HYDROPONICS: Great potential, but with challenges

Wageningen UR researcher Tycho Vermuelen examines a trial deep flow system.

Wageningen UR researcher Tycho Vermuelen examines a trial deep flow system.

In Europe, substrate culture and hydroponics are regarded as separate growing techniques, where substrate-less culture is known as hydroponics. In this article, the author examines the business case for developing new hydroponic systems that increase crop yields, and he highlights the bottlenecks and challenges for large-scale system developers.


Using no substrate at all for growing crops—hydroponics—has been around for ages. Yet it never became commonplace in the Netherlands, a country where soilless cultivation is the norm. How did that come about, and are the Dutch missing out? In recent years there has been renewed interest in these ‘new’ production systems, leading to a series of studies growing different crops. What progress has this research brought us, and is there the potential for a large-scale breakthrough?

The reason for this interest is similar to initiatives in system developments in the past: prevention or reduction of root diseases, recirculating water to minimise emissions and, above all, to get better control of the root zone to deliver higher yields and product quality. However, such benefits need to be quantified in order to make a business case for new systems.

Depending on the root disease, crop damage can be estimated, while direct costs of soil fumigation and crop protection are known for specific crops. The benefits due to steering nutrition better can be reasoned through an understanding of plant physiology: improved control in the root zone leads to less nutrient deficiencies and less plant stress from drought or over-saturation. Not having to use overhead irrigation will also lead to lower fungal infection, and less energy use for greenhouse climate control.

On the basis of multiple tests on a small scale, we found that 20-30% more growth can be achieved in hydroponic systems compared to soil-bound cultivation. Taking into account the cost of the systems, these yields make hydroponics an interesting business case.

The potential of developing new systems has brought a number of growers and technical suppliers together, cooperating in eight consortia of different size to design new hydroponic systems. Over the past 10 years, research has delivered enough insights into nutrient (pH) regulation, oxygen supply, uniformity, as well as irrigation systems, substrate choice and general hygiene.

We realise that variety selection is of critical importance: not every variety will grow well on substrate, and that’s okay. As long as a number of market-relevant varieties do well there is a base for further development.

In addition, some technological developments have occurred in recent years. One is the concept of cross-NFT, where each plant has its own water stream, rather than having to drink the ‘waste’ of the neighbouring plant, as is the case in conventional NFT systems. Likewise, there is more understanding of the different floating ‘rafts’ for deep flow systems—the raft is the main point of difference among the consortia.

Finally, in recent years, there has been a strong focus on developing resilient growing systems: to prevent situations where plants could be weakened (stressed), or where pathogens can flourish. Although this scrutiny has not led to conclusive strategies, it has given a better understanding of plant-disease interaction.

The current research-scale experiments, as well as in demonstration facilities (1000-2000 m2), have identified the following bottlenecks towards profitable new systems:

• Planned production—being able to plan the production cycle

• Logistic systems—having a sound logistical system

• Micro life and resilience—understanding the dynamics of micro-organisms and their interaction with plant development, which would also translate to disease resilience; and

• Scale—overcoming the often negative impact of scale.

More growth can be achieved in hydroponic systems compared to soil-bound cultivation.

More growth can be achieved in hydroponic systems compared to soil-bound cultivation.

Planned production
Over the years, we managed to design uniform growing systems, or at least understand what leads to non-uniformity in order to make a proper cost-benefit analysis of the technology. Uniformity has to do with the fact that all plants receive equal conditions in terms of water, temperature, light, nutrients and humidity (VDP). Known errors in uniformity are the salt build-up in ebb/flow systems, nutrient changes throughout an NFT-gully, and the challenges in getting uniform overhead irrigation with sprinklers (and cleaning the system to keep the uniformity up). With an understanding spatial uniformity, the next step is to understand optimal planning of crop cycles.

In hydroponics, temperature fluctuates more than in soil-bound systems. The water temperature follows the greenhouse temperature and can, if so desired, be controlled (at a cost). The temperature influences crop development, and is therefore a factor to deal with in crop management. This factor—or apparatus—should be properly understood. At the moment, the interaction of activity of the seedling (grown in warm or cooler situations), the fluctuation of water temperature, and traditional growing conditions (light, temperature, humidity), lead to mismatches in crop planning.

Deep flow systems have the potential for better crop planning, but they require a learning curve to make good use of them. And this learning is not optional. The structure of the horticultural chain requires that a grower should be able to schedule production.

Logistic systems
Logistics is one of the main reasons for developing substrate-less systems. Logistics include functions such as planting, transplanting and harvesting. Once the plants are no longer fixed in the ground, but in a small holder or on a strip, suddenly, anything is possible.

