@article{BoeneckeLueckRuehlmannetal.2018,
  author    = {B{\"o}necke, Eric and L{\"u}ck, Erika and R{\"u}hlmann, J{\"o}rg and Gr{\"u}ndling, Ralf and Franko, Uwe},
  title     = {Determining the within-field yield variability from seasonally changing soil conditions},
  series = {Precision Agriculture},
  volume    = {19},
  journal   = {Precision Agriculture},
  number    = {4},
  publisher = {Springer},
  address   = {Dordrecht},
  issn      = {1385-2256},
  doi       = {10.1007/s11119-017-9556-z},
  pages     = {750 -- 769},
  year      = {2018},
  abstract  = {Crop yield variations are strongly influenced by the spatial and temporal availabilities of water and nitrogen in the soil during the crop growth season. To estimate the quantities and distributions of water and nitrogen within a given soil, process-oriented soil models have often been used. These models require detailed information about the soil characteristics and profile architecture (e.g., soil depth, clay content, bulk density, field capacity and wilting point), but high resolution information about these soil properties, both vertically and laterally, is difficult to obtain through conventional approaches. However, on-the-go electrical resistivity tomography (ERT) measurements of the soil and data inversion tools have recently improved the lateral resolutions of the vertically distributed measurable information. Using these techniques, nearly 19,000 virtual soil profiles with defined layer depths were successfully created for a 30 ha silty cropped soil over loamy and sandy substrates in Central Germany, which were used to initialise the CArbon and Nitrogen DYnamics (CANDY) model. The soil clay content was derived from the electrical resistivity (ER) and the collected soil samples using a simple linear regression approach (the mean R-2 of clay = 0.39). The additional required structural and hydrological properties were derived from pedotransfer functions. The modelling results, derived soil texture distributions and original ER data were compared with the spatial winter wheat yield distribution in a relatively dry year using regression and boundary line analysis. The yield variation was best explained by the simulated soil water content (R-2 = 0.18) during the grain filling and was additionally validated by the measured soil water content with a root mean square error (RMSE) of 7.5 Vol\%.},
  language  = {en}
}
@article{LueckRuehlmannKirchmann2011,
  author    = {L{\"u}ck, Erika and R{\"u}hlmann, J{\"o}rg and Kirchmann, Holger},
  title     = {Properties of soils from the Swedish long-term fertility experiments VI. Mapping soil electrical conductivity with different geophysical methods},
  series = {Acta agriculturae Scandinavica : Section B, Soil and plant science},
  volume    = {61},
  journal   = {Acta agriculturae Scandinavica : Section B, Soil and plant science},
  number    = {5},
  publisher = {Taylor \& Francis Group},
  address   = {Oslo},
  issn      = {0906-4710},
  doi       = {10.1080/09064710.2010.502124},
  pages     = {438 -- 447},
  year      = {2011},
  abstract  = {Swedish long-term soil fertility experiments were used to investigate the effect of texture and fertilization regime on soil electrical conductivity. In one geophysical approach, fