Manufacturing Simple and Inexpensive Soil Surface Temperature and Gravimetric Water Content Sensors

This method allows anyone to build sensors that can measure the temperature and moisture of the top five millimeters of soil, a dynamic and difficult area to measure. Simultaneously measuring microclimate in the soil surface allows the assessment of how temperature and soil moisture influence organisms, gas fluxes, and other components of ecosystem function at the soil surface. The soil surface is particularly prone to large fluctuations in temperature and moisture and may be disproportionately important in regulating overall ecosystem activity.

The sensors provide new insights into how soil moisture surface biota are regulated by and respond to changes in temperature and moisture, which has been difficult to study in the past. To prepare the thermocouple cable, strip the cable jacket four to give centimeters from the end of the cable, and strip the newly exposed small diameter sheaths five millimeters from the ends of the wires, and cut the ground wire off at the cable jacket so it is not exposed beyond the jacket. Wearing the appropriate protective equipment, ARC weld the exposed tips of the wires together and tug gently on the wires to test the strength of the weld, and to make sure that the tips do not separate.

Dip the ARC welded tips of the thermocouple cable into liquid electrical tape to cover the exposed metal of the wires, and at least three millimeters of the small diameter wire sheaths. After allowing the electrical tape to dry for at least four hours, or as recommended by the manufacturer, cut a piece of approximately 3.3 millimeter heat shrink tubing that is long enough to cover the electrical tape on the small diameter sheaths and at least one centimeter of the thermocouple jacket, and insert the wires into the tube, then move the tube back over the cable jacket. To prepare the soil moisture cable, strip the cable jacket five centimeters from the end of the cable and cut the ground wire off at the cable jacket so it is not exposed beyond the jacket.

Strip one centimeter of the inner small diameter sheaths from the ends of the soil moisture wires and twist the exposed metal of each wire to consolidate the small strands. Then, using the appropriate skin and eye protection, apply solder to the exposed metal at each wire end to tin the small, twisted strands. Next, cut a piece of 10 millimeter diameter heat shrink tubing to be approximately one centimeter longer than the distance at which the cable jacket was stripped to the end of the tinned wires, and place this tube over both wires.

Slide the tubing back over the cable jacket and cut two 1.5 centimeter pieces of 3.3 millimeter heat shrink tubing. Place one piece of tubing over each wire and apply solder flux to the prongs of the two-prong socket strip. Solder the tinned ends of the wire to the ends of the two-prong socket strip, being sure to keep the two ends separated so they are not touching.

Move the two pieces of 3.3 millimeter diameter heat shrink to the base of the two-prong socket strip, so that all of the metal parts are covered. Use the heat gun to adhere the heat shrink tubes, taking care not to overheat and melt the solder underneath the tubes. Move the 10 millimeter diameter heat shrink tube to one millimeter from the end of the two-prong socket strip so that it is covering the socket strip, the small diameter wires, and some of the cable jacket, and use the heat gun to fix this heat shrink tube in place.

To modify the eight-prong terminal strip, orient the strip so the top prongs are curving away from view and use wire snips to cut the second, fourth, and seventh prongs from the left, just below the black plastic contact strip. Measure five millimeters below the black plastic contact strip and mark the third, fifth and sixth prongs from the left at five millimeters, then snip these prongs at the five millimeter marks. For sensor head assembly, cut two one centimeter pieces of approximately 13 millimeter diameter heat shrink tubing and slide one over each of the thermocouple and soil moisture cables.

Move the ARC welded end of the thermocouple wires over the top of the third clipped prong so that the tip of the thermocouple is oriented with the end of the end of the clipped prong, and bend the wires so that they follow the top curve of the prong. Slide the 3.3 millimeter diameter heat shrink tube up over the curved part of the prong and the thermocouple wires and check that the heat shrink tube is also covering part of the thermocouple cable jacket. Use a heat gun to adhere the heat shrink tube in place and use fingers to squeeze the part of the heat shrink tube that is over the curved prong.

Insert the top curved ends of prongs five and six into the two-prong socket strip and move the top 13 millimeter diameter piece of heat shrink tube toward the sensor head until it is positioned approximately one centimeter from the head. Use a heat gun to fix the tube in place, taking care to keep the socket strip firmly connected to prongs five and six, and to the thermocouple wire on prong three, and affix the other 13 millimeter diameter piece of heat shrink tubing a few centimeters behind the previous piece of heat shrink tubing. When fixing the heat shrink tube in place, a good connection between the two-prong socket strip and prongs five and six of the modified sensor head is critical.

Then, apply liquid electrical tape to all of the sides of the thermocouple wire and prong three, and to all of the sides of the socket strip connection, ensuring that all of the exposed metal is covered. Do not, however, cover the 5 millimeter clipped prongs associated with the connection. Here, dry-down calibration data for two samples of each of three soil substrates, each with its own probe, are shown.

The regressions for the silt loam soil samples were distinct from the other two soil substrates, therefore, applying the regression equation of the silt loam soil to moss biocrust or vice versa would lead to dramatically different values. On the other hand, the relationship between gravimetric water and the probe resistances for the fine sand soil and the moss biocrust were similar. As there can be variation within the substrates, it is important to obtain a large enough sample size to produce an accurate calibration curve and to create individual calibration curves for all of the sites.

In these graphs, the average temperature and gravimetric water content from heated and controlled plots for two separate rain events that occurred in early May, 2018 can be observed. The average temperatures in the warmed plots were consistently higher than the average temperatures of the controlled plots. Over the course of these two rain events, the resistivity sensors in the heated plots registered less soil moisture than the controls and the heated plots dried more quickly.

It is important to ensure that a good connection has been made when ARC welding the thermocouple wires and when connecting the second strip to the sensor head. We have installed these sensors for their use in multiple warming experiments, as understanding how temperature treatments affect soil moisture readings is critical for interpreting soil surface microclimate data. In conjunction with other instruments, these sensors have allowed investigation of how temperature and moisture at the soil surface affect fundamental soil processes, like carbon dioxide efflux to the atmosphere.

This new link between surface soil microclimate and soil efflux has been critical to our understanding of how dryland soils may create feedbacks to global change.