Soil Electrical Conductivity Measurements Research Articles

Application of Soil Electrical Conductivity to Precision Agriculture

Due in large measure to the prodigious research efforts of Rhoades and his colleagues at the George E. Brown, Jr., Salinity Laboratory over the past two decades, soil electrical conductivity (EC), measured using electrical resistivity and electromagnetic induction (EM), is among the most useful and easily obtained spatial properties of soil that influences crop productivity. As a result, soil EC has become one of the most frequently used measurements to characterize field variability for application to precision agriculture. The value of spatial measurements of soil EC to precision agriculture is widely acknowledged, but soil EC is still often misunderstood and misinterpreted. To help clarify misconceptions, a general overview of the application of soil EC to precision agriculture is presented. The following areas are discussed with particular emphasis on spatial EC measurements: a brief history of the measurement of soil salinity with EC, the basic theories and principles of the soil EC measurement and what it actually measures, an overview of the measurement of soil salinity with various EC measurement techniques and equipment (specifically, electrical resistivity with the Wenner array and EM), examples of spatial EC surveys and their interpretation, applications and value of spatial measurements of soil EC to precision agriculture, and current and future developments. Precision agriculture is an outgrowth of technological developments, such as the soil EC measurement, which facilitate a spatial understanding of soil–water–plant relationships. The future of precision agriculture rests on the reliability, reproducibility, and understanding of these technologies.

Application of Soil Electrical Conductivity to Precision Agriculture

Due in large measure to the prodigious research efforts of Rhoades and his colleagues at the George E. Brown, Jr., Salinity Laboratory over the past two decades, soil electrical conductivity (EC), measured using electrical resistivity and electromagnetic induction (EM), is among the most useful and easily obtained spatial properties of soil that influences crop productivity. As a result, soil EC has become one of the most frequently used measurements to characterize field variability for application to precision agriculture. The value of spatial measurements of soil EC to precision agriculture is widely acknowledged, but soil EC is still often misunderstood and misinterpreted. To help clarify misconceptions, a general overview of the application of soil EC to precision agriculture is presented. The following areas are discussed with particular emphasis on spatial EC measurements: a brief history of the measurement of soil salinity with EC, the basic theories and principles of the soil EC measurement and what it actually measures, an overview of the measurement of soil salinity with various EC measurement techniques and equipment (specifically, electrical resistivity with the Wenner array and EM), examples of spatial EC surveys and their interpretation, applications and value of spatial measurements of soil EC to precision agriculture, and current and future developments. Precision agriculture is an outgrowth of technological developments, such as the soil EC measurement, which facilitate a spatial understanding of soil–water–plant relationships. The future of precision agriculture rests on the reliability, reproducibility, and understanding of these technologies.

Variograms of Ancillary Data to Aid Sampling for Soil Surveys

To provide reliable estimates for mapping soil properties for precision agriculture requires intensive sampling and costly laboratory analyses. If the spatial structure of ancillary data, such as yield, digital information from aerial photographs, and soil electrical conductivity (EC) measurements, relates to that of soil properties they could be used to guide the sampling intensity for soil surveys. Variograms of permanent soil properties at two study sites on different parent materials were compared with each other and with those for ancillary data. The ranges of spatial dependence identified by the variograms of both sets of properties are of similar orders of magnitude for each study site. Maps of the ancillary data appear to show similar patterns of variation and these seem to relate to those of the permanent properties of the soil. Correlation analysis has confirmed these relations. Maps of kriged estimates from sub-sampled data and the original variograms showed that the main patterns of variation were preserved when a sampling interval of less than half the average variogram range of ancillary data was used. Digital data from aerial photographs for different years and EC appear to show a more consistent relation with the soil properties than does yield. Aerial photographs, in particular those of bare soil, seem to be the most useful ancillary data and they are often cheaper to obtain than yield and EC data.

