In-situ soil water potential sensor and nutrient sensor

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Chen, Yuncong
Major Professor
Dong, Liang
Que, Long
Castellano, Michael
Lu, Meng
Pandey, Santosh
Committee Member
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Electrical and Computer Engineering
Water potential is a main factor to determine the dynamics of water and impacts plant growth and yield. Due to cavitation formation inside the sensing unit at absolute negative pressures, conventional water potential sensors exhibit a relatively small measurement range of -0.1 MPa, which is much smaller than the plant permanent wilting point of -1.5 MPa. Chapter 2 of this dissertation presents a miniature water potential sensor with an improved measurement range down to -0.8 MPa. In order to minimize cavitation and improve the measurement range, an ultra-shallow water reservoir sandwiched between a nano-porous ceramic membrane and an ultra-smooth and hydrophilic SiO2 diaphragm is developed. A printed-circuit-board with an optical distance detection unit is integrated to the external side of the diaphragm for monitoring the displacement of the diaphragm when the sensor is exposed to an unsaturated water environment and the water reservoir is under negative pressure. The high air-entry-value (AEV) of the nano-porous ceramic membrane helps to prevent air bubbles from coming through the pores of ceramic; the ultra-smooth and hydrophilic SiO2 diaphragm decreases the chances of trapping air bubbles in the corners or crevices inside of the water reservoir. The small form fact of the sensor benefits from the seamless integration of an ultra-shallow water reservoir, a nano-porous ceramic membrane, and a miniature optical displacement detection unit makes it capable of in-situ, continuous monitoring of soil water potential variations near the roots of plants. The proposed miniature sensor is used for laboratory soil water potential measurement near corn roots for two weeks and installed into a corn field for in-field water potential sensing as well. Next, in Chapter 3, a new soil water potential sensor with an osmotic solution filled in its reservoir is presented. By replacing the water in the reservoir with an osmotic solution, this sensor provides a larger measurement range of soil water potential benefiting from the built-in osmotic pressure of the solution. However, to minimize the influence of temperature on the volumetric change of the osmotic solution, the sensor is designed with an active sensing element and a reference element. These two elements are identical, except that the reservoir of the reference element is sealed. This reference element only responds to temperature variations, thus the temperature influences could be compensated by subtracting the signal from the active element with that from the reference element. This sensor is used to continuously monitor the dynamic change in soil water potential over multiple days. Thirdly, in Chapter 4, a multi-ion sensor is developed for detecting nitrate, phosphate, and potassium ions. An artificial neural network (ANN) algorithm is applied to reduce the effect of interfering ions on the accuracy of the sensor. The accuracy of the sensor response interpretation for ionic activity estimation is compared between with- and without-neural network processing. The results show that the adoption of ANN is able to improve the accuracy in estimating specific ion concentration, compared to the direct ion concentration measurement using the sensor only. The present method is validated in detecting and quantifying nitrate, phosphate, and potassium ions in soil water, plant fluids, and tile drainage water from croplands. Finally, in Chapter 5, a miniature soil nitrate sensor is demonstrated for in-situ, continuous and long-term field nitrate monitoring for 60 days. The sensor consists of a working electrode and a reference electrode. The working electrode is formed from a nanocomposite of poly(3-octyl-thiophene) and molybdenum disulfide (POT−MoS2) coated on a patterned Au electrode and covered with a nitrate-selective membrane. The reference electrode was comprised of a screen-printed silver/silver chloride (Ag/AgCl) electrode covered by a Nafion layer to prevent chloride (Cl−) leaching in long-term measurements. The sensor is characterized for measuring NO3- concentrations in the adjusted soil samples in the laboratory in well-controlled conditions. No significant differences are shown between the sensor measurement results and the theoretical NO3- concentrations of the soil samples. Then the sensor is deployed in fields for 60 days for in-situ monitoring of soil NO3- concentrations. By using the in-situ soil sensors with a high sampling rate for NO3- concentration measurements, the dynamic changes of the soil NO3- have been studied.
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