Cherish Bauer-Reich, NDSU Center for Nanoscale Science and Engineering

A common issue causing reduced corn yields is salinity.  Salinity can be difficult to manage due to changing levels within a field and with depth.  A proposed solution to this problem is a biodegradable sensor that can be placed directly in the ground to provide real-time information about salinity conditions.  Our proposed solution, the SEED (Sensing Earth Environment Directly) sensor, has the potential to help monitor salinity and other soil conditions.  The SEED sensor is a biodegradable sensor that can be placed and left in the soil.  A soil-located sensor can give real-time soil conditions faster and more efficiently than mobile testing units, particularly after crops are growing.  The SEED sensor is made out of corn-based plastics and contains no materials or batteries that would contaminate the soil or cause toxicity to plants.  Construction from these materials eliminates the need to recover sensors at the end of their useful life.

The purpose of the funded project is to develop a larger ground-based sensor to study soil salinity levels.  These initial sensors are larger than the final conception of the SEED sensor, and they are made from traditional electronic materials rather than biodegradable materials.  These initial prototypes are needed to study how well ground-based sensors work compared to other salinity measurement methods, such as an EM-38.  Creating these sensors help investigators understand the challenges of in-situ soil measurement before miniaturizing the technology.

The Electronics Technology Group at NDSU’s Center for Nanoscale Science and Engineering developed several prototype sensors designed to work in pairs to measure temperature and  a broad range of conductivity values.  While the sensors cannot measure salinity directly, conductivity can be used with temperature and moisture levels to determine salinity. The sensors were 2 ¼ inches on a side and made of traditional electronics materials.  Traces and antennas were created from copper on glass-reinforced epoxy laminate.  Some types of Radio Frequency IDentification (RFID) chips can be used without batteries by harvesting energy from an antenna.  This principle was used to build the salinity sensors, so an RFID-based sensor chip was attached to the sensor board.  This chip provided the electronics necessary for the sensors to communicate with an above-ground computer attached to an RFID reader as well as read conductivity levels from the soil.  Three other components were added to the sensors for tuning all of these components and the copper boards were coated in a water-proof material to prevent corrosion of the copper as well as soil contamination.  Finally, a pair of probes was attached to one side of the sensor.  The probes are used to measure the conductivity of the soil between them.  The sensor is shown in Figure 1.

After they were constructed, all sensor boards were tested in the lab to confirm conductivity measurement accuracy.  Any boards that did not take controlled measurements were rejected.  The final testing left six pairs of sensors.  Five of these pairs were waterproofed in a traditional plastic material while one pair was coated with a corn-based plastic.  The sensors were distributed in pairs at six well sites at the SHARE farm.  The sensors measured conductivity at a depth of approximately two inches, and the sites range from highest to lowest salinity in the field.

A sensor reader was made into a portable device that could attach to a laptop for field monitoring.  Portable readers typically have low power, making it difficult to read sensors through moist soil.  A higher-power wall-mount reader was outfitted with a handle and two portable batteries, as shown in figure 2.  A laptop was connected to the reader via an Ethernet cable so that the computer could control the reader to make measurements.

Initial measurements made at the SHARE farm were plagued by wet soil with moisture levels outside of the sensor-reading range.  Monitoring will continue through the summer and fall, hopefully allowing for measurements with more ideal conditions.  Funding for the continued monitoring is being provided by c2sensor, a North Dakota start-up company that has licensed the patent on the SEED sensor technology and plans to continue its development.