Thoughts on Soil Sensor Installation from a German Precisionist
Many researchers carefully choose the right instrumentation for their projects, but when it comes to installing the soil sensor into the soil, they are less than careful about the process. Researchers need to know how to install sensors in a way that will allow them to get the most accurate data the instruments are capable of.
Georg von Unold has almost two decades of experience installing all types of soil sensors and a German eye for precision that is unmatched in our experience. As the president and founder of UMS (now METER Ag), a German company that develops and manufactures precision soils instrumentation, and a close friend, we thought there would be no one better to share a couple of ideas on careful installation. Here’s what he had to say:
Pick the Right Place to Install your Sensors
When we develop research instrumentation we look at the accuracy and the resolution of our instruments from a technical point of view. However, the heterogeneity of research sites can be so vast that we have to take care to select a research site that is representative from a scientific point of view of the results we would like to publish. We do this first by analyzing the biosphere above the soil that is visible to us, and then perhaps doing some auguring into the soil at various sites to investigate what might be going on in different areas of the field. If you are researching on a farm, it is important to ask the grower where he’s had good and bad harvest results, where he’s needed to irrigate, and where he’s had problems with erosion. Always interview people who know the history and specifics of the sites first, because if the sites are flooded or at risk for landslides, it will be a bad choice for long-term monitoring. Investigating the right place for your sensors before you install will save you time and help you obtain the most applicable and accurate data for your research.
Be Careful with the Way you Install Sensors
One of our research projects used tensiometers to try and determine how water flowed through gravel. We knew that gravel would have bad capillary contact because the stones would have holes between them. So we decided to make a slurry of fine material from this gravel soil and put it in the installation hole so that the tensiometer would have better capillary contact. It was a good idea, but it led to misleading results. What we ended up with was a kind of water reservoir with fine material around the tensiometer which had nothing to do with the true moisture situation in the gravel. The tensiometer gave us wonderful readings: very constant but with no dynamics that would have been typical for a gravel soil. When we took it into the lab to investigate, we realized we’d built an artificial soil around our tensiometer. We weren’t measuring the gravel but were measuring our artificial error which we had created so carefully. The other thing we found is that over the course of time our slurry would move away from the tensiometer, and within a few years, the tensiometer would be simply hanging in a big gap. This project also contained fine, heavy soils. Eventually, we realized that we needed an auguring tool that would not push the soil away or compact the soil where we placed the tensiometer because compaction would mean different hydraulic behavior. So we asked our friends at a Dutch company to make us an auger that was shaped in a form that wouldn’t change the natural soil density that we wanted to measure.
It is important to be careful when you install sensors. For example, if you have a clay soil and you auger a bigger hole than your tensiometer, you will have a water tube around your sensor. If your soil flooded, the water would flow down your shaft to where your tensiometer is placed, and then what are you measuring? Thus it is necessary to seal the shaft or to prevent access of surface water to a deeper horizon.
Beware of Simple Mistakes
You can also make simple mistakes with other types of soil sensors, such as temperature probes. You need to remember that if you want to measure temperature at a depth of one meter below the surface, the thermal conductivity is strongly dependent on the kind of soil and the moisture of the soil. If, for example, you put a temperature probe wired with copper wires in a dry sand or gravel, you will get an average value of the temperature of the sunlight exposed hot cable. The reason is that the copper is leading the temperature down to where you measure and has a much higher conductivity compared to dry, coarse soil. Thus it is important to think through your installation processes because it is likely you will have a different installation method in a clay soil versus a gravel soil.
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