the hidden issues that exist with soil-based growing."

Authors: Tom Bennett (Plant Scientist), Dr. Jennifer Bromley (Head of Plant Research & Development) and Jamie Burrows (Founder & Chief Executive Officer)

Overview

In any new industry, proponents are often challenged to validate the core value propositions they offer. In the case of controlled environment agriculture (CEA), this often relates to the use of energy, and whether the benefits of indoor agriculture outweigh the perceived energy costs compared to growing outside. This question is being dealt with through progressive improvements in production systems, LED lighting and other technologies. Another common question regards the soilless nature of indoor growing – does this have any effect on the quality of crops grown, and if so, how does this affect human health? In this article we’re going to examine this question in detail using evidence from published, peer reviewed papers, in combination with data and know-how from Vertical Future’s operations and R&D hub.

Plant Nutrient Content

To understand if there are nutritional differences between soil and non-soil grown crops, we first need to establish what is meant by ‘nutrition’ in the context of humans and plants.

Nutrients, in the context of the human diet, specifically refers to the minerals and vitamins that that are essential for our metabolism and wellbeing, but which we ourselves cannot produce. We acquire these vitamins and minerals from our diet, of which plants are the ultimate source. Like humans, plants also rely on their environment to supply minerals that they themselves cannot produce. These are typically mineral ions such as nitrate, phosphate, calcium, magnesium, sulphur, copper, iron, zinc and others, many of which are also essential in human diets. Plants acquire nutrients from their environment via their root systems, which transport macro- and micronutrients dissolved in water using specialised ion transporters at the root surface. It therefore follows that nutrient availability in the plant’s environment directly affects the nutrient status of the plant, and thus the nutrient content of food crops1.

Plant Nutrient Content in Soil-Based Growing Systems

While it is certainly true that fertile soils can produce nutritious crops, it cannot be assumed that the nutritious quality of all soil-grown crops is equal. It is well documented that environmental conditions and farming practices, including organic practices, affect the nutritional quality soil-grown food crops. Nutrient-stressed crops grown on marginal land and infertile soils are known to have lower nutritional quality2,3. Zinc deficiency in humans living in certain regions of the global south is associated with zinc-deficient soils in those areas2, serving as an example of how nutrient deficiency in a plant’s environment can impact human nutrition.

Similarly, certain micronutrients that are essential for humans are known to be non-essential for plant growth, namely the trace elements selenium and iodine4. The presence of these micronutrients in food crops is dependent on the existing soil nutrient status, which is poor in many parts of the world. The content of these minerals in crops can be boosted through the application of these minerals to soil1, though where farmers see no yield increase upon application of these minerals, their incentive to apply them is limited.

Photo Credit: Vertical Future Limited (c) 2016-2021

Plant Nutrient Content in Soilless Growing Systems

In a soilless growing system such as hydroponics or aeroponics, plant nutrients are dissolved in water and supplied directly to exposed plant roots, with the plant sitting in a biodegradable substrate for structural support.

With these methods, so called biofortification can be used to tailor the mineral make-up being supplied to plant roots, creating a nutritional ‘diet’ that maximizes both the nutritional content and growth of crops. The precision of these methods of irrigation makes it possible to tailor that diet to specific crops and their stage of growth, targeting both the plant’s primary and secondary metabolisms to create different responses and define different end characteristics that could not be as precisely achieved in soil-based growing. When combined with tailored ‘light diets,’ such as those used in Vertical Future’s systems, the breadth of outcomes and impact of soilless growing is further accentuated.

The advantages of this method of growth can be seen when comparing independent nutritional analysis of Vertical Future’s hydroponically grown spinach to publicly available mineral data on soil grown spinach. Here we have seen that our spinach contains comparable or increased levels of iron, manganese, potassium and zinc. Such results are also in part down to an added benefit to plant nutrition of soilless growing in a controlled environment – the potential to grow a wider variety of cultivars. By removing the pressure that pathogens put on crops in outdoor growing, higher yielding and potentially more nutritious varieties can be grown that would otherwise be susceptible to field-borne diseases. The fact that vertical farming allows for production in locales that are closer to the site of consumption (particularly in cities) reduces the time that crops spend in storage between harvest and consumption, further increasing nutrient levels and/or nutrient bioavailability at the point of consumption7.