Many system developments started with a logistics concept, and rightly so. Think of MobyFlower (mobile transplanting), the mobile rose and gerbera growing systems (harvesting at one location in the greenhouse), and Dry Hydroponics (planting at the one end of the pond and harvesting at the other end). Regardless of whether it was a success, these initiatives show that business operators need a unique (preferably patentable) concept to base their investments on. While research from collective funding focuses on the plant needs, corporate investment can focus on patentable technologies that utilise these plant-insights. Integrated system development requires collaboration between the two.

At present, there are only a few commercial turn-key systems on the market including NGS (New Growing Systems), Hydroplan and Dry Hydroponics. This leaves room for new initiatives and concepts. Companies interested in operating in this market should aim for (patentable) solutions to the following challenges:

• Generic system for leafy vegetables. Different product-market combinations require different planting distances and production time. Static systems limit growers in their flexibility. For example, deep flow rafts currently have fixed holes that will only work for specific combinations;

• A system that prevents crop damage during transplanting. Transplanting often causes root or leaf damage, which slows growth for two to four days. A smooth transition—careful handling, and enabling a micro climate in the receiving root zone equal to the old root zone—benefits plant health and growth;

• A system or method of moving plants (planting or spacing). One that is cost-effective.

Some earlier system challenges have been tackled, although not always commercially viable; such as keeping rain water out of the basin, having a light-tight basin, reliable oxygen distribution, and dealing with changes in crop weight during production.

Micro Life and resilience
Substrate-free cultivation systems are not sterile. When working with so much water, it is no longer feasible to purify the water several times a day. In a large touristic aquaria (where money plays less of a role), it is possible to decontaminate the water at a rate of 100% per hour; but not in horticulture. A basin will, therefore, behave as a complex ecosystem. Three weeks after a very clean start, while using only UV-treated rainwater, we found over 100 species of bacteria and dozens of species of fungi.

The colonisation of the basins by micro-organisms seems at first like a random process. In a major screening of several basins we found that there is no one single ecosystem, which evolves in deep flow basins, given similar starting conditions. In general, the micro-organism composition is dominated by one to four species, but not always the same in different basins. It is unclear whether populations will become more similar over time, given similar cultivation practices.

Next, we see an effect of active inoculation with certain micro-organisms. Inoculating a basin with either beneficials or water from another basin will affect the micro-organism population in the receiving basin. Inoculation with beneficials seems to have a positive effect on plant growth and disease resilience. It is not yet clear which organisms bring the positive effect, and what principle of plant-microbe interaction leads to better plant development. Possible positive interactions could be: direct antagonism of pathogens, competition against pathogens by colonisation of the roots, or by using up all the free sugars in the water, or assisting the roots in nutrient uptake.

Our initial findings show that the micro life in water matters. As a general hypothesis, it seems better to inoculate with beneficial organisms while keeping total microbiological low. This activity can be controlled by removing sugars and organic matter from the basins and keeping water temperatures low (<25 degrees C). Killing organisms using UV or peroxide and releasing dead organic material back into the system for others to feed on seems not to be a successful strategy, according to this hypothesis. However, build-up of organic matter is unavoidable (leaves, broken-off roots, dirt, algae growth), so frequent removal of debris is essential. Lastly, bacteria that are encapsulated in a slime layer on basin walls and irrigation lines are difficult to remove and are still very able to multiply. It is not clear whether these encapsulated bacteria become a ‘show stopper’ or just a factor to be dealt with.

Because many of the problems of substrate-less cultivation occur in the root zone, research focus needs to be on creating a healthy rooting environment.

With the first hypothesis in place, it is time to do systematic research.

At a research scale, it is always difficult to get sick plants, while at (semi) commercial scale plants too often get sick. This also applies to pests. This phenomenon is more than just the monoculture-aspect of crops and, therefore, the easy spread of pests and diseases.

Also, water with high densities of pathogens is more likely to cause mayhem on a large scale than in a small basin or bucket. While temperature, irradiation, VPD, oxygen and nutrient availability are strong influencers on plant resilience, it seems to be the tidiness of the system that tips the balance: small systems are often very neatly made, while large-scale systems have gaps and leaks that lead to algae formation, debris that enters the system, and they are generally more messy.

Final remarks
In the Netherlands, several consortia are developing hydroponic systems, often in cooperation with researchers, to capitalise on the opportunities of increased production. The greatest challenges rest in the area of crop planning, logistics, plant resilience and growing on a large scale. In addition, entrepreneurs will have to make the link with the market to position these systems as sustainable innovations, producing healthy crops for consumers.

About the author
Tycho Vermeulen is a researcher at Wageningen UR Greenhouse Horticulture (www.glastuinbouw.wur.nl), with a focus on developing novel production systems in the greenhouse sector. Ω

September 2015 / Issue 159