SITE-SPECIFIC MANAGEMENT Site-Specific Management Zones Based on Soil Electrical Conductivity in a Semiarid Cropping System

geographic information systems (GIS) for spatial analysis and mapping, variable-rate applicators, and input preSite-specific management (SSM) can potentially improve both ecoscription maps to define management zones and direct nomic and ecological outcomes in agriculture. Effective SSM requires metering devices controlling input rates (Eliason et al., strong and temporally consistent relationships among identified man1995). While the first three components are currently agement zones; underlying soil physical, chemical, and biological parameters; and crop yields. In the central Great Plains, a 250-ha dryland available, the last, an effective and economical basis for experiment was mapped for apparent electrical conductivity (ECa). defining site-specific inputs, is lacking. In response to Eight fields were individually partitioned into four management zones this need, significant research effort has been directed based on equal ranges of deep (ECDP) and shallow (ECSH) ECa (aptoward evaluating a variety of individual and combined proximately 0–30 and 0–90 cm depths, respectively). Previous experiGIS databases as frameworks for identifying stratified ments documented negative correlations between ECSH and soil propwithin-field management zones (regions of similar proerties indicative of productivity. The objectives of this study were to duction potential). These include kriged soil test point examine ECSH and ECDP relationships with 2 yr of winter wheat (Tritidata (Mulla, 1991); soil survey maps (Robert, 1989); cum aestivum L. ) and corn (Zea mays L.) yields and to consider the topography (Kravchenko et al., 2000); remote sensing potential applications of ECa–based management zones for SSM in (McCann et al., 1996); topography and remote sensing a semiarid cropping system. Within-zone wheat yield means were (Tomer et al., 1995); topography, remote sensing, and negatively correlated with ECSH (r 0.97 to 0.99) and positively farmer experience (Fleming et al., 1999); electrical concorrelated with ECDP (r 0.79–0.97). Within-zone corn yield means ductivity sensors (Sudduth et al., 1997; Lund et al., 1999); showed no consistent relationship with ECSH but positive correlation and yield maps (Eliason et al., 1995; Stafford et al., with ECDP (r 0.81–0.97). Equal-range and unsupervised classification methods were compared for ECSH; within-zone yield variances de1999). These approaches to SSM have met with varying clined slightly (0–5%) with the unsupervised approach. Yield response degrees of success that are often highly soil or region curves relating maximum wheat yields and ECSH revealed a boundary specific. line of maximum yield that decreased with increasing ECSH. In this Because some factors affecting crop yields occur unsemiarid system, ECSH–based management zones can be used in SSM predictably, including weather, human error, and equipof wheat for: (i) soil sampling to assess residual nutrients and soil ment malfunction (operator error, plugged spray nozattributes affecting herbicide efficacy, (ii) yield goal determination, zles or planters, herbicide drift, weed pressure, poor and (iii) prescription maps for metering inputs. seed viability, etc.), the potential impact of SSM may be limited in some years. At best, it will optimize the interactions between soil and inputs of nutrients, seed, D uniform management across a field, withinor pesticides by targeting soil indices related to producfield variability in crop yields is a well-recognized tion potential that are measurable, relatively stable, and phenomenon. For this reason, whole-field management manageable. The productivity of a given soil is deteris increasingly viewed as inefficient because it results in mined by the cumulative effect of natural factors inthe overapplication of inputs in low-producing areas and volved in its formation, including climate, topography, suboptimal application in areas with high-production parent material, biological activity, and time (Jenny, potential. Site-specific management—the spatially di1941), and management history. Management history rected management of soils, crops, and pests based on can significantly affect the range and spatial heterogenevarying conditions within a field (Larson and Robert, ity of soil chemical properties beyond that attributable 1991)—provides an alternative to the use of the field to natural processes. This is particularly true in organic as a primary management unit. Increasing fertilizer and systems where input applications are typically less unipesticide costs, coupled with environmental concerns form than in conventional systems (Cambardella and stemming from their use, conceptually advance SSM as Karlen, 1999). a means to improve economic (Griffith, 1995; Reetz While variations in individual soil factors have limited and Fixen, 1995) and ecological outcomes in agriculture utility for SSM, their combined impact on water and (Wallace, 1994; Castelnuovo, 1995; Larson et al., 1997). nutrient use efficiency is highly relevant to both producThe implementation of SSM requires real-time and tion potential and environmental concerns, such as NO3 accurate global positioning system (GPS) equipment, leaching (Bouma and Finke, 1993) and soil acidification (Malhi et al., 1991). Fields can be mapped for multiple C.K. Johnson, B.J. Wienhold, J.F. Shanahan, and J.W. Doran, USDAARS, 120 Keim Hall, Lincoln, NE 68583-0934; D.A. Mortensen, Dep. of Crop and Soil Sci., Pennsylvania State Univ., 116 ASI Building, Abbreviations: ECa, apparent electrical conductivity; ECDP, deep apUniversity Park, PA 16802; Received 14 Nov. 2001. *Corresponding parent electrical conductivity; ECSH, shallow apparent electrical conauthor (cjohnso2@bigred.unl.edu). ductivity; GIS, geographic information system; SSM, site-specific management. Published in Agron. J. 95:303–315 (2003).