The Basic Study for Real-Time Measurement of Soil Electrical Conductivity in Paddy Field

The Basic Study for Real-Time Measurement of Soil Electrical Conductivity in Paddy Field

Estimation of Soil Solution Electrical Conductivity from Bulk Soil Electrical Conductivity in Sandy Soils

Studies of solute transport through soil and attendant environmental impacts are hampered by the lack of methods for continuous monitoring of solute concentration. Measurement of bulk soil electrical conductivity (ECb) using time domain reflectometry (TDR) is a promising technique, but it is indirect, and estimation of solute concentration from such measurements requires a model relating soil solution electrical conductivity (ECw) to ECb Several models of varying complexity exist, but further testing is required to determine their relative merits and applicability. This study was conducted to determine the efficiency of various models that use different methods of estimating the tortuosity factor, Fg, in order to estimate ECw from ECb All models assume that the ratio of ECb/ECw is proportional to soil water content, θ, with Fg as the coefficient of proportionality. In this study, two types of models were compared, those in which Fg is obtained from soil hydraulic properties and those in which Fg is estimated as it is in gas diffusion models, except with θ rather than porosity as the independent variable. Measurements were conducted in a sandy soil, across a range of ECw from 0.10 to 0.56 S m−1 The models in which Fg is obtained from soil hydraulic properties did not perform as expected. The results with the gas diffusion analog models were variable; the most successful of these was based on the 1959 model of Marshall, in which Fg is a power function of θ, θb Optimal results were obtained with , not far from Marshall's suggested value of 0.5 for gas diffusion. We conclude that there is no benefit to the use of soil hydraulic properties in estimating ECw from ECb measurements, at least for sandy soils, where simpler relationships appear to provide superior results.

A Simple Approach to Determine Reactive Solute Transport Using Time Domain Reflectometry

Time domain reflectometry (TDR) possesses potential for determining solute‐transport parameters, such as dispersion coefficients and retardation factors for reactive solutes. We developed a simple method based on peak‐to‐peak measurements of water and solute velocities through the soil using TDR. The method was tested by carrying out unsaturated leaching experiments in the laboratory on two soil columns packed with a South Pacific soil from Maré, which is a ferrasol with variable surface charge. One column was left bare and the other was planted with mustard. Pulses of CaBr2 and Ca(NO3)2 were applied to the surface of either wet or dry soil and then leached by water from a rainfall simulator applied at a steady rate of between 30 and 45 mm h−1 Water and solute transport were monitored by collecting the effluent. Contemporaneous in situ measurements of the water content and electrical conductivity were made using TDR. Transport parameters for the convection–dispersion equation, with a linear adsorption isotherm, were obtained from the flux concentration and the solute resident concentrations measured by TDR. Anion retardations between 1.2 and 1.7, and dispersivities between 1 and 9 mm, were found. Retardations also were calculated using our simple approach based on TDR‐measured water and solute front velocities. These used TDR measurements of soil water content and bulk soil electrical conductivity with time, and were similar to those obtained from the effluent. The agreement suggests TDR could be a valuable in situ technique for obtaining the parameters relating to reactive solute transport through soil.

Simultaneous measurement of water retention and electrical conductivity in soils: Testing the Mualem‐Friedman Tortuosity Model

A conceptual model that accounts for the influence of pore geometry is needed to obtain soil water conductivity from bulk soil electrical conductivity measurements in unsaturated soils. Mualem and Friedman [1991] proposed such a model, based on the hypothesis that the tortuosity factor affecting bulk soil electrical conductivity is identical to that defined for prediction of (odel output) of two soils were measured simultaneously over a wide range of soil water conductivity using a multistep outflow (MSO) experiment in combination with time domain reflectometry. The model with only one free parameter can describe the measured data. Predictions indicate a large dependence of the model outcome on the value of the fitting parameter. The fitted tortuosity parameters were used for inverse modeling of the water flow during the MSO experiments, obtaining unique estimates of the hydraulic conductivity.

