Use of Electromagnetic Induction Tools in salinity assessment/appraisals in eastern Colorado

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Courtesy of Geonics Limited

Abstract
Electromagnetic induction (EMI) is a relatively low-cost and rapid method for measuring, assessing spatially soil salinity. The two studies were conducted to evaluate data obtained with a single-frequency (EM38 meter) and multi-frequency (GEM300 sensor) EMI instruments and to relate apparent conductivity measured by these instruments with the more conventional conductivity of the saturated soil extract. These two studies were geo-referenced for soil correlation sampling, salinity mapping and future monitoring of salinization and/or degradation. The one study area (54 hectares) utilized a subset sample area for the comparison procedure; the other site (323 hectares) was conducted solely with the EM38 instrument. Data was incorporated into modeling programs and further used into mapping software to develop two-dimensional maps.

Correlation coefficients between the two instruments ranged from 0.8039 to 0.8617. Although the GEM300 sensor predicted somewhat less accurately the conductivity of the soil samples collected and also produced higher apparent conductivity measurements, spatial patterns of apparent and electrical conductivity produced by the two instruments were similar, reasonable and practical for the end user, the agricultural producer.

Introduction
Across the western United States salinity is occurring at an alarming rate on irrigated lands. This 'silent killer' is a major cause of reduced crop production, degraded crop quality, and degraded soil and water quality. In these areas, changes in soil salinity need to be measured, fully assessed and monitored to make future evaluations so as to minimize the advance of salinization and soil degradation. Strategies for resource agencies associated with irrigated growers have relied upon traditional soil sampling methods to ascertain the level of salinization which has not been very helpful to determining the quantifying of salinity degradation in a spatial sense. In the early 1980s, electromagnetic induction (EMI) gained acceptance as a useful method for rapid field identification and mapping of soil salinity (Johnston et al., 1997). In the past 20 years EMI has offered a set of tools to assess and assist in monitoring the advance of salinization.

Electromagnetic induction uses electromagnetic energy to measure and map spatial and temporal variations in the apparent conductivity (ECa) of soils. Apparent conductivity is a weighted, average conductivity measurement for a column of earthen materials to a specific depth (Greenhouse and Slaine, 1983). In areas of salinized soils, such as here in the major river valleys of Colorado, 65 to 70 percent of the variance in apparent conductivity can be explained by changes in the concentration of soluble salts alone (Williams and Baker, 1982). Moderate to high correlations have been found between apparent conductivity and saturated paste extract (ECe), the most accepted and accurate method of determining soil salinity (de Jong et al., 1979; Williams and Baker, 1982; and Wollenhaupt et al., 1986). Studies have demonstrated that EMI can provide reasonably accurate estimates of soil salinity (Williams and Baker, 1982; van der Lelij, 1983; Diaz and Herrero, 1992).

Since the early 1980s, agronomists, conservationists, and soil scientists have used EMI meters developed by Geonics Limited.1 In the 1990s, Dualem, Inc., Geophysical Survey Systems, Inc., and Veris technologies developed additional EMI instruments. Our work in Colorado has concentrated on the use of the Geonics EM-38 for the assessment of salinity in agricultural fields. Our second emphasis is to utilize these tools with geographical positioning system (GPS) instruments, summarize these data and employ computer software to quantify the conductivity data into a spatial map for the agricultural producer. This apparent conductivity (ECa) measured by the EMI instruments is related to the electrical conductivity of the saturated paste extract (ECe) using equation developed by the scientists at the United States Salinity Laboratory, Riverside, California.

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