Although last week finally brought a few days of rain in the Netherlands, most greenhouse horticulture businesses have long been relying on irrigation water from alternative sources. This water often contains some sodium, and when drain water is reused, sodium concentrations can gradually increase. However, according to researcher Jim van Ruijven of Wageningen University & Research, this doesn't necessarily mean that growers should start discharging water. "Most crops can tolerate more sodium than previously assumed," he explains in this contribution from sector organisation Glastuinbouw Nederland.
After yet another extended dry spell—and with most of the summer still ahead—the question arises whether such prolonged dry periods are becoming a trend or are merely coincidental. "Based on national weather data, there is definitely a trend," says Van Ruijven. "On an annual basis, total rainfall is increasing slightly. The average is already above 850 mm, which is about 50 mm more than thirty years ago. However, we also see that the October–March period is getting wetter, while less rain falls during the summer months. This shift is due to climate change, which is altering weather patterns in Northwestern Europe. And when it does rain in summer, it's often short and intense."
© Glastuinbouw Nederland
Wider drainage and collection capacity
In the long term, greenhouse businesses would be wise to expand their rainwater drainage and storage systems. Greenhouse builders are already responding by designing wider gutters and downspouts. Basins and silos also need to be enlarged—depending on the crop—to as much as 2,000 to 3,000 m³ per hectare.
"For new constructions, this is a solid starting point," Van Ruijven notes, "but most existing operations lack the space for it. As a result, growers are often reliant for extended periods on supplemental irrigation water from alternative sources, which frequently contains some sodium. In high concentrations, sodium can damage crops. Rising sodium levels are therefore the main reason why growers occasionally discharge drain water and replenish with fresh, relatively sodium-free water."
EcoNutri
Out of concern for potential crop damage, conservative thresholds have traditionally been used. However, with increasing pressure to reduce nutrient discharge and the growing need to use irrigation water and fertilizers more efficiently, it's worth exploring whether discharging can be delayed. This is exactly what the European project EcoNutri is investigating. Researcher Tommaso Barbagli of Wageningen University & Research in Bleiswijk has been studying this across several crops. Van Ruijven, as team leader for Rootzone Dynamics, has been involved from the start.
"Right now, research is being conducted on strawberries," he reports. "Tomatoes, gerberas, roses, Phalaenopsis, peppers, and cucumbers have already been studied. So far, we consistently see that safe threshold levels are significantly higher than the limits most growers currently observe. These thresholds do vary by crop. For Phalaenopsis, a safe level in the root zone is 6 mmol/l; for cucumbers, 8 mmol/l or more; for gerberas, 10 mmol/l; and tomatoes show no issues even at 15 mmol/l. We also know that plants take up more sodium when it's present in the nutrient solution, but this generally doesn't cause harm. So there's ample room to adjust discharge thresholds and extend water recirculation."
Tips
Van Ruijven remains unconcerned for now about sodium levels reaching damaging thresholds. He offers growers the following tips:
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Consult the EcoNutri subproject publications on the Glastuinbouw Waterproof website. "Understanding the details of the research can clear up a lot of doubts."
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Using reverse osmosis to purify source water? Don't push the system too hard. "That reduces its efficiency, and you might still end up with more sodium than expected."
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Dealing with persistently high sodium levels? Consider investing in selective sodium removal systems to treat drain water. "These significantly reduce wastewater volumes. Just be aware that depending on the system, some potassium and nitrate may also be lost in the process."
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Planning to discharge to the sewer? Always purify the water first and check whether the discharge can be done responsibly. "This may seem obvious, but if multiple businesses are connected to the same system, simultaneous discharge can cause localized problems. It's best to coordinate with your neighbors."
Source: Glastuinbouw Nederland