A geostatistical method for soil salinity sample site spacing

A calibrated Four-Electrode Probe (FEP) was used for inexpensive and indirect determinations of salinity-sensor Electrical Conductivity (EC) in a plot at Cauto Valley, Cuba. Two transects were made in the North–South (N–S) and East–West (E–W) directions. Laboratory measurements of soil EC were also made from samples taken on a 50-m spaced square grid. A linear semivariogram was obtained for the salinity-sensor EC measurements at the E–W transect, which agrees with the topographical slope of the plot and with the expected soil salinity variation. It also coincides with the spatial structure of laboratory-measured soil EC. A cross-validation analysis has shown that EC semivariograms obtained from FEP measurements can characterize the soil EC spatial variation in a similar way as semivariograms of laboratory-measured soil EC. Thus, the distance between samples for soil salinity maps can be based on the semivariogram's range of salinity-sensor EC measurements.

Identification of Contaminated Soils by Dielectric Constant and Electrical Conductivity

To develop effective decontamination methods, characterization and identification of contaminated soils are needed. However, current methods of environmental soil characterization involve either soil sampling and analysis for targeted species in the laboratory or soil electrical conductivity measurements. Soil sampling and analyzing in the laboratory involves the risk of sample contamination during handling and testing. Furthermore, it is destructive. To overcome deficiencies of current identification and characterization methods, the use of dielectric constant and electrical conductivity of the soil-fluid system has been suggested. However, little is known about the dielectric behavior of a contaminated soil-fluid system. Thus, the objective of this study is to investigate the possibility of using dielectric constant and electrical conductivity to characterize and identify contaminated fine-grained soils. To investigate the usefulness of the preceding concept, the dielectric constant and electrical conductivity of kaolinite, bentonite, and a local soil are determined at various ion concentrations, organic liquids, and moisture content. Results show that both dielectric constant and electrical conductivity of soil-fluid system are mainly controlled by those of pore fluid.

Nutrient accumulation and nitrate leaching under broiler litter amended corn fields

Abstract Alabama's broiler chicken (Gallus gallus) industry produces large amounts of waste, which are disposed of by application to crop and pasture land. Land application of litter (manure and bedding) from broiler production can lead to contamination from losses of nutrients accumulated in soil. A study was conducted on 2 and 4% slopes from 1991 to 1993 at Belle Mina, Alabama, to determine the effects of broiler litter (BL) on soil elemental concentrations and nitrate leaching under a corn (Zea mays L.) ‐ winter rye (Secale cereale L.) cropping system amended with either: l) 9 mg#lbha‐1 of BL, 2) 18 mg#lbha‐1 of BL, or 3) commercial fertilizer (F) at a recommended rate. Soil was sampled to 100 cm prior to corn planting and subsequent to com harvest. Soil leachate samples were collected biweekly with wick lysimeters installed at a depth of 100 cm. Litter applications increased concentrations of soil organic carbon (C), extractable phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), copper (Cu) and zinc (Zn). Post harvest soil sampling indicated leaching of soil nitrate that was generally highest under BL18. Soil electrical conductivity measurements were highest under BL18, but values were not in the range considered detrimental to crops. Nitrate‐N (NO3‐N) concentrations measured in soil percolate at 1‐m depth on the 2% slope were higher under F than litter treatments. Both the F and BL18 treatments produced some NO3‐N concentrations above the primary drinking water standard, but averaged only 8.3 and 4.8 mg#lbL‐1, respectively. The BL9 treatment consistently remained under 10 mg NO3‐N#lbL‐1 with a mean concentration of 1.3 mg#lbL‐1. Overall, litter applied a 9 mg#lbha‐1 produced agronomic results comparable to F and appeared to be the optimal rate of application under the conditions of this study.

Reservoir Characterization Using Surface Geochemistry, Sorrento Field, Cheyenne County, Colorado: ABSTRACT

The survey was performed in July of 1994 as part of the Colorado School of Mines Reservoir Characterization Project. In addition to the geochemical survey, the study included a complete geologic and engineering evaluation, a 3-D seismic survey, an electrical resistivity survey, and a radiometric survey. Geochemical methods included soil gas hydrocarbons, pH, Eh, and soil electrical conductivity measurements. The data were analyzed and interpreted for concentration and compositional anomalies. Field analog comparisons provided additional support for the compositional characterization. With the exception of local lineaments, defining anomalies based on direct hydrocarbon concentration proved difficult due to field depletion. The indirect parameters of pH, Eh, and conductivity exhibited a greater degree of anomaly definition by identifying areas of varying reservoir potential and exposing a complex system of faults and fractures. Compositional analysis of the data yielded information regarding reservoir lithology and hydrocarbon type. Surface hydrocarbon ratios were compared to gas compositions from known Pennsylvanian and Mississippian wells within Cheyenne County. Discrimination of these reservoirs, by analog comparison, was possible within the Sorrento Field limits. The final report, released by CSM in August of 1995, asserted that the hydrocarbons and pH parameters reliably detected the major trends. The conclusion stated thatmore » the geochemical anomalies were consistent with the geologic and geophysical interpretations.« less

