Snack tomato nursery Vitensa of Vierpolders, the Netherlands, uses a monitoring system with multiple wireless slab sensors in two of its greenhouse sections in conjunction with a smartphone app. This gives grower Dennis van der Knaap a 24/7 picture of the water content, the EC and the temperature in the root environment. “I get to see discrepancies between the slabs immediately without having to check in with the climate computer,” he says. “Plus, more sensors provide a more reliable picture.”

Dennis van der Knaap and his partner Robin Grootscholten grow red, yellow, orange and brown snack tomatoes on 7.2 hectares. The nursery is connected to the geothermal heat grid in Vierpolders, which went live last year, and does not use grow lights. For the past few years they have also been running a second nursery in nearby Brielle together with another grower.

Van der Knaap has overall responsibility for cultivation. He was one of the first people to use the GroSens MultiSensor system, which was launched around 3½ years ago by substrate specialist Grodan. “We are always happy to carry out trials, and this system seemed like a good idea to us,” he says. “We started in one 7,000 m2 section, where we use three sensors. I also have a separate hand-held meter for taking extra measurements in different places. I don’t do that every day but I do do it quite regularly, particularly when a slab starts producing a different crop profile.”

Reliable picture

Each sensor unit has six sensors which are inserted into the side of the slab and which permanently record the water content, the EC and the temperature in the slab. Because the sensors are at different heights in the slab – three at the top and three at the bottom – they give a more reliable picture of the situation in the slab than would be obtained from just one sensor. In addition, using multiple sensor units provides a representative picture for the whole section.

All the units are fitted with transmitters which transmit the data wirelessly to a central receiver connected to the climate computer. The associated software displays the data in easy-to-read graphs. Van der Knaap: “It displays both the average measurements and the values from each unit, so any discrepancies between the slabs are immediately visible.”

Handy app

To make the system more user-friendly, in 2016 the manufacturer launched e-Gro, a special app for smartphones. Five users including Van der Knaap were involved in the development process from the very beginning. Although the app had not yet been fully refined at the start, the grower says, he has been using it since August last year and finds it a very handy tool.

“It gives me a real-time picture of what’s happening, whether I’m in the greenhouse, at home relaxing or out with friends,” he explains. “The alarm function is particularly handy. If the water content, the EC or the temperature exceeds a pre-set limit, you receive a notification. That might be when the slab dries out too quickly, for example. As soon as the alarm sounds, I check the app to see what’s going on and it gives me the opportunity to take any corrective action that may be needed. Thanks to this app I don’t have to keep checking in with the climate computer to monitor the situation. But if I really want to twiddle the knobs, I still have to do that.”

Van der Knaap points out that there are several occasions in each crop when, with hindsight, he would like to have intervened. “Usually it would be a case of bringing forward an irrigation cycle or having an extra one in the afternoon. What is less common, but what I reckon every grower has to contend with about once every five years, is the system breaking down or even getting turned off accidentally. Then it’s really handy to get an alert.”

Logical next step

Grodan has had two slab sensors in its range since 2014: the flexible, simple hand-held meter and the semi-permanent MultiSensor system.

According to substrate advisor Marco Peeters, hundreds of MultiSensor systems and apps have since been installed worldwide, most of them in the Netherlands. “To be honest, we have been taken completely by surprise by the success of these systems,” he admits. “We never expected them to take off quite so quickly. But it does show that the market is ready for this innovation. It is a logical next step in our philosophy of Precision Growing, which we have been actively promoting for the past five years or so.”

Other monitoring systems

Before his previous crop was planted, Van der Knaap was already using three extra sensors in another section of the greenhouse. He now takes permanent measurements in six slabs, whereas he previously only had a fixed water content meter in one slab. “We still use this method in two of our other six irrigation sections, and we use scales in the others,” he adds. “Of course, we would ideally like to be using GroSens everywhere, but there is a price tag attached to that. I can get by with all the methods so we don’t need to switch over in one go. Incidentally, we’re using this system at our other nursery in Brielle now as well.”


In 2014 a wireless measuring system was launched that displays the water content, the EC and the temperature in rockwool slabs in minute detail. Hundreds of these systems, with the extended version with multiple slab sensors per greenhouse or section, have since been installed all over the world. Last year the package was extended with an app for smartphones which includes an alarm function. Grower Dennis van der Knaap now takes permanent measurements in six slabs.

Text and images: Jan van Staalduinen.