Wireless sensor gives representative image of water content in slab

Measuring water content of just one slab is not enough says grower
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Wireless sensor gives representative image of water content in slab

Dutch tomato grower Kees Stijger, of Honselersdijk, believes that measuring the water content in a single substrate slab is not enough. Instead, he feels that a new wireless unit for measuring moisture, with its multiple sensors spread over several watering sections, gives a more representative picture of his entire crop. He grows red, orange and yellow medium-sized tomatoes on stone wool on 2.5 ha.

To understand the distribution of the water content in the Cultilene Optimaxx stone wool slabs the tomato grower uses the new module RootView from Klimlink, an analysis package for climatic data by Klaver4ICT. In a cut section of slab, the colours relates to the water content so you can see at a glance the distribution of the water in the slab.
To know the actual water content, measurements need to be taken in the stone wool slab. For this purpose the greenhouse has four wireless measuring units each with five sensors. Every sensor has three equally long probes, which are pushed into the stone wool slab at different heights and places, also under the stone wool block and in the side of the slab. Each slab has 15 different measuring points for moisture level, temperature and EC. "This gives a good picture of what is happening in the slab,” says Kees Stijger.

Analyse the measurements

The values from the slab are sent to a sensor that hangs on one of the greenhouse columns. The data that has been collected is sent via a wireless connection to a universal database. This also contains data from the climate computer, such as radiation, water supply, volume and EC of the drain water. In this way the measurements from the slab can be analysed in combination with the climate data. An average figure is taken from the sensors from the different measuring units to provide a representative and reliable picture of the water content in the slabs.
“By knowing the water level at the top as well as at the bottom of the slab the grower deduces the flow of water during the course of the day. By then playing with the water supply, the system makes it possible to reduce the drain percentage,” says Wim van Vliet, of Klaver4ICT.

Watering strategy

With these measurements the tomato grower has insight into his watering strategy. With this knowledge, he can determine the watering, such as when to start supplying water in the morning and the volume of water to give. The grower can also see the EC-gradient in the slab.
“If you look carefully at the EC, you can adjust the watering accordingly. You can allow the drain percentage to depend each day on the EC and water content of the slab. As a result you have less drain water to disinfect and possibly discharge. And that is s big advantage,” says Stijger. “Even more importantly is that the root structure remains good, so there are no problems from water shocks: you prevent the roots from drowning. If there is a good root structure in the slab, the crop above ground is also much easier to manage. That is reflected in the crop growth and fruits."

Slab dynamics

Because additional measurements don’t automatically provide more information, the grower wanted a different form of presentation. Van Vliet set about presenting the mass of data in a simple form. In the summer of 2015 this resulted in a new module that provides insight into the distribution of the water content in the slab itself.
Van Vliet: “In a vertical cut section in the length of the slab we can see the water content in the form of different colours. The grower can see at a glance the distribution of the water in the slab. This distribution, or the ‘slab dynamics’, varies from one moment to the next and therefore is easy to show in an animation.”

Graph line water content

As well as the slab dynamics the module also shows all the relevant data from the climate computer in a 24-hour-graph. With a reading line it’s possible to point to every time the slab dynamics and the graph values are at the desired level. In the graph of the water content it is possible to show part of the slab as well as an average for the entire slab. The grower can make graphs of the average at the top of the slab, the average at the bottom of the slab, the difference between the top and bottom of the slab and the water content measured per sensor. In this way he can keep a close eye on the saturated bottom layer and the difference between the top and bottom of the slab.
The graph quickly gives the grower a complete picture for the entire day, but he can also zoom in on the slab dynamic for any given moment. The module automatically shows the values of the corresponding slab dynamics at that moment.

Conditions in the slab

Thanks to the new module, the grower receives quick feedback. Stijger: “During the spring, with the changeable weather, it’s important to know what is happening in the slab, especially with Next Generation Growing, when you use the screen more to save energy. Because there is less transpiration under a closed screen, you have to make sure that the slab is not too wet.”
The aim is to further use the data from the climate computer and the slab measurements to make separate calculations, for example, the radiation in relation to the water supply, the moisture content, the transpiration and the amount of drain. “The radiation determines the amount of water you give, but it also depends on transpiration,” says the grower.

New ideas

The module ensures that the moisture sensors don’t essentially have to be connected to the climate computer to provide a complete analysis. With the combined display of graph and slab dynamics Rootview is at the forefront of developments. If the tomato grower could decide, he would prefer to receive a graph of the water gradient via the climate computer.
Van Vliet: “As far as we are concerned, the data doesn’t have to only come from the climate computer, but data can also go to it. For example, this would offer the opportunity to make a connection that could shorten the duration of the watering. The data provided can also be used to give advice or to follow a certain strategy.”
As well as new insights the module also yields new questions and needs. For example, the grower would like insight into the possible fertiliser accumulation in the slab. The EC in the slab is measured but so far there is no form of presentation. Van Vliet is currently working on a way to visualise the EC distribution in the slab.

Summary

Dutch tomato grower Kees Stijger uses a new module that allows him to visualise the distribution of the water content in multiple stone wool slabs. This is done by showing the colour in relation to the water content in a cut section of the slab. In order to measure the water content, EC and temperature, five sensors, each with three probes, are stuck into the side of the substrate slab. With data from the climate computer such as radiation, water supply, amount and EC of the drain water, the measurements from the slab are analysed further.

Text/photos: Harry Stijger

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