Field measurements of soil electrical conductivity and water content by time-domain reflectometry

The ability to easily and simultaneously measure temporal distributions of both soil water content and soil salinity is desirable when conducting experiments designed to evaluate management practices in irrigated agriculture where saline waters are used. Soil salinity is related to soil electrical conductivity. Soil electrical conductivity and water content in a ridge-furrow tilled loamy sand irrigated with a salt solution were measured using time-domain reflectometry (TDR). The salt solution was 0.026 mol kg −1 NaCl which has the electrical conductivity of about 0.3 S m −1. A computer controlled multiplexer was used to acquire electrical conductivity and water content data from a metallic cable tester. Information in TDR traces related to water content and electrical conductivity was not appreciably affected by impedance mismatch within a rotary switch, used as a multiplexer. Soil water content and electrical conductivity of soil solution estimated from TDR agreed well with sampled data.

Frequency domain analysis of time domain reflectometry waveforms: 2. A four‐component complex dielectric mixing model for soils

Although time domain reflectometry (TDR) is becoming accepted as an important tool for the measurement of soil water content and bulk soil electrical conductivity, a major part of the method is based on empirical relationships. An improved understanding of dielectric measurements on soils may give more insight into soil properties other than soil water content and bulk soil electrical conductivity. Frequency domain analysis of TDR waveforms enables the measurement of the frequency dependent complex dielectric permittivity of soils. The frequency dependent complex dielectric permittivity of soils can be described with a four‐component complex dielectric mixing model based on the volumetric mixing of the refractive indices of the soil components. The four soil components in the model are air, solids, bound water, and free water. Results indicate that the apparent dielectric permittivity obtained from the travel time of the TDR pulse in the soil is the dielectric permittivity at the highest measurement frequency of the cable tester, probe, and soil system. The model based on the volumetric mixing of real permittivities underestimates the measurements in situations with high values of the imaginary part of the dielectric permittivity. Because the model based on the mixing of the complex dielectric permittivities can describe the data, we conclude that the apparent dielectric permittivity is influenced by the imaginary parts in the dielectric, permittivities of the soil components. Combination of the four‐component complex dielectric mixing model with the complex dielectric permittivity obtained from the frequency domain analysis of TDR waveforms gives a tool for modeling the bulk soil electrical conductivity by separating the conductivity of the soil water into a bound water conductivity and a free water conductivity.

Use of electromagnetic measurements of apparent soil electrical conductivity to predict boulder clay depth : Brus, D J; Knotters, M; van Dooremolen, W A; van Kernebeek, P; van Seeters, R J M GeodermaV55, N1/2, Oct 1992, P79–93

Use of electromagnetic measurements of apparent soil electrical conductivity to predict boulder clay depth : Brus, D J; Knotters, M; van Dooremolen, W A; van Kernebeek, P; van Seeters, R J M GeodermaV55, N1/2, Oct 1992, P79–93

The use of electromagnetic measurements of apparent soil electrical conductivity to predict the boulder clay depth

Bulk soil electrical conductivity ( EC a) was measured under both dry and wet conditions by using the EM38 and EM31, in order to assess the potential of these instruments for measuring of the boulder clay depth ( D bc beneath coversand. The relationship between D bc and EC a was examined by fitting an exponential model (using D bc as a response variable) and a spherical model (using EC a as a response variable). The parameters of the exponential model were estimated by the maximum likelihood method, which makes it possible to take into account the censored observations of D bc. It was shown that the D bc predictions can be improved significantlly by stratification according to land-use. Nevertheless the residual standard deviation was rather large (43 to 67% of the predicted value), probably because the spatial variation in soil moisture content is not fully circumscribed by the stratification. Predicting D bc in two and four depth classes, gave only satisfactory results in the case of two classes: approximately 80% of the class predictions were correct. The results of the measurements of EC a, using the EM38 instrument, were better under wet conditions than under dry conditions. Additionally, under wet conditions, those with the EM38 were better than those with the EM31. Models with two measurements of EC a were not or only slightly better than models with a single measurement. The accuracy of the predictions with the spherical model was comparable to that of the exponential model.

