Apparent soil electrical conductivity measurements in agriculture

The field-scale application of apparent soil electrical conductivity (ECa) to agriculture has its origin in the measurement of soil salinity, which is an arid-zone problem associated with irrigated agricultural land and with areas having shallow water tables. Apparent soil electrical conductivity is influenced by a combination of physico-chemical properties including soluble salts, clay content and mineralogy, soil water content, bulk density, organic matter, and soil temperature; consequently, measurements of ECa have been used at field scales to map the spatial variation of several edaphic properties: soil salinity, clay content or depth to clay-rich layers, soil water content, the depth of flood deposited sands, and organic matter. In addition, ECa has been used at field scales to determine a variety of anthropogenic properties: leaching fraction, irrigation and drainage patterns, and compaction patterns due to farm machinery. Since its early agricultural use as a means of measuring soil salinity, the agricultural application of ECa has evolved into a widely accepted means of establishing the spatial variability of several soil physico-chemical properties that influence the ECa measurement. Apparent soil electrical conductivity is a quick, reliable, easy-to-take soil measurement that often, but not always, relates to crop yield. For these reasons, the measurement of ECa is among the most frequently used tools in precision agriculture research for the spatio-temporal characterization of edaphic and anthropogenic properties that influence crop yield. It is the objective of this paper to provide a review of the development and use of ECa measurements for agricultural purposes, particularly from a perspective of precision agriculture applications. Background information is presented to provide the reader with (i) an understanding of the basic theories and principles of the ECa measurement, (ii) an overview of various ECa measurement techniques, (iii) applications of ECa measurements in agriculture, particularly site-specific crop management, (iv) guidelines for conducting an ECa survey, and (v) current trends and future developments in the application of ECa to precision agriculture. Unquestionably, ECa is an invaluable agricultural tool that provides spatial information for soil quality assessment and precision agriculture applications including the delineation of site-specific management units. Technologies such as geo-referenced ECa measurement techniques have brought precision agriculture from a 1980's concept to a promising tool for achieving sustainable agriculture.