Simultaneous Measurement of Soil Water Content and Salinity Using a Frequency-Response Method

where ε equals permittivity, εr equals relative permittivity, ε0 equals permittivity of free space (8.85 10 12 F Laboratory tests were conducted to simultaneously measure soil m 1), ε r equals dielectric constant (the real part of relawater content and salinity using a four-electrode Wenner array sensor. The sensor was modified to enhance the capacitive effect. Soil bulk tive permittivity), ε′′ r equals equivalent dielectric loss takdensity and the depth to which the electrodes penetrate into the soil ing into account the conductive loss (the imaginary part were strictly controlled during the experiments. Sinusoidal current of relative permittivity), and j √ 1. signals with a constant amplitude and frequencies ranging from 1 Hz The complex form of soil permittivity is analogous to to 15 MHz were sent to the outer electrodes of the Wenner array, impedance, which is composed of a resistive and a reacwhereas voltage outputs were measured from the inner electrodes. tive (capacitive and inductive) component with a phase Frequency-response data were analyzed using the partial least-squares difference of 90 . Soil permittivity is strongly affected method to establish calibration models for simultaneously predicting by frequency. In fact, both the real and imaginary parts water content and salinity from the frequency-response patterns. The of relative permittivity are functions of frequency (Eq. R2 values for predicting water content and salinity at the 30-mm penetration depth reached 0.89 and 0.91, respectively, whereas the [3] and [4]). Thus, the frequency, at which the dielectric root-mean-square errors for the volumetric water content and salinity permittivity is measured, plays a strong role on the effecmeasurements were 0.019 m3 m 3 and 0.173 cmol kg 1, respectively. tiveness of the measurement. Test results showed that, in general, the calibration models predicted ε′′r ε′′d /( ε0) [3] the water content more accurately than salinity. The depth to which the sensor penetrates into the soil has a strong effect on the measurement ε r ε′′r /tan [4] accuracy. This study has demonstrated that the modified Wenner array sensor and the frequency-response method have a potential for where ε′′d equals dielectric loss, /( ε0) equals conductive simultaneously measuring soil capacitive and conductive properties. loss, equals electrical conductivity (EC), equals anHowever, numerous difficulties, including contact resistance, depth gular frequency, and loss angle. control, and the effect of soil type, will need to be addressed to The capacitive (dielectric) behavior of soils has been improve the measurement accuracy. used to measure soil volumetric water content. A capacitance probe uses a pair of electrodes that insert into the soil to form a capacitor. This capacitor and an inductor S plays a key role in crop production as a physical in an oscillator circuit determine the oscillation fresupport and a reservoir of water and nutrients. Sitequency. Changes in volumetric water content vary the specific crop management decisions for optimized input capacitance and, in turn, change the oscillation frerates of water, fertilizer, pesticides, and seeds are largely quency. Test results indicated that solution ionic conbased on physical, chemical, and biological properties ductivity within soil water always causes increases in of soils. Traditional soil surveys and accompanying soil the dielectric permittivity measurement, indicating the databases are too general for use in site-specific farming effect of the imaginary part of relative permittivity on systems, and the current method of intensive grid samsoil dielectric characteristics (Robinson et al., 1998). pling requires a sizeable investment of money and time. The conductive behavior of soils has been used to Therefore, inexpensive sensors that are capable of meameasure bulk soil EC with sensors of various designs, suring multiple soil properties in real-time are needed. including a contact-type design, which places four elecSoil permittivity is a good indicator of several importrodes in a Wenner array configuration (Fig. 1) on the tant soil properties closely related to crop productivity. immediate surface of soil, and a noncontact, nondestrucSoil relative permittivity is composed of a real compotive design that uses the electromagnetic induction prinnent, the dielectric constant, and an imaginary compociple (McNeill, 1980). These sensors operate in the low nent, the equivalent dielectric loss (Kraus, 1984) (Eq. frequency range. Wenner array sensors have been tested [1] and [2]). extensively and were found highly accurate in measuring soil salinity, because soil salinity is the major factor ε εrε 0 [1] determining bulk soil EC. However, since soil water εr ε r jε′′r [2] content also has a strong effect on bulk soil EC, measurement of soil salinity using this type of sensor has to N. Zhang, G. Fan, and K.H. Lee, Dep. of Biological and Agricultural be made under known soil water conditions, such as “2 Engineering; G. Kluitenberg, Dep. of Agronomy; and T.M. Loughin, to 3 d after irrigations” (Rhoades and Ingvalson, 1971). Dep. of Statistics, Kansas State Univ., Manhattan, KS 66502. Contribution number 03-54-J of the Kansas Agricultural Experiment Station, A commercial soil EC sensor using the Wenner array Manhattan, KS. Received 7 Aug. 2003. *Corresponding author (zhangn@ configuration has been applied in many precision agriksu.edu). culture experiments for fast field measurement. HowPublished in Soil Sci. Soc. Am. J. 68:1515–1525 (2004).  Soil Science Society of America Abbreviations: EC, electrical conductivity; PLS, partial least squares; RMS, root mean square; TDR, time domain reflectometry. 677 S. 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