APPLIED PHYSICS LETTERS
VOLUME 80, NUMBER 15
15 APRIL 2002
Determination of the humidity of soil by monitoring the conductivity with indium tin oxide glass electrodes Francisco Fabregat-Santiago, Noemı´ S. Ferriols, Germa` Garcia-Belmonte, and Juan Bisquerta) Departament de Cie`ncies Experimentals, Universitat Jaume I, 12080 Castello´, Spain
共Received 15 October 2001; accepted for publication 11 February 2002兲 A number of approaches to the determination of water content of soil use a dielectric permittivity measurement, which is accurate but involves expensive and complex devices. We explore the use of indium tin oxide, a wide-band gap semiconductor with high conductivity, long-term stability, and low cost, to build a cheap sensor of low humidity values for automatic irrigation systems. The electrical resistance determined by an ac impedance method discounts the effects of electrode contacts and provides a reliable measure of the water content, even when the chemical conditions of water vary widely. The reason for this is that, at low values of humidity, the conductivity is controlled by a surface conduction mechanism that is largely insensitive to the electrolytic properties of added water. The results appear promising for applications that require the detection of very low levels of humidity in soil. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1469215兴
The automation of irrigation in modern farms, greenhouses or buildings has extended over the last years, in order to increase the irrigation efficiency and reduce the maintenance expenses. Frequently, automatic irrigation systems are based on prescheduling which does not account for the exact conditions. Thus, water wasting may follow a sudden rain or crop losses can result from unexpected dry atmosphere conditions. Therefore, there is an increasing need for simple inexpensive automatic monitoring of water content in soil for agriculture applications. Reliable techniques of determination of the water content have been developed based on the measurement of dielectric permittivity, such as time domain reflectometry1 and capacitance probes.2 However, these measurements operate at very large frequencies 共above MHz兲 and involve complex and expensive devices. Furthermore the dielectric permittivity systems are mainly aimed at high volumetric water contents 关with (%)⫽100(V w /V s ), where V w is the total volume of water in the total volume of all soil components, V s 兴. This limits their applicability in dry environments, such as mediterranean, subtropical, and other semiarid climates, where plants are adapted to low . Another class of devices is used for measuring the atmospheric moisture. These sensors are based on electrical resistance measurements of porous materials such as polymers and ceramics. These devices are sensitive to environmental humidity owing to conduction through the water molecules that adsorb onto the internal surface.3–5 The behavior of the porous devices in periodically irrigated earth would be limited by their endurance against external hazards, the chemical attack, and the progressive occlusion of the voids by mineral particles. The feasibility of such mechanism for the agricultural applications, requires a reliable method of resistance measurement, which will be stable against reasonable
variation in the water chemistry. They should also withstand harsh underground conditions during years without degradation. As a candidate to build this device, we studied indiumdoped tin oxide 共ITO兲, which is a degenerate semiconductor usually employed as a transparent conducting substrate in many electrochemical and photoelectrochemical applications. The electrochemistry of ITO is well known. It does not react with water unless one applies more than 0.5 V either anodic or cathodic. ITO withstands heavy heterogeneous currents without degradation and remains stable also when it is immersed in acidic or basic solution. Furthermore, glass covered with ITO film is available in large inexpensive sheets that are produced on an industrial scale. We report here on laboratory tests conducted to investigate the resistivity of the wet soil measured with ITO glass electrodes by the ac impedance method. Pieces of 1.5 ⫻1.5 cm ITO glass 共Delta Technologies, Ltd., series resistance 10 ⍀/䊐兲 were used. A longitudinal groove ⬃500 m wide separated the conducting film to two independent halves. Contacts were attached to each part with silver epoxy and thermally sealed with a commercial thermoplastic to
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Author to whom all correspondence should be addressed; electronic mail:
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FIG. 1. Schematic representation of the sensor/electrode.
0003-6951/2002/80(15)/2785/3/$19.00 2785 © 2002 American Institute of Physics Downloaded 22 May 2002 to 147.156.1.162. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
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FIG. 2. Complex plane representation of the impedance of the ITO electrode immersed in soil in a drying series. Water content 共%兲: 15.4 共䊊兲, 11.4 共䉮兲, 8.81 共䊐兲, 7.67 共〫兲, 6.04 共䉭兲, 3.20 共˝兲. The inset shows the bode plot of the impedance, Z⫽Z ⬘ ⫹Z ⬙ , Z ⬘ (䊊) and Z ⬙ (䉮), for ⫽15.4%.
avoid spurious reactions 共Fig. 1兲. The sensor was embedded in a container filled with ⬃2 kg of soil, making a direct contact with the soil. The water content variation was determined by weight. The ac impedance was measured between the coplanar halves of the electrode with standard electrochemical equipment, applying a sinusoidal amplitude of 100 mV. The ac impedance responses at various values are shown in Fig. 2. Taking into account the complexity of the measured system, the results are reasonably reproducible in successive cycles of watering and drying. The impedance spectra contain two types of processes. The low frequency response is characteristic of a diffusion process, and in other cases, it approaches a distributed capacitive process. This slow process is probably related to the buildup of electrode polarization. As shown in Fig. 2, the electrode process is completely separated from the bulk response by the frequency-resolved measurement, so that one can avoid contact effects rather easily by this method. The high frequency arc is the parallel combination of a resistor R p and a capacitor C p . The parameters R p and C p were obtained by fitting the high frequency arc with standard software. The arc diameter, which is related to the value of resistance R p , decreases with increasing 共Fig. 3兲. The inset of Fig. 3 shows the reciprocal of capacitance C p . These values were found to increase from 1.2⫻10⫺10 F for the dry
Fabregat-Santiago et al.
