RE SEARCH Potatoes and soil moisture: when should I start irrigating? sensor is unlikely to represent the average soil moisture situation of a plot. Soil moisture sensors of great importance Nevertheless, soil moisture sensors are of great importance: they provide an excellent representation of the dynamics and thus indirectly the consequences of dehydration. If the moisture values quickly drop to a set critical limit, a signal is given to start irrigation. That is better than following a gut feeling: something is better than nothing. But in most cases, it will not be a representative value for a plot. The available soil moisture depends on the water retention capacity of the soil. A sandy soil cannot retain much moisture (100 millimetres/metre of soil depth) and a sandy-loam soil even more so (up to 250 millimetres/soil depth). The critical soil moisture in which stress will occur also depends on the drought susceptibility of the variety, and the weather. Most potato varieties absorb the moisture easily until 35 percent of the available soil moisture has disappeared (the so-called FAO p factor is 0.35). At that point, the root zoneis 65 percent full and at lower values the plant starts to stress. In terms of soil suction, this corresponds to approximately -300 to -500 centimetres. High crop evaporation is more likely to inhibit the water uptake and increase the critical soil moisture value for stress. The plant knows that it has a ‘heavy job’ transporting water at top speed with a high evaporation of 6 millimetres per day and will close the stomata earlier to deal more efficiently with its cell moisture. Conclusion three is that accurate information should be available on the local soil properties and the current weather conditions to determine the moisture content at which irrigation should be carried out. A simple rule of thumb doesn’t do justice to the complex reality. For this reason, precision farming focuses on gathering detailed information on organic matter content and the local water retention capacity. Using drones with a thermal sensor Thermal sensors are an alternative method for measuring the spatial distribution in soil moisture. This can be done via a hand-held metre, a drone or with satellites. Thermal satellites measure the temperature with pixels of 100 metres x 100 metres and that is too coarse for most Dutch plots. Moreover, Figure 2: Thermal uptake (°C) and translation to volumetric soil moisture (cm3/cm3) (source: Aurea Imaging). thermal satellites are very sensitive to clouds The national platform for DroneWorkers, which provides growers with data with the eBee drones, was therefore introducing more drones with a thermal sensor this summer. A 24-hectare plot can be covered within 20 minutes, even in cloudy conditions. Drones measure on the centimetre scale and give fine variations of the crop and the soil moisture (see Figure 2). A potato plant with a moisture deficiency quickly heats up a few degrees, which is detected by the thermal sensor. This too, is an indirect measurement. A higher temperature cannot be translated 1:1 to less soil moisture. Temperature has a clear day cycle that is associated with the position of the sun and the prevailing wind, while the soil moisture only changes gradually. Energy balance models are needed to correct these effects. These algorithms are not error-free and often need to be calibrated against advanced flux measurements of which there are only a few locations in the Netherlands. The potato may also show thermal warming due to factors that have nothing to do with soil moisture such as lack of nutrients, pests and diseases and low humidity. Conclusion four is that the images of thermal drones provide a lot of extra information, but that this can not be translated directly into soil moisture; expertise is necessary. Although robotic drones can technically be used to fly autonomously every day (similar to the robotic lawn mower), aviation authorities do not yet allow this in the Netherlands. A pity, because otherwise the temperature of each plant could be measured every day. A potato plant with a moisture deficiency quickly heats up a few degrees, which is detected by the thermal sensor on a drone. New generation of water balance models at plant level Hydrologists feel at home with applying water balance models, because all water flows are then integrated with each other. How much water is added by sprinkling and how much water disappears by evaporation? Wageningen University has been developing soil moisture simulation models since the 1970s, but these are mainly used by the government as policy instruments. They are not intended for farming practice. A new generation of water balance models at plant level has recently been developed by SME companies. The Netherlands has many innovative companies that develop new measuring techniques and models for the agricultural sector. For example, an Aurea Imaging model calculates the soil moisture in the root zone Potato World 2018 • number 4 25 Pagina 24
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