Time Domain Reflectometry Measurements of Water Content and Electrical Conductivity of Layered Soil Columns

AbstractThe major argument for measuring the soil salt concentration and water content at the same position is their high correlation and spatial variability. Time domain reflectometry (TDR) was used for simultaneous measurement of soil water content, θ (derived from the soil dielectric constant ε), and bulk soil electrical conductivity, σa (from the attenuation of a transmitted pulse), for uniform and layered soil profiles in the laboratory. The purpose of the study was to test the different concepts of travel time and attenuation by using independent estimates of θ and σa. Both parallel (two‐rod) and simulated coaxial (three‐rod) probes resulted in essentially the same ε. The three‐rod probe measurements were easier to interpret and did not necessitate an impedance‐matching transformer. The TDR measurements of θ for layered profiles were not always accurate, especially when wet soil was overlying dry soil, due to erroneous interpretation of the TDR trace. In this case, the pulse travel time should be measured at the inflection point, which is sometimes difficult to identify, and not at the minimum of the TDR trace. This study reports a new method of calculating σa, which is based on a direct measurement of the transmission‐line load by TDR. This new method, which is simpler than the previous ones and independent of multiple reflections, was found to correlate better than the previously published methods with another, independent method for σa measurement (four‐electrode technique).

Determining Soil Salinity from Soil and Soil‐Paste Electrical Conductivities: Sensitivity Analysis of Models

AbstractModels and relations used to determine soil salinity (ECe) in the field from measurements of bulk soil electrical conductivity (ECa), or from saturated soil‐paste electrical conductivity (ECp), and estimates of other influential soil parameters were analyzed to evaluate the degree to which salinity appraisal is affected by inaccuracies made in the estimates. Results show that the values of the parameters which can not be easily measured in the field (i.e., bulk density (ρb), particle density (ρs), clay percentage (% clay), and total and “immobile” volumetric soil water contents (θw and θws, respectively) can be estimated sufficiently accurately for the purposes of practical soil salinity appraisal.

New Calibrations for Determining Soil Electrical Conductivity—Depth Relations from Electromagnetic Measurements

AbstractSoil salinity can be determined in the field from measurements of soil electrical conductivity (ECa). Measurements of depth‐integrated ECa can be made remotely using electromagnetic induction (EM) techniques. A means of determining ECa for the soil depth intervals of 0 to 30, 30 to 60, 60 to 90 cm, etc., from the EM measurements is needed for salinity appraisal. The EM and ECa measurements were made at about 900 sites in the San Joaquin Valley of California. This large data set was used, along with rigorous statistical techniques, to obtain empirical coefficients used in equations to predict ECa by depth intervals within the soil profile from EM readings taken above ground. Predictions were found to be more accurate using these new coefficients rather than those previously available.

886108 Matrix nonuniformity effect on measurement of apparent bulk soil electrical conductivity : Nadler, A Soil SciV145, N3, March 1988, P157–162

886108 Matrix nonuniformity effect on measurement of apparent bulk soil electrical conductivity : Nadler, A Soil SciV145, N3, March 1988, P157–162

The time-domain reflectometry method for measuring soil water content and salinity

This paper discusses the physical principles and use of time-domain reflectometry as a new tool for studying water and solute transport in unsaturated soils. In-situ measurements of water content and bulk soil electrical conductivity are shown to give results that are comparable with those obtained by conventional non-destructive techniques. An equation is presented that relates the bulk soil electrical conductivity to the soil solution electrical conductivity. Also derived are constraints that water content and electrical conductivity place on the use of time domain reflectometry sensors.