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13.2.2 EM-ECA Device

The device used to measure EM-ECa was the Geonics soil conductivity meter EM38 (Mississauga, ON, Canada; www.geonics.com), which is the sensor most often used for such measurements in agriculture (Sudduth et al., 2001). The device may be used in one of two measuring modes, in vertical mode (coil axes perpendicular to soil surface, denoted EMV) or in horizontal mode (coil axes parallel to soil surface, denoted EMH). The respective effective measuring depths are approximately 1.5 m and 0.75 m. When measuring, the device was placed in a plastic cylinder, which was mounted on a plastic sledge and towed behind an off-road vehicle for continuous measurements (ca. 200 measurements ha-1). A differentical Global Positioning System (DGPS) receiver (Ceres, CSI Wireless Inc., Canada) was attached on the sledge to deliver the geographical coordinates of the ECa measurements.

13.2.3 Measurements

On 30 April 2003, 310 soil samples were taken from the topsoil (0 to 20 cm) in a 20 x 20 m grid, covering about 38 percent of the field (12 ha) and analyzed for ignition loss, pH, P-AL (AL denotes the ammonium lactate/acetic acid mixture used for extraction; Égner et al., 1960), K-AL, Mg-AL, Ca-AL, Na-AL, and K-HNO3. A selection of 154 soil samples were additionally analyzed gravi-metrically for water content, and mechanical analysis of texture was performed on forty of these samples. From the latter group, twenty-one samples were extracted with 1 M ammonium acetate, and exchangeable Ca2+, K+, Mg2+, and Na+ were analyzed spectrometrically (ICAP 1100, Thermo Jarrell Ash Corp, US). Exchangeable H+ was determined by titration (NaOH) and cation exchange capacity (CEC) calculated as the sum of the five cations. Soil organic matter (SOM) was determined from ignition loss, by correcting for clay content, where the clay content was estimated by the finger method in those samples that lacked ordinary texture analysis. The same day as the soil sampling, EM-ECa was measured in the field with the EM38 in horizontal mode only. The rationale for using this mode is that the relative contribution to the signal from the topsoil is larger for EMH than for EMV (McNeill, 1980), and that EMH has been found to be superior to EMV in practice, when using the EM-ECa technique to map topsoil properties on fairly comparable soils (Korsaeth, 2005a, 2005b).

13.2.4 Statistics and Data Analyses

The ECa data were not transformed, as this is seldom done in commercial practice. The statistical software package MINITAB (Release 14.13, www.minitab.com) was used for basic statistics (calculating means, standard deviations, and correlations), one-way analysis of variance (ANOVA), and Fisher's least significant difference (LSD) test for comparisons. ArcView GIS (version 3.2, www. esri.com) was used to produce the maps.

Soil properties that showed a reasonably strong correlation with EMH (Irl > 0.5), and from which at least forty observations were available (thus excluding the exchangeable cations), were selected to test the standard procedure used for commercial soil survey. On the basis of measured ECa, the selected data were grouped separately into classes with intervals of 2 mS m-1 (groups with less than three observations were excluded). Thereafter, one-way ANOVA and Fisher's LSD test were used to test for differences between groups with respect to each of the selected variables. The rationale for using an interval of 2 mS m-1 was that experience from commercial mapping of a range of different soil types in Norway has shown that an interval of this size is normally most suitable.

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