In addition, soilless growing has the advantage of removing heavy metals from a plant’s growing environment. Most plants accumulate heavy metals as their cation transporters aren’t sufficiently selective to tell the difference between zinc (essential for plant and human health) and cadmium and arsenic (not essential to plant health and toxic to humans)5. Globally, there are more than 5 million sites across the world where soil contamination with heavy metals has been reported6. Moreover, the structure and sheer size of outdoor, soil-based farming means monitoring and controlling such contamination is a challenge. Indoor, soilless production systems, which use ‘neutral’ substrata, offer the possibility to better control, monitor, and segment contamination – in all forms, including heavy metals.

Food Safety and Post-Harvest Washing

While soil-based agriculture can produce nutritious produce, it also carries an inherent risk of crops becoming contaminated with pathogens that damage human health. This contamination can stem from both from the soil itself (as in certain organic practices), or from the extensive human handling of crops as they are processed from farm to shop. Food safety has been an increasingly hot topic for consumers in recent years, driven by several salad-linked e-coli outbreaks in the US8. While the risk of contaminated products reaching consumers is limited by washing salad and herb products in water (often with the addition of citric acid or chlorine)9, this itself consumes vast volumes of fresh water (up to 6,500L of water per tonne of produce for certain herbs). Newer generations of vertical farm, such as systems developed by Vertical Future, offer highly automated harvesting and packaging, reducing the risk of contamination from manual intervention. By growing in a controlled, clean environment, soilless growing systems can minimise or eliminate the need for such washes.

The Wider Impacts of Soilless and Soil-Based Agriculture on Human Health

Our global food system is inextricably linked with greenhouse gas emissions, biodiversity, water quality, insect populations and soil health, all of which ultimately impact human health.

In the UK, depending on the time of year, between 60 and 100 percent of some food categories are imported, including leafy greens (salads), lettuces, strawberries, and other crops that are suitable for indoor agriculture. As pressure on arable land continues to increase, not just in the UK but in other countries across the globe, so does the intensification of farming methods, the use of pesticides, herbicides, and fungicides, and the subsequent pressure on fertile soil. Soil-based agriculture is finite in the context of food production and available land, whereas indoor, soilless agriculture is substantially less finite due to being able to produce between 100-1,000X more produce per unit of land, with the only major limitation being the support structure (building) required and the weight implications.

By efficiently growing in a controlled environment, land that would otherwise be used for crop production is spared. This enables soils to regenerate their organic matter through lower-intensity, low tillage agriculture and reduced pesticide use which enables soil microbiota to recover and increased use of cover crops such as legumes which support soil regeneration, whilst still maintaining production levels through indoor production.

Indoor agriculture, while not intended to be a replacement for broadacre, outdoor, soil-based agriculture could – considering the above– also be regarded as a safeguard and promoter of the natural world, and as a consequence, human health.

While it is difficult to challenge the assertion that soil is important in terms of delivering nutrients to plants, there is no evidence that shows that soilless production systems have any less of an impact on plant health and delivery of nutrients. On the contrary, the control, precision, transparency, and other key factors associated with soilless growing, especially in fully-controlled environments, confronts many of the hidden issues that exist with soil-based growing.

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References

  1. Welch, R.M. et al., 2013: Linking Agricultural Production Practices to Improving Human Nutrition and Health
  2. Welch, R.M., 2001: Micronutrients, agriculture and nutrition; linkages for improved health and well being.
  3. Bruulsema, T.W. et al., 2012: Fertilizing crops to improve human health: a scientific review
  4. Gupta, M. & Gupta, S., 2017: An Overview of Selenium Uptake, Metabolism, and Toxicity in Plants
  5. Yoneymama, T. et al., 2015: Route and Regulation of Zinc, Cadmium, and Iron Transport in Rice Plants (Oryza sativa L.) during Vegetative Growth and Grain Filling: Metal Transporters, Metal Speciation, Grain Cd Reduction and Zn and Fe Biofortification
  6. Shentu, Z. H. E. et al., 2015: Heavy Metal Contamination of Soils: Sources, Indicators, and Assessment
  7. Kramer A. 1977. Effect of storage on nutritive value of food
  8. Carr, T. 2020. Unclean greens: how America’s E.coli outbreaks in salads are linked to cows
  9. Murray, K. et al., 2017 Food Quality and Safety, Volume 1

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