Field-Scale Electrical Conductivity Mapping for Delineating Soil Condition

water resources and enhancing soil quality (Wallace, 1994). An essential first step to the successful implemenTraditional sampling methods are inadequate for assessing the tation of site-specific management is the evaluation of interrelated physical, chemical, and biological soil properties responsible for variations in agronomic yield and ecological potentials across new technologies at the field-scale (Vanden Heuvel, a landscape. Recent advances in computers, global positioning sys1996). tems, and large-scale sensors offer new opportunities for mapping Complex inter-relationships exist between physical, heterogeneous patterns in soil condition. We evaluated field-scale chemical, and biological soil properties and their reapparent electrical conductivity (ECa) mapping for delineating soil sponse to land management; these factors are responsiproperties correlated with productivity and ecological properties. A ble for crop productivity and ecological potential (Bauer contiguous section of farmland (250 ha), managed as eight fields and Black, 1994; Gardner and Clancy, 1996; Olson et in a no-till winter wheat (Triticum aestivum L.)–corn (Zea mays al., 1996). Soil condition is the combined characteristics L.)–millet (Panicum miliaceum L.)–fallow rotation, was ECa mapped of a given soil that define its level of function as a (≈0to 30-cm depth). A geo-referenced soil-sampling scheme sepamedium for crop production and a contributor to air rated each field into four ECa classes that were sampled (0to 7.5and 7.5to 30-cm depths) in triplicate. Soil physical parameters (bulk and water quality. In this paper, we define ecological density, moisture content, and percentage clay), chemical parameters potential as the complementary interactions between (total and particulate organic matter [POM], total C and N, extractthe soil biological community and the soil environment able P, laboratory-measured electrical conductivity [EC1:1], and pH), that optimize soil condition and are determined by land biological parameters (microbial biomass C [MBC] and N [MBN], management. and potentially mineralizable N), and surface residue mass were signifDifferent approaches have been used to detect and icantly different among ECa classes (P 0.06) at one or both depths map soil condition patterns related to spatial variation (0–7.5 and 0–30 cm). Bulk density, percentage clay, EC1:1, and pH in productivity. Lark (1997) used intensive grid sampling were positively correlated with ECa; all other soil parameters and (20-m intervals), based upon soil texture and depth, to surface residue mass were negatively correlated. Field-scale ECa classiidentify seven map units across a 6-ha field. He found fication delimits distinct zones of soil condition, providing an effective basis for soil sampling. Potential uses include assessing temporal imsignificant differences among map units for several pacts of management on soil condition and managing spatial variation yield-related soil properties including percentage moisin soil-condition and yield-potential through precision agriculture and ture and organic matter, mineral N, and pH at 0to 20site-specific management. cm depths (P 0.003). However, this type of intensive grid sampling is both labor intensive and costly, making it impractical at the farm-scale. Francis and Schepers T heterogeneous nature of soil across a land(1997) used selective soil sampling based on soil color, scape has long been recognized; however, the lack texture, depth, slope, and erosion characteristics to proof sensitive tools to detect subtle shifts among soil propduce fertilizer recommendation zones. These zones eferties has limited spatial delineation of this variability. fectively partitioned concentrations of the nonmobile Recent technological advances in computer hardware nutrients P, K, and Zn. Studies, such as these, underand software, global positioning systems, and sensors score a need for cost-effective technology to assess spafor field-scale measurements offer new opportunities to tial variation in soil condition at the field-scale. map the complex patterns in soil condition that underlie Laboratory measurement of EC1:1 is a useful integand define agronomic yield potential. Field-scale sensor rator of soil physical, chemical, and biological factors maps may provide a basis for soil-sampling strategies that regulate soil function (Smith and Doran, 1996). that accurately reflect spatial variation. Such sampling Geo-referenced in situ estimates of ECa are now being strategies may be useful in temporal analyses to monitor made at the field scale using both direct contact sensors ecological trends and for managing inherent soil varito measure resistance and noncontact sensors based ability through precision agriculture. Site-specific manupon electromagnetic induction technology (Dolittle et agement has the potential to maximize agricultural proal., 1995; Jaynes et al., 1995; Jaynes, 1996). These two duction and economic return while conserving soil and approaches provide highly correlated measures of ECa and both have been shown to correlate with crop productivity at topsoil depths to 90 cm (Fritz et al., 1999; C.K. Johnson, J.W. Doran, B.J. Wienhold, and J.F. Shanahan, USDASudduth et al., 1999). ARS, 120 Keim Hall, Lincoln, NE 68583-0934; H.R. Duke, USDAARS, AERC-CSU, Ft. Collins, CO 80523-1325; K.M. Eskridge, Univ. Measured soil ECa is determined by clay type and of Nebraska, 103 Miller Hall, Lincoln, NE 68583. The USDA-ARS, percentage, soil moisture (in conjunction with pore size, Northern Plains Area is an equal opportunity/affirmative action employer and all agency services are available without discrimination. Abbreviations: EC1:1, laboratory-measured electrical conductivity usJournal Series No. 13291. Received 5 Sept. 2000. *Corresponding ing a 1:1 soil:water ratio; ECa, field-scale apparent electrical conductivauthor ([email protected]). ity; MBC, microbial biomass C; MBN, microbial biomass N; POM, particulate organic matter. Published in Soil Sci. Soc. Am. J. 65:1829–1837 (2001).