The increase in scale that has been taking place over the past ten years calls for a different way of watering. Including on the technical side – after all, how do you make sure all the plants in those long rows are getting enough water? Can you even get it all the way round? Systems are being designed for pressure loss and pumping capacities are being ramped up. What’s more, growers are opting for drip hoses with a smaller diameter. And all with the aim of getting the water and the nutrient solution to the plants faster.
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Although it is difficult to measure, it is very important to have a good understanding of irrigation, the experts say. It is not only the quality and composition of the water that affects the crop but also the circulation speed. A proper understanding of the latter tells the grower when the new fertiliser recipe will reach the plants at the back of the greenhouse, for example, and how to get it there as efficiently as possible.
Economical with water
There may be no shortage of water in the Netherlands, but growers still have to use it sustainably. We are already doing quite a good job of that, tomato grower Marcel van der Knaap of Rimato in Honselersdijk and Revaho specialist Edwin Rijpsma believe. Dutch growers only need a tenth of the amount of water used in Spain to produce a nice tomato. And we’re quite proud of that.
But the fact remains that there is always room for improvement. Perhaps not in terms of litres of water or kilos of product, but rather by doing that little bit extra that your competitors don’t do. Van der Knaap: “By optimising our watering, we want to be harvesting the same quality at the front of the row as at the back and just as quickly. We still stand to gain in that area.”
Little and often
“It’s not so much about the total amount of water you use,” Rijpsma says, “but rather about the frequency and the amount you use in each cycle. Watering more often and in smaller quantities improves the conditions below the plant – in the slab and the roots, for example.”
For the time being this idea is mainly based on intuition, since no link has officially been made between watering little and often and a better production outcome. Not yet, anyway, because if it were up to the irrigation specialist, that link would soon be proved. “The intuition stems from things we do already know for sure, such as the fact that a good oxygen level in the water is important for root quality, or that an ideal moisture level in the slab improves a plant’s performance.”
Watering in smaller cycles enables the grower to better control the balance between oxygen and water. In specific terms, if a tomato grower gives 200 cc per plant at 10 l/m<sup2/day in summer with 2.5 plants/m2, that works out at 20 large cycles. With small cycles – 100 cc – the number doubles to 40. “This method improves the total balance of oxygen in the slab,” says Rijpsma. “When your water contains less than 30% oxygen, the roots have problems. But water can’t absorb unlimited amounts of oxygen; there is a maximum. The ‘shortfall’ is supplemented by the slab via the air. If you swamp that same slab with a big splash of water, the air doesn’t get through to the roots and an imbalance occurs.”
There is also a hunch that more frequent cycles reduce the risk of dangerous blockages in the pipes. Rijpsma: “Irrigation water contains all kinds of fertilisers and small pieces of dirt. When large amounts of these flow slowly through the pipe all the time, blockages are more likely to occur. When you dose more often, the system cleans itself and what reaches the plant is actually the mixture and the quantity you intended. We think this refreshing action has a positive effect on crop yield. The production speed increases and production becomes more uniform down the whole length of the row.”
Same amount of water
Overall, the amount of water given remains the same. “The plant’s needs are still the main priority,” the tomato grower says. “The amount of drain is also the same. And when to water? Well, each grower or crop advisor approaches that differently anyway. We sometimes do a quick cycle at night, for example. If the slab dries out too much, we will simply turn the tap on for a bit in the late evening. This is simply about acting fast.”
By combining smaller pipes and drippers with a higher watering frequency, you can constantly achieve the best oxygen/water ratio in the slab and therefore at the roots. “That’s something we firmly believe, but there is still a lot we don’t know. For example, what happens to the fertilisers in the slab that leach out? You should really measure that separately in each growing section, along with any differences in yields. We would urge people to focus on this. Watering is still too often underestimated, but it’s so very important.”
Tested in practice
With a better awareness of the impact of oxygen on the root environment, you can optimise the associated irrigation systems. After all, as is so often the case, people tend to operate on auto-pilot. Rijpsma: “That’s something we have noticed even in the types of drip hoses people buy. Some growers routinely go for 20 mm pipes and drippers with a delivery rate of 3 or even 4 litres per hour. Why? Because they’ve been doing that for years.”
Van der Knaap, on the other hand, installed 16 mm high flow pipes and drippers of 2 litres per hour from the outset when they built their new greenhouse in 2009. “And they have had good experiences with that. If we can actually provide the numbers to back up those experiences, other growers will start adopting this method and we can work together to take production to a higher level.”
Optical sensor measures minute oxygen fluctuations
There’s no doubt in Dutch water specialist Arie Draaijer’s mind: the watering strategy is the last major area of optimisation available to growers. “We simply don’t yet know enough about the effect watering has on the root environment in the various substrates – mainly because we haven’t been able to measure it, as most sensors use oxygen themselves.”
“Two things are important when it comes to the root environment,” says Draaijer of Sendot. If the amount of oxygen decreases, the amount of water and nutrients taken up by the roots also decreases. And if oxygen is absent for a long time, roots can die off and there will be an increased risk of diseases. “You don’t want that, obviously. Growers often find themselves confronted with a difficult balancing act. In most substrates, we know that the oxygen content of the slab drops during the day under the influence of temperature and solar radiation – sometimes even right down to zero. In that case, the oxygen supply in the water simply won’t be enough. On the other hand, more watering creates a constantly saturated slab, which is also not what you want. So the challenge is to allow the slab to dry out enough without causing drought stress.”
Last December Sendot launched a handy tool that measures oxygen in the root environment. The next step is to integrate this measurement data into the watering strategy. The FluoMini Pro is an optrode (an optical sensor) with a fluorescent coating that reacts to both heat and oxygen.
Draaijer: “Our sensor doesn’t use oxygen itself, so it is ideal for use in substrates, from potting soil to rockwool. The grower simply inserts the tip of the tool into the substrate and the sensor stores the data. It can store up to 2 to 3 weeks’ worth of data, which the grower can view in an app. I have watched several of our customers use this system. It’s great to see: every dripper cycle, every night cycle – you can keep a close watch over everything.” The manufacturer is currently working on ways to connect its sensor to the climate computer.
We still know very little about the effect of irrigation on the plant. Nobody really knows exactly what type of cycle or frequency produces the best results in the crop. There are hunches, however. Dosing smaller amounts more often may improve the root environment, get the water and fertilisers circulating faster and reduce the risk of blockages. New sensors should start backing up these results.
Text: Jojanneke Rodenburg. Images: Studio G.J. Vlekke.