Introduction

The use of sensor techniques provides a time- and cost-effective alternative to traditional soil sampling and laboratory analyses, in order to monitor spatial and temporal soil variation. One sensor-based, noninvasive technique is to measure apparent electrical conductivity (ECa) of a soil profile by means of an electromagnetic (EM) induction approach. The device selected for the EM measurements in this paper was the Geonics EM38 (Mississauga, ON, Canada; www.geonics.com), which is the EM-ECa sensor most often used in agriculture (Sudduth et al., 2001). The device may be operated in one of two measurement modes that give an effective measuring depth of either 0.75 or 1.5 m.

EM-ECa has been used successfully as an indirect indicator of important soil physical and chemical properties, such as soil salinity hazard (Williams and Baker, 1982), soil water content (Khakural et al., 1998), and topsoil inorganic N content (Korsaeth, 2005). Whereas these studies focus mainly on soil variation in the lateral plane, others have used the EM-ECa technique with

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