RESEARCH PAPER
Influence of soil physical properties on electrical parameters of Elec-trodes|SoilSsystem in aspect of the determination of soil moisture and salinity
 
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Institute of Agrophysics, Polish Academy of Sciences, Doświadczalna 4, 20-290 Lublin, Poland
Publish date: 1993-05-06
 
Acta Agroph. 1993, (0), 5–107
KEYWORDS
ABSTRACT
The possibility to monitor soil water status is an indispensable condition to control as well as to model processes occuring in the soil-plant-atmosphere continuum. Taking into consideration that only these sensors can be integrated in modern data acquisition systems that are read electrically, it becomes clear that the sensors for soil moisture and salinity must be electrical. Continuously undertaken since the end of the previous century attempts to evaluate soil moisture from soil electrical conductivity or electrical permittivity as derived from the measurements of an ElectrodeslSoil System (ESS ) impedace has not brought electrical methods sufficiently reliable to be widely applied. Calibration curves, i.e. the soil moisture-electrical resistance or moisture-electrical capacitance rela-tionships have proved to be strongly affected by frequency of the applied electrical field as well as by the soil temperature, salinity, texture and bulk density. The theory of dielectrics when applied lo soil cannot explain such phenomena as: enormous rise of electrical resistance and capacitance of the ESS with the frequency decrease, excessive influence of temperature and salinity on electrical capacitance of the ESS, and also a particularly strange phenomenon that: at the same geometry of the system and the same frequency of the applied electrical field, ESS with pure solution can reveal smaller electrical capacitance than that with saturated soil. Because of increasing interest in utilization of electrical properties of soil in order to gain information on the soil moisture and salinity, a search for a model explaining behavior of the HSS, when applied as a sensor for soil moisture and/or salinity, was undertaken. Part 2 of the presented dissertation describes an attempt to find a qualitative model of such a sensor, i.e. a model of the ESS (Electrodes|Soil System). The HSS under consideration is a system consisting of two bare electrodes and soil within their electrical field. The proposed model follows equivalent electrical circuits of impedance of the electrodelsolution interface like that applied in electrochemistry for considering kinetics of electrode processes. The model is based on clectrical polarization phenomena, including the interfacial one. It was shown that the ESS should be considered as an electrolytic cell where the soil pore water is the electrolyte. Its electrical impcdance is the resultant of several component subimpedances arising from a diversity of phenomena causing the sensor electrical polarization. It was shown that within the range of commonly applied frequencies below 108 Hz, readings of electrical capacitance, C, of the sensor (i.e. the ESS) were totally masked by interfacial pseudocapacitance while readings of the sensor electrical resistance, R, were affected by interfacial phenomena unless a special impedance bridge was applied which compensates for capacitive component of the sensor impedance. Numerical solution of the model was verified by comparison of its computer generated impedance-frequency characteristics with experimental data. The data concerned measurements of the C and R in-parallel equivalents of the ESS impedance, read within the frequency band ranged from 5*10-1-105 Hz. Then the numerical model was solved for a wide band of frequencies 10-8-1014 Hz. The solution brought to the conclusion that for the electrocapacitive measurements of the soil moisture the ap-plied frequency should fall within the range of 108—1010 Hz, whereas for the electroresistive measurements of the soil moisture and/or salinity the frequency should fall within the range of 103-107 Hz, Recent advances in generation of fast rising pulses has resulted in the application of Time Domain Reflectometry (TDR) for measurements of soil electrical permittivity (thus also moisture) and electrical conductivity (thus also salinity). Ihis technology is particularly suitable for the deter-mination of electrical permittivity of conducting media, like the soil, because it operates with a pulse having its leading edge composed of frequencies from the range 108-1010 Hz, recommended for readings of electrical permittivity of the soil. The technology of TDR is continuously developed in The Institute