soil to 1.8⫻10⫺10 F for the water-saturated soil. The polarization at the Helmholtz layer in the semiconductor surface is known to provide values about 5 F/cm2. Thus the much lower C p value obtained can not be related to contact phenomena. Therefore, the high frequency arc reflects the electrical properties of the space between the electrode plates. A comprehensive examination of the characteristics of a wide range of different humidity sensors3 concluded that all operate by means of the same physical mechanism. At low humidity, small ions present in the surface of the grains possess a high local charge density and a strong electrostatic field, providing good sites for the chemisorption of water molecules. Thus, upon exposure of the sensor to the atmosphere, strongly bound water molecules occupy the available sites quickly. Once formed, this layer is not further effected by exposure to humidity. Subsequent layers of water molecules are physically adsorbed. The physisorbed water dissociates because of the high electrostatic fields in the chemisorbed layer: 2H2 O↔H3 O⫹ ⫹OH⫺ . Charge transport occurs when H3 O⫹ releases a proton to a neighboring water molecule which accepts it while releasing another proton, and so forth. This is known as the Grotthuss chain reaction. At high humidity, liquid water condenses in the pores, and electrolytic conduction takes place in addition to the protonic transport in the adsorbed layers. This general picture3 satisfactorily explains the electrical response of the ITO sensor. The change of mechanism, from a predominance of proton hopping at the grains surface, to electrolytic paths through interstitial water, is observed in Fig. 3, where the conductivity reaches a plateau at ⬎10%. This view is further supported by the similarity of the impedance patterns found in Fig. 2, to those usually obtained in hydrated porous materials such as zeolites4 or ceramics.5,6 Furthermore, the dielectric permittivity of microporous materials increases with the penetration of water, and Fig. 3 shows that this is observed in our system. It has been suggested7 that the increase of the dielectric constant of the medium implies a decrease of the free energy of dissociation. This raises the number of H3 O⫹ charge carriers, and as a result the conductivity increases. According to this model, the logarithm of the measured electrical resistance varies linearly with the reciprocal of the dielectric constant. This theory7 is reasonably obeyed by our data, as shown in the inset of Fig. 3. Additional tests were conducted to examine the ITO sensor operation in different conditions. Several cycles of watering and drought with acidic water 共pH 5.5兲 produced variations in resistance R p of 10%. The same process was carried out after the addition of commercial liquid fertilizers to the water 共pH 3.7兲. In this case, after three cycles, the measured resistance doubled its value. In order to measure the conductivity of free water 共with dissolved soil species兲 the sensor was placed in a small permeable box stuck into the soil. It was observed that a variation of the acidity of the poured water between pH⫽5 and 8 induced a small change in R p lower than 5%. The reason is probably that the soil itself provides a large reservoir of dissolved species, determining the measured conductivity. Finally, measurements of R p ( ) with a much more soft and porous type of soil showed
FIG. 3. Variation of the resistance 共high frequency arc兲 of the ITO electrode immersed in soil, with respect to the water content, in a drying series. The inset shows the logarithm of the resistance with respect to the reciprocal capacitance. Downloaded 22 May 2002 to 147.156.1.162. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
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Appl. Phys. Lett., Vol. 80, No. 15, 15 April 2002
a very similar trend as that of Fig. 2, but with consistently higher values of resistance. While the quantity R p has been determined in the laboratory by fitting the impedance spectra, for applications in the field a simple and sufficiently accurate procedure is available. Inspection of the bode plot of the impedance 共inset of Fig. 2兲 shows that the real part Z ⬘ traces a plateau corresponding to the valley of the imaginary part Z ⬙ . Therefore, R p can be determined measuring the electrode resistance at a single frequency, choosing this frequency near the minimum of Z ⬙ . Moreover, the high frequency relaxation time varies weakly with water content, due to the opposite sense of variation of R p and C p with . Thus, the minimum of Z ⬙ shifts along the frequency axis only weakly, and as a consequence, it is possible to determine R p for a wide range of water contents using a uniquely selected frequency of about 104 Hz. In conclusion, in the range 5%⬍ ⬍10%, the ITO sensor shows high sensitivity; the readings of R p vary by two orders of magnitude in this range 共Fig. 3兲. The conductivity appears to be determined by the mechanism of proton hopping in the grains surfaces that is generally found in porous
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humidity sensors. In our case, the porous structure of the soil itself is used as an integral part of the electrical sensor. Therefore, while probably unsuitable for an absolute determination of water content, this device appears promising for tracking the dryness of the soil. This should be confirmed by more extensive tests in the field, which are beyond the scope of this report. This work was supported by la Comisio´n Interministerial de Ciencia y Tecnologı´a under project PB98-1045, and the authors acknowledge discussions with Ignacio Morell.
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