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At the Greenway tomato nursery in De Lier, the Netherlands, each slab doesn’t have just one slit but two: one for each plant. A simple extra cut at the bottom of the rockwool slab ensures a more even EC and more uniform distribution of the nutrients in the slab. Erik Volkering, the nursery’s cultivation manager, has been doing it this way for five years and is very happy with the results. A seminar earlier this year was devoted to this subject, as this method is by no means standard.

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Greenhouse horticulture will have to be virtually emission-free in ten years’ time. By 2027, European growers must ensure that their drainage water is completely free from residues of fertilisers and crop protection products. Researchers are currently working on a cultivation method that involves the plant finishing up all the nitrate and phosphate in the substrate slab. But a lot depends on achieving the right balances.

A layperson may think it’s easy to grow crops with zero emissions. Just stop fertilising a few weeks before the end of the growing period so that the plants use up the last of the fertiliser – and voilà: the slab is clear, the water is clean and the greenhouse is producing emission-free.
Of course, that’s not what actually happens in capital-intensive greenhouse horticulture. The crop has to continue to produce, preferably right up until the very last day – the day before the crop is cleared. So it needs a good supply of nutrients right up until then.

The right level

“The main aim of an end-of-cultivation strategy is to end the growing and production season emission-free, i.e. with no nitrate and phosphate left in the slab. But the plant needs to continue to receive enough nutrients and water right up until the penultimate week of cultivation,” says researcher Chris Blok of Wageningen University & Research in the Netherlands. “You can quite safely stop providing nutrients in the very last week and leave the plant to fend for itself after that. But in the four or five weeks leading up to that, it’s important to maintain feeding and water levels. That way you will keep production and quality at the right level right up until the end.”

Targets and settings

Blok and his colleagues Romain Leyh and Marco Bustamante are working on a method that entails gradually reducing the supply of nutrients, but in such a way that the plant doesn’t run short of anything. Their research, which is being done on sweet peppers, is being funded via the Dutch Foundation for Water Board Research. A number of companies, including substrate specialists Grodan, are involved in it.
To begin with, the researchers set target values for week -5, week -3 and week -1. The negative week number indicates the number of weeks before the end of cultivation. To achieve the target values, the researchers chose settings for the nutrient supply. An example based on nitrogen supply: 1) The target value for week -5 is 15 mmol N/litre. 2) This should have fallen to 10 mmol/litre by week -3. 3) By week -1, the slab should only contain 5 mmol/litre. 4) So in week 0, when cultivation ends, the slab will contain no more N at all.
To achieve this, the set N supply must decrease: 1) from 15 mmol N per litre in week -5, 2) to 5 mmol/litre in week -3, 3) and finally to 1 mmol/litre in week -1. The researchers have developed similar schemes for phosphate (P), EC and water content in the slab.

Results needed fast

Can it work? Will it mean the grower ends up with a nutrient-free slab? The answer is yes, but the method requires the grower to be very vigilant. Blok: “You will have to take measurements in the slab regularly to keep an eye on how things are going. For example, you might choose a target value that is too ambitious, which would mean that you are supplying too few nutrients. The plant would then run short and production would suffer. If that happens, you would need to be able to make adjustments quickly: you can’t wait a week for the results of the measurement to arrive, you would need them within a couple of days. In the trial department we work with electronic meters which give us the water and EC results we need fast.”

Too much versus too little

As stated, all elements the plant needs must be present in sufficient – but never excessive – quantities until just before the end of production. Researchers Leyh and Bustamante: “To check that, we measure the nutrient levels in the slab twice a week. We focus mainly on whether N and P are being used up because these elements are banned from being discharged in the drain water. If the measured value differs from the target value by more than 25% – up or down – it is important to adjust the settings. Because if the measured value is too high, you won’t get the slab clear in time. If the value is too low, the crop will run short of nutrients.”

Souring the slab

In a departure from general nursery practice, the researchers are using chloride as an anion in their trials. They have to do this because as the amount of nitrate supplied drops, the supply of cations, particularly potassium, needs to remain slightly higher. So instead of providing nitrate, they supply chloride as an anion.
Another strategy that helps to clear the slab is to play around with the pH value. If you keep the pH value deliberately low with acid and ammonia, the slab turns acidic. This releases the last residues of precipitated phosphate, causing the plant to use up this nutrient as well. Blok: “This way you can get the plant to finish up all the leftovers, as it were.”

Precisely the right balance

Researcher Ruud Kaarsemaker of Groen Agro Control was involved in the research. He calculates the plant’s nutrient uptake and advises the researchers so that they can input this information alongside the target values and settings. Kaarsemaker points out that nutrient uptake should remain stable for as long as possible. “The plant should continue to grow and produce. But as the amount of food and water the plant receives decreases, the system and therefore the plant become more and more susceptible to disruptions. So it’s a question of looking for precisely the right balance.”
This method is not yet ready for use in practice. However, what is clear is that the nutrient and water supply needs to be adjusted in such a way that the slab is actually empty by the time cultivation finishes. Aspects such as weather influences also have to be taken on board in the targets and settings.
According to Blok, the method should also be highly automated, otherwise it will be too much work for the grower. “An attractive aspect of this method of working is that it doesn’t require any investment on the part of the grower. The result is achieved entirely from controlling the nutrient flow. We are essentially telling the plants: ‘It’s time to start clearing your plate’.”

Summary

Researchers at Wageningen University & Research and Groen Agro Control in the Netherlands are working on a nutritional regime in which the amount of feed and water given to the plants gradually decreases towards the end of cultivation. This forces the plant to finish up everything in the slab, thus making cultivation completely emission-free. The trick is to adjust the amount of nutrients given to ensure that production and product quality remain at the required level.

Text and images: Jos Bezemer and Studio GJ Vlekke.