With a little more effort you can improve even more. This is the approach behind new trials aimed at optimizing water and fertiliser use with the specific aim of reducing fertiliser emission to the environment. It sounds rather demanding until you realise that the quality of the end product can also improve, while maintaining or even increasing yield. Excess drain as an insurance policy for the cultivation is an out-dated idea.
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It’s a domino effect. Modern greenhouses that have been specially developed to save energy and reduce CO2 emissions have a different climate than before. It becomes humid faster. This, in turn, affects the plants, which as a result take up less water. Less water consumption means that the transportation of fertilisers is at risk. One change has an influence on several processes.
Several trials are running at Wageningen UR Greenhouse Horticulture, Bleiswijk, the Netherlands, and the adjacent GreenQ Improvement Centre, to put the spotlight on less water consumption and a reduction in the total emissions of fertilisers. Eelke Hempenius, of Grodan (supplier of stone wool substrates) and Ellen Beerling, of Wageningen UR, describe the research in which the emphasis is on participation by suppliers. “This synergy is needed,” believes Beerling. “By their [the suppliers] involvement, solutions for emission-free growing are more quickly taken up in practise.”

Fewer obstacles

This year Hempenius is using an area that contains a lit (by LED) tomato crop (Komeett). Here he can scale-up a number of trials that were carried out earlier elsewhere on a small scale. “We have the vision that optimal cultivation begins in the propagation phase. For example, seed should be able to germinate uninhibited. The fewer obstacles present, the easier it is for a healthy crop to develop.”
Since 2006 the company has been producing a stone wool block with an improved capillary action, so that the roots are well distributed and grow more at the top of the block. The researcher’s role is to study and optimise root growth. The step from propagation in the block to rooting in the slab needs to occur unhindered. “I’d like to see the roots at the top spread through the slab as quickly as possible. They shouldn’t grow directly downwards but instead remain at the top where the most oxygen is available.”

Controlled growth

The tomato trial involves a derivative of the Elite system, in which each plant has its own slab. In this trial there are three plants per slab. A block of 10x15x7.5 cm sits on a 10 cm high slab that has two different layers (dual density). The two layers ensure that the plant firstly creates well branching roots at the top before they start growing downwards.
“We want the plant to grow in the most controlled way possible because when the plant starts to make roots very quickly, growth of other plant parts comes to standstill. This creates an imbalance,” explains Hempenius. Hempenius believes with the right young plant specification and block / slab combination there is no need for a pre-planting phase, however it still needs to grow in a controlled way.
The question is, how much water and fertilisation does the plant really need for optimal growth? Wageningen UR Greenhouse Horticulture has created a model that sets down how much a plant should transpire to ensure the proper transportation of nutrients to all parts of the plant.

Excess drain is not necessary

If the plant grows in a controlled way it is easier to steer the watering in a specific direction. Therefore two different watering systems were purposefully chosen for this trial: A two litre dripper system was compared with a three litre system. The trial with two litre drippers aimed to steer the EC in the slab to 5.5 – 6, with a maximal of 20% drain. The treatment with the three litre drippers aimed to steer the slab towards an EC of 3.5 – 4 with 30-35% drain.
Attention focused on the system with two litres and a high EC. As expected, a tomato plant develops well at an EC of 5. “There is a relationship between the EC level and the water content in the slab,” explains Hempenius . “Therefore if I can steer more critically I wonder if an excess of drain is really necessary.” The idea behind the need to create drain comes in part with the desire to achieve more uniformity in the slabs as a kind of insurance policy for the cultivation.
Steering with a higher EC makes this trial attractive because it improves the taste and quality of the fruits. The researcher recognises that talking about EC-steering is still a grey area. This can still be very interesting because it could make it possible to link together taste and production. That is often still contradictory.

Without emissions

Drain reduction is one thing but the goal of Ellen Beerling, with her project ‘Emission-free Greenhouse’, is the total elimination of greenhouse water discharge to the environment. “Together with our partners we are convinced that in fruit vegetable production it is possible to have emission-free production provided that some conditions are met,” she says. They recently completed the first trial which involved a cucumber crop. The project is continuing with peppers. She is comparing two compartments, one with emission-free production and production according to the current emission standards.
In recent years a lot of knowledge has been gained as to why growers discharge water and how this can be prevented. Testing and demonstrating these solutions is carried out in the emission-free greenhouse. Beerling: “We use irrigation water low in sodium and we disinfect it prior to it going to the fertiliser dosing untiwith ozone asozone also breaks down growth inhibitory substances.”
The basis for an optimal crop in this greenhouse is centred on the Precision Growing strategy developed by Grodan. By taking accurate measurements in the root environment it is possible to optimally steer the watering and fertilisation strategy. Frequent analyses of the fertiliser in the irrigation water and drain make it possible to add nutrients in a precisely controlled fashion. In this way the plant gets exactly what it needs.
Complete recirculation takes places from the beginning. The use of a flat-bed filter eliminated the need to back flush filters, an often overlooked reason for greenhouse water discharge..


Beerling measures the yield and quality of the fruits, water consumption, emission of nutrients and crop protection products and calculates the costs. “This trial also has a demonstration function, which involves many important players in the horticultural sector. But we also ask ourselves if we have overseen any bottlenecks. Of course we want to demonstrate what we already know but simply by doing things here we learn a lot.”


Optimum use of water automatically means optimum use of fertilisers. Research into responsible water usage in a tomato crop, aims to show that the combination of a good stone wool slab and a well-designed irrigation system means initial drain volumes can also be reduced. It needs to be made clear that in an emission-free greenhouse, full recirculation is possible with at least the same yield and quality.

Text/photos: Pieternel van Velden