How sustainable are soilless growing systems?
In my previous post I looked at the literature regarding the nutritional content of fruits and vegetables from soilless production systems (hydroponics). There is still not so much data on it, but it seems that this form of production can keep up with food produced in soil. While writing that post, I was more and more wondering how environmentally sustainable such a production could be. So in this article I will summarize what has been found regarding this question. Since soilless production is often performed in the context of controlled environment production in greenhouses, I will also expand to the sustainability of this form, which uses temperature and lighting control mechanisms.
Resource efficiency
With resource efficiency I mean how much of the inputs are needed for the production of a unit of fruit/vegetable. The resources comprise water, nutrients, substrate, space, energy and others. I will go through some of them to assess advantages and disadvantages with regards to soilless production.
Water
When a closed soilless system is used, the water stays mostly in the system and does not leak uncontrolled out of the system. Comparing to open soilless or soil systems, a closed system uses much less water, since it doesn’t get lost in the soil and evaporation from soil is reduced [1]. A study simulated the water use of of a conventional and hydroponic lettuce production and found the hydroponic system uses 13 times less water than the conventional production [2]. Another study compiled water usage of different tomato growing systems and also found that closed systems use considerably less water [3]. But even when grown in soil, but in a greenhouse, less water was needed comparing to open field production.
The fact that closed soilless systems have a high water efficiency makes them interesting for areas with a climate with low water availability.
Nutrients
As discussed in the previous article, in a closed soilless production systems, the nutrients can be recycled and don’t get lost in the soil and waterways. At the same time the adjustment of the nutrient solution is complex and typically requires to have different partial nutrient solutions, which will be mixed together. These solutions have to be produced, requiring energy and resources, but at the same time, even in many production systems with soil, such fertilizers are also used.
An alternative to chemically produced fertilizers are organic fertilizers such as composts or slurries. However, in soilless systems the applicability of organic fertilizers may be limited. Firstly, it may be challenging to adjust the nutrient solution with such a complex mixture of natural fertilizer like compost or slurry. Secondly, some components in these fertilizers first need to undergo a transformation by microorganisms, before plants can use it. This transformation in soilless systems may be very limited due to lower number of microorganisms. However, some researchers already tried to introduce microorganisms in hydroponic systems to use organic fertilizers and it seemed to work for them [4].
So, in principle, the nutrients can be used very efficiently in closed soilless systems, since losses are very limited, but it is unclear to what extent organic fertilizers could be practically used.
Growing substrate
Instead of soil, in soilless systems the plants grow in a substrate, which provides some structure for the roots and serves as a buffer that hold some of the nutrients applied. Whatever the substrate used in a soilless system, be it rockwool, peat or coconut fibres, it has to be produced and transported somehow. After its use, the substrate is discarded and there, some substrate have raised concerns since they don’t degrade, such as rockwool [5]. For other substrates like coconut fibres the questions arises if it makes sense ecologically to transport it over long distances. So to me it seems that using a local resource such as soil comparing to any substrate still has certain advantages. Substrates come with another advantage though: they are typically devoid of any soil life and therefore don’t contain pathogens.
Space
Soilless systems are very flexible regarding space. They can be stacked as vertical gardens or built on a sealed surface. This reduces the requirement for space comparing to classical field production. A compromise could be to use soil like a substrate and place it in containers, as we already do with potted plants. So probably not much of a difference between soil and soilless here. However, another important point to consider when stacking plants vertically is whether they get enough light, in order to grow well. In vertical structures, the light may not reach the plants optimally because of shading and sun trajectory.
Plant protection
One of the biggest advantages of soilless systems is the absence of soilborne pathogens. These organisms can be detrimental to production and result in huge losses. Therefore, in open field production the rotation of crops is very important in order to avoid a build up of certain pathogens. By planting plants of different families over time, a pathogen that thrives on a particular crop will decrease when this crop is not available. This could also reduce the need of pesticides, and it is therefore common practice in organic agriculture. In some cases, soil is treated with steam or chemicals to get rid of infestations of pathogens. In soilless production the substrate doesn’t contain soilborne pathogens and therefore plant protection measures can be reduced, but this is still no guarantee that no root diseases will occur. And in addition to that, the aquatic environment in soilless systems can favour other types of pathogens, normally not present in soil [6].
An advantage of protected production, be it in soil or soilless, is better control of other sources of pathogen contamination. Wind can bring spores of microorganisms which may grow on plants causing damage, and rain modifies microclimate and thus contributes to the growth of plant pathogens.
Energy
The production and transport of inputs (e.g. water and nutrients) to the growing systems uses some energy already. The production of nitrogen fertilizer for example is quite energy intense (see my other post). Since closed soilless systems have almost no loss of fertilizers, therefore needing less of them, which in turn uses less energy. However, what is unique to soilless systems are the circulation of tubes and pumps, which constantly require electrical energy. This in fact makes them very dependent on electricity and huge losses can occur should there be a power outage.
A more considerable amount of energy is used in temperature control and artificial lighting. In greenhouses heating is quite common and supplementary lighting is also often employed. In some cases, crops are produced with artificial lighting only. A study estimated that a hydroponic lettuce production with heating and supplementary lighting produces ten times as much lettuce but consumes 80 times more energy than conventional production [2], which makes hydroponic production much less energy efficient.
Conclusion
There are many factors to consider to evaluate environmental sustainability. Soilless systems use less water and nutrients if they are closed, but then they require special substrates, expensive infrastructure and lots of energy. If greenhouses are heated and use artificial lighting, the energy consumption increases dramatically, which decreases the sustainability of such a system. But it also depends where the energy is coming from. In some cases greenhouses are coupled with waste incineration plants, so they can use the heat energy directly [7]. Current soilless systems are also limited to a few, valuable crops (e.g. lettuce, tomatoes, strawberries) and it is unlikely that the three calorie crops – wheat, rice and maize – will be produced in soilless systems on a large scale, since infrastructure costs are high and the market price is much lower for these three crops. For example, agricultural researcher Louis Albright from Cornell University says that using hydroponic systems with artificial lighting to grow enough wheat to make a loaf of bread would cost US$ 23 (!), which is way above what we would be willing to pay for [8].
In arid areas with poor soils, where otherwise no food could be produced, soilless production may be an option if energy can also be harvested on site, e.g. through photovoltaics. But these unnatural systems, as productive as they may be, are also quite fragile and depend on very specific inputs. They cannot sustain themselves, as natural systems can.
Soilless production is sometimes considered environmentally friendly, because it uses less water and nutrients, but we should be careful to try to consider all external effects. Still, I find soilless systems interesting and I will follow their development. At the same time, they are very far from natural systems and I think production in soil will remain very important. Therefore, I think it is crucial that we keep searching for ways of production that is water, nutrient as well as energy efficient. This may or may not include methods for soilless production, however, regenerating our (degraded) soils should have a high priority, since we depend on them for most of our food.
Let us know what you think about this topic!
And stay healthy 🙂
Christoph
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