Split-root system harvests sodium and removes it via the crop

Growing with higher sodium concentration looks possible
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Split-root system harvests sodium and removes it via the crop

The shift towards closed growing systems is forcing growers to take a critical look at their sodium figures. Would higher sodium levels affect the crop? And could sodium be “harvested” and removed from the system that way? The answer to both questions is “yes”, initial Dutch research results indicate.

When the sodium concentration in drain water rises above 5 mmol/l, almost every grower will discharge the water. Ideas on acceptable concentrations are based loosely on the results of research carried out in the past, in which a very generous safety margin was applied. But nowadays we live in different times: zero-emission growing is getting ever closer and discharging drain water costs money. So now is a good time to take another look at the margins within which you can work safely. This was the background to the sodium study carried out by Wageningen University & Research in the Netherlands as part of the “Prevention and Control of Leaching from Greenhouses” research programme.

Current standards

Sodium is not an essential element for the plant and can be toxic in high concentrations. It also competes with the uptake of potassium and calcium. Too much sodium in the irrigation water can cause blossom-end rot in fruiting vegetables by inhibiting calcium uptake.
“There are three input streams,” project manager Wim Voogt explains. “Sodium can enter in the water, with fertilisers and in some organic substrates such as coco. The crop absorbs some of it and the rest leaches out into the drain water. Some crops, such as cucumber, tomato, aster, carnation and gerbera, absorb a lot of sodium. Others absorb barely any, such as rose, orchid and sweet pepper.”
Drain water containing sodium can be reused providing the concentration is not too high. Voogt: “That begs the questions: Are the standards from the past still relevant today? And can you ‘teach’ the plant to handle salt?” The EC of the recirculating water partly stems from the nutrient solution and partly from ballast salts. “With tomato and cucumber, you need an EC of at least 1-1.7 mS/cm for the nutrient supply. But growers often work with an EC of 2.5-3, or even more for tomatoes. So there’s leeway for extra salt there,” he says.

Extreme values

To explore the limits, he first carried out a trial with sweet pepper, with tomato following this year. Sodium was added to the basic nutrient solution in increments rising from 2 to 10 mmol/l (10 mmol is extremely high and is regarded as unacceptable in practice).
But the surprising result was that the sweet peppers performed well even at the highest level (see figures 1 and 2). Voogt: “The yield per square metre, the number of fruits and the fruit weight remained the same at all concentrations. Because we anticipated problems with calcium uptake at high Na concentrations, with the associated higher risk of blossom-end rot, we increased the calcium level in the nutrient solution in some of the treatments. But even that turned out to be unnecessary. So our conclusion was that it is possible to grow with higher sodium levels. The Supervisory Committee for Research followed the study with a critical eye but didn’t see any problems. This year we will be looking at tomato with even more extreme values, up to 15 mmol/l.”

Sodium concentrations

The second part of the research project looks at the question of whether you can “harvest” sodium. If you can store the element safely in the crop, it will ultimately end up in the composter and you will be rid of it. The more you can remove this way, the less you will need to discharge.
The plant can take up more Na if you allow the concentration to rise at the roots, but this inhibits nutrient uptake. So you need a workaround: a split-root system (SRS) (see diagram). One half of the roots gets the regular nutrient solution and the other half gets the drain water with rising Na concentrations.
Voogt again: “We know from previous research with these types of systems that water uptake drops as the EC rises, while nutrient uptake increases the more you supply, in other words, the higher the EC. Based on this idea, we want to develop a cultivation system in which half the roots are in a gutter with a normal nutrient solution and low sodium, allowing the plant to take up water and nutrients freely. The other half of the root system is in a gutter with rising sodium concentrations. The proportion of the drain water to be discharged is added to the second gutter.”

Principle works

When presented with a high supply, the plant will take up high levels of sodium and there will be less remaining in the system. The researcher this year ran trials with tomato and cucumber with Na in a range of 0-15 mmol/l with two EC increments (2.8 and 4.2) in one half of the roots, and with no Na and an EC of 2.8 in the other. The results were surprising (see figures 3 and 4). “This way you can remove sodium from the system with no negative impact on growth. We certainly don’t have answers to all the questions yet, but we now have proof that the principle works,” the researcher says.
There is a catch, however. Despite the fact that the two halves of the root system are separate, sodium was found to have made its way into the other gutter. “It travels up the xylem and passes into the stem. Then it flows down again through the phloem and is secreted by the roots, but not in high enough amounts to reverse the removal effect. In net terms, a lot more sodium still finds its way into the leaves than is secreted,” he says.

Leave more leaves

To begin with, the growers on the supervisory committee were sceptical about how the system could be implemented in practice. But it is feasible, according to Voogt: “You only have to equip a small part of the greenhouse with a split-root system. That’s plenty. The annual costs aren’t too high and it’s a good way of reducing the amount of water that has to be discharged. The project still has a year to go, so we have plenty of time to flesh this out.”
The system should also be ideal for Mediterranean regions where irrigation water is often salty. But you can also harvest more salt without technical adaptations, he adds. “As long as leaves transpire, sodium goes into them. Tomato growers currently aim for 11-15 leaves on the plant to keep the leaf/fruit ratio constant. But if you leave the leaves on the plant for longer, you can get more sodium out of your system. That could be a reason to leave a few more leaves on the plant.”

Summary

Growing with higher sodium levels looks possible for sweet pepper. Trials with tomato will follow this year. A split-root system enables sodium to be harvested out of the system and removed with the crop. In this system, one half of the roots gets the regular nutrient solution and the other gets the drain water with rising Na concentrations. Both growing with higher sodium levels and storing sodium in the crop reduce the need to discharge drain water.

Text: Tijs Kierkels
Images: Wilma Slegers and Wageningen University & Research

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