of Agrophysics, Polish Academy of Sciences and brought to a TDR apparatus for simultaneous measurements of the soil electrical permittivity, conductivity and temperature, from which the soil moisture and salinity can be determined, as discussed below in part 3 and 4. Part 3 of the dissertation concerns reduction of influence of soil matrix on dielectric readings of the soil moisture. Samples of soils, soil-like, and also other capillary-porous materials were analysed using TDR with the aim of determining the contribution of material bulk density, thus also porosity, to the electrical permittivity-water content conversion function. The study showed that bulk density, thus also porosity, substantially affects the permittivity-moisture relationship. Two equivalent, empirical, normalized conversion functions were found - one accounting for bulk density and the other for porosity effect. Each of them, when applied to the dielectric TDR determinations of moisture, doubled precision of the determinations, independent of the material bulk density, thus also porosity. Part 4 of the dissertation concerns the possibility of interpretation of simultaneous readings of soil electrical permittivity and conductivity in terms of the soil salinity. When analyzing relationships between soil bulk clectrical conductivity, ecG, and its bulk relative electrical permittivity, e, it was found (for the investigated mineral soils) that within the range of volume water content, θ, from about 0.2 up to saturation the derivative δecG/δε is moisture independent and is directly proportional to the soil salinity. It was found that the variable SX=3ecG/δε, determined from in situ measurements of ecG(θ>0.2) and ε(θ>0.2), can be considered as an index of relative salinity of the soil. It was shown that knowing the salinity index, SX, and also sand content, it was possible to calculate electrical conductivity of soil pore water which is a widely accepted measure of the soil salinity. This way the possibility for nondestructive monitoring of salt migration in the soil became realistic. This possibility was experimentally verified in a column experiment by recording time and spatial variability of non-steady salt flow parameters (break-through curves). Original methodical solutions of the measurements and their experimental embodiments are described in part 5 of the dissertation. It contains detailed description of: a variable ratio arm impedance bridge for the determination of the capacitive-resistive impedance of the ESS, a programme for numerical solution of the frequency dispersion of the ESS impedance, TDR in application for the determination of electrical permittivity and conductivity of soil.
METADATA IN OTHER LANGUAGES:
Polish
Wpływ fizycznych właściwości gleby na elektryczne parametry układu elektrody-gleba w aspekcie pomiaru jej wilgotności i zasolenia
agrofizyka, gleby, wilgotność, zasolenie, badania gleboznawcze, pomiary elektryczne, właściwości fizyczne, elektrody
Możliwość monitoringu parametrów wody glebowej jest warunkiem koniecznym kontrolowania oraz matematycznego modelowania procesów zachodzących w kontinuum gleba-roślina-atmosfera, a więc i prognozowania ich następstw. Biorąc pod uwagę, że integrowalne we współczesnych cy-frowych systemach akwizycji danych są wyłącznie czujniki "czytane" elektrycznie, należy dysponować elektrycznymi czujnikami agrofizycznych parametrów wody (patrz część l). Istotne spośród tych parametrów -wilgotność gleby oraz jej zasolenie, łatwe do bezpośredniego zmierzenia w laboratorium, są zarazem najtrudniejsze do odczytu elektrycznego. Niniejsza rozprawa dotyczy poszukiwań metody pomiaru elektrycznych parametrów gleby oraz sposobu ich interpretacji w kategoriach wilgotności i zasolenia. Dla ułatwienia identyfikacji dyskutowanych zmiennych ich symbole, oddzielone przecinkami, są wtrącane w tekst. Np.: "względna elektryczna przenikalność gleby, ε, decydująca o elektrycznej pojemności układu elektrody|gleba..,", "elektryczna konduktywność gleby, ekG, zależna od wilgotności gleby, θ, oraz od elektrycznej konduktywności elektrolitu (wody glebowej), ekE,..." itp. Dla zaznaczenia granic faz występujących w szeregu elektrochemicznym tworzonym przez układ dwu elektrod w glebie, przyjęto stosowany w elektrochemii sposób dzielenia nazw tych faz pionową kreską. Np.: "układ elektrody|gleba...", "granica elektrod|roztwór..." itp. Ten sposób podkreśla występowanie bezpośredniego kontaktu elektrycznego pomiędzy dyskutowanymi fazami.Trzon rozprawy tworzą cztery zasadnicze części: 2,3,4, i 5 w skrócie omówione niżej.
 
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