‘Sailing a steady course is key to an optimal water balance’

Richard van der Stoep and Remy Maat on watering in practice

The water balance is one of the three pillars under the Plant Empowerment concept and plays a crucial role in keeping the plant in balance. Cultivation manager Richard van der Stoep of the Delphy Improvement Centre in Bleiswijk is well aware of this. Together with Saint-Gobain Cultilene, he is constantly dotting the i’s and crossing the t’s in this respect. Stability is the creed here. “If you do extreme things in cultivation, you get extreme reactions.”

Richard van der Stoep makes no bones about it: an optimal water balance is essential for optimal plant growth. “The water balance – the balance between the intake of water and evaporation – is very important for the construction of new cells. When the plant absorbs water, it also absorbs nutrients and elements, which determines its growth and development. Many cultivation problems can be traced back to an insufficient intake of water and thus of nutrients.”

According to Van der Stoep, the water balance is inextricably linked to the energy and assimilate balance. “If there is no energy supply to the plant, for example, then there is no evaporation and the plant therefore does not take in any water. And when the plant is under water stress – because there is too little water available or the EC is too high – and a lot of energy is supplied, it will close its stomata. The plant then absorbs less CO2 and reduces photosynthesis. In short: the water, energy and assimilate balance interact strongly. They are three essential elements for achieving optimal plant balance and growth.”

Constant control
Van der Stoep is actively working on optimising his water balance. For example in the research greenhouse, where the programme ‘Kas als Energiebron’ looks at the possibilities for fossil-free tomato cultivation. The department in question comprises 1000 m2 of Merlice tomatoes being grown on stone wool. In order to achieve an optimal water balance, the cultivation manager works closely with Cultilene, which supplies the substrate, CARA MET sensors and knowledge. “We grow on Exact Air slabs, with three plants and six drain holes per slab. As far as the irrigation strategy is concerned, we rely on the results of our slab sensors and on Hoogendoorn’s Aquabalance. This sensor system shows the net weight of the substrate slabs and thus also provides insight into evaporation levels,” says Van der Stoep.

The cultivation manager uses a day length of eighteen hours in the research greenhouse. The lights go on just after midnight. “We then set a reduction of ten percent, so there’s at least enough oxygen in the substrate and around the roots. This percentage, which is linked to the slab’s saturation point after artificial lights are turned on, is usually reached after 3.5 to 4.5 hours. The Aquabalance determines the right moment to make the first reduction. This system lets you guarantee the correct reduction; the lower limit is fixed and you prevent the slabs from becoming too dry. In other words: there is constant control. On average, we perform three or four smaller reductions.”

Cumulative drain
From sunrise, the drain or saturation point of the slab are worked towards in three to four steps. This point varies from day to day and depends, among other things, on the window position, pipe demand, etc. “The Aquabalance determines how best to work towards this saturation point. In practice this means that the frequency of watering is increased. At this time of year, the saturation point is reached at around 11:00 a.m.”

From the moment the saturation point is reached, Van der Stoep strives to realise sixty percent drain per step. “Time between steps depends on the reduction we want between steps and therefore on the evaporation. But we always stick to about sixty percent drain per step. Basically, we cumulatively end up with a 25 percent drain every day.”

Remy Maat, Manager Application at Cultilene, emphasises that 25 percent cumulative drain is a good target in this specific situation. “When we control this, all the parameters in the slab in this greenhouse remain virtually the same. It also keeps the EC well under control.”

Heat ‘harvesting’
According to Van der Stoep, a relatively large amount of watering takes place in the research greenhouse: the average step size is between 25 and 30 cc per litre of stonewool per dripper. In this greenhouse, this is the ideal amount of water to achieve the right cone size for optimal slab refreshment. One of the objectives is fossil-free cultivation. For this reason, the greenhouse is equipped with an active dehumidification system. “We use this to ‘harvest’ heat from the greenhouse air, which we store and reuse. Because you remove moisture and therefore energy, the plant will produce moisture and evaporate more, so you have to provide more water. On the other hand, we save energy. So, on balance, we make gains in sustainability.”

Yo-yo effect
According to Van der Stoep, the strategy outlined is successful. “The crop is nicely balanced at the moment. The head is strong enough, the cluster is strong and bends nicely and the setting also goes well.” Both the cultivation manager and Maat emphasise the importance of a uniform step size; too much variation is not desirable. “We keep the steps as stable as possible, to keep the EC stable and under control”, says Van der Stoep. “If you are going to play with your step size, which many growers are inclined to, you can get a yo-yo effect in terms of EC. And that is reflected in your substrate, and ultimately in the growth you achieve. Our EC is very stable, which translates into constant growth and development.”

Maat confirms that, “The more stability and regularity you achieve, the more pleasant that is for the plant and for the crop. If you do extreme things in cultivation, you get extreme reactions. And they’re usually negative; of course you don’t want that. With a stable step size and frequency, you keep the EC under control and offer the plant what it needs under all circumstances. You also achieve a good distribution in the slab. We also see that here at the Delphy Improvement Centre: the root distribution through the slab is good. In addition, it helps to keep the cumulative drain constant; in this case at 25 percent. Draining is necessary but not a goal in itself. It helps to control and monitor the conditions in the slab. As already mentioned: If we control this in the greenhouse, you can be sure that the conditions in the slab – and therefore the EC, too – will remain constant. Initially, it is always a bit of a puzzle to know where the tipping point is, which cumulative drain percentage you can use to create regularity. And what is the right strategy in terms of step size and frequency to achieve this. Once you’ve found these, you can control what the plant needs.” If, for example, the EC needs to be raised, you can adjust the water strategy to increase the EC.

This is underlined by Van der Stoep: “Sailing a steady course; therein lies the secret of an optimal water balance. And, as already mentioned, it also contributes to a good energy and assimilate balance, and thus to a correct plant balance and optimal crop growth.”

An optimal water balance: a tip from Remy Maat
“In order to achieve an optimal water balance, you first need to know where you stand. That is why sensor measurements in the slab are crucial. Not once a day, but continuously in different places in the greenhouse. In this way you can constantly keep track of the water content, EC and temperature. This enables you to take the right action when the plant really needs it. In other words: you predict what’s going to happen and anticipate it. That’s how to make sure you stay on course and don’t have to make corrections afterwards.”

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