Quality of produce grown without soil
When we buy fruits and vegetables, sometimes it is not clear whether they come from plants that have ever touched the soil or not. I wanted to research a bit more about the topic of soilless food production and in this article, I will summarize and share with you what I have found in the scientific literature. Namely, I am interested to understand the difference between the quality of food produced in our out of soil. I will first introduce briefly soilless production systems and then continue with evaluating studies analyzing nutritional value of such produce.
There are four vital factors for plants growing successfully: soil, sun, air and water. The soil provides structure where the plant can anchor the roots, further the soil provides air and nutrients that are fundamental for the growth. The sun is the source of energy, with which the plants build up carbohydrates from carbon dioxide in the process of photosynthesis. Finally, water helps to transport nutrients, regulate the temperature and stabilize the plant body. And water also takes part in metabolic reactions. While in Nature all these factors are vital for most plants, in artificial systems it is possible to grow plants without soil – hence the name soilless agriculture.
There are different forms of soilless agriculture:
- Hydroponics
- Aeroponics
- Aquaponics
In hydroponics the soil is replaced with a substrate, for example coconut fibers or stone wool. To this substrate a well adjusted nutrient solution is regularly added, such that the plants can grow optimally (strictly speaking hydroponics is growing in nutrient solution). On the other hand, in aeroponic systems, the plants are mounted on a support system and its roots are hanging freely in air and are regularly sprayed with a nutrient solution. Finally, aquaponics is a combination of a fish tank and a plant production. The waste from the fish production serves as nutrients for the plants, which are typically hanging in a support system, similar to aeroponic systems. Soilless production of food is very common in greenhouses, where an intense production is desired and thus, the control of many growing conditions is beneficial.
One might think that growing plants without soil is a modern method of cultivation, however, such methods were already described in Egyptian hieroglyphs hundreds of years B.C. [1]. But with today’s modern technology, soilless production can be arranged quite efficiently, benefiting from certain advantages comparing to in-soil production. The biggest advantage is to have more control over the environment, and thus, pest and pathogen control is easier. Further, the nutrient solution can be adjusted precisely. On the downside, soilless systems can be rather complex and come with high costs for the installation, therefore it is mostly interesting to grow value crops like tomatoes, peppers or lettuce. Here is a non-exhaustive list of advantages and disadvantages:
Advantages of soilless agriculture
- Control of nutrition
- No fertilizer leaching (the solution can be kept in a recycling loop)
- No soil erosion
- Reduced disease and weed control needed
- Better ergonomics, when fruits can be picked at a comfortable height
- No need for fertile soil
- Longer shelf life of produce
Disadvantages
- Requires production of substrate and nutrient solutions
- Recurrent costs for substrate and nutrition
- Energy input to keep system running
- High initial investment cost
- Knowledge intense
- Possibility on missing out on other factors, which are present in natural systems
Soilless production has increased strongly in the past years [2,3] and many people are eating it on a regular basis even without knowing it. Therefore, in this article the main focus is to assess the quality of produce grown without soil. In a future post we will talk about another important aspect regarding soilless cultivation – sustainability.
Quality
In principle, a hydroponic system has a lot of control over the nutrition accessible. So let’s see what researchers found regarding the quality of soilless produce.
Tomatoes and peppers
An article of 2009 summarized many studies, which explored quality parameters such as mineral and vitamin content, taste, sugar content and others [4]. In most studies, no difference between conventional soil and soilless cultivation was observed, but some studies indicated better vitamin content, taste and sugar for produce grown without soil.
In peppers, the results are similar: higher sugar, more phenolic compounds and higher vitamin C content along with higher antioxidant activity in produce from soilless systems, compared with a production with low-input of resources and organic production [5]. However, such a difference can also depend on the pepper variety, as some may be better suited for soilless production [6].
Beyond the growing period, ripening is also important for fruit quality. One study looked at the ripening of hydroponically produced tomatoes and found that fruits, which ripened on the vine had more of the antioxidant lycopene [7].
Lettuce
Vitamin C and E were observed to be increased in lettuce produced in soilless compared to in-soil production along with an increased content of phenolic compounds [8, 9]. In another study, a sensory panel assessed taste, odour and visual quality of lettuce, but did not find differences between in-soil and soilless [10]. One advantage of soilless production is the absence of contamination with soil pathogens. This can also contributes to a better quality after storing [8]. In some cases, lettuce is not cut, but sold with the roots, which also helps to maintain the quality over longer time.
Strawberries
For strawberries, a similar picture emerges: increased Vitamin C and E and increased phenolic compounds in produce from soilless production [11]. The content of bioactive compounds in the fruits can even be adjusted by the nutrient solution applied. For example, a study showed that limiting phosphorus and iron increased the contents of phenols in the fruits [12]. In addition to the differences in the nutrient content, participants in a study were also able to discriminate between hydroponically and soil grown fruits and most prefered the soilless [11]. This shows that also the taste of fruits grown without soil can even outperform conventionally grown strawberries.
Conclusion
This short overview shows that produce grown without soil can at least keep up in the analyzed nutrients as well as in taste. This may not be true for every hydroponically produced fruit, as hydroponic cultivation is complex and mistakes can be made. And while we know many things about plants and human nutrition, there are probably still many things we don’t know, which could play an important role in the nutrition of plants and ourselves. The basic factors influencing nutrient profile and taste of fruits and vegetables are: plant variety, accessible nutrients and growing conditions such as light, temperature and humidity. Some systems apply artificial lighting, which thus may also modify the quality. Sometimes the manipulation of nutrients can increase an interesting compound in a plant. However, this may lead to a decrease of yield. For example, increased salinity in the nutrient solution can encourage a plant to produce more antioxidants but at the same time limit biomass production [13].
As many factors are affecting the compounds in produce, the limited research available doesn’t really allow for a definitive conclusion regarding the quality of food produced in soilless systems. It’s true, soilless production doesn’t sound very natural, but on the other hand there are certain benefits to it. Notably the reduced pressure of pathogens and pests, which in turn allows to decrease plant protection measures. And the use of no or fewer pesticides eventually benefits the quality of the produce, by reducing the toxic load on the food we eat. Since this reduction of pesticides is also a hallmark of organic produce, it would be interesting to see a comparison of quality between soilless and organic agricultural systems (and also compare to permaculture systems).
For a healthy diet, considering the current state of knowledge, it seems more important to eat enough fruits and veggies no matter whether grown in soil or without (although it is good to make sure they are organic to reduce the toxic load). It’s also not always obvious if a produce comes from a hydroponic or conventional system, since labelling is may not be required, so going for organic gives some certainty that it contains less toxins. In many countries, vegetables and fruits from soilless production cannot be considered organic (as for example in Switzerland [14]). One exception to this are the United States of America, where it is possible to have organic food from soilless production [15].
This post focused on the nutritional value of food from soilless production. During the process of writing, I wondered more and more about the sustainability of soilless systems, encouraging me to look more into it in the next post. So stay tuned! 🙂
Let us know what you think about this topic!
And stay healthy 🙂
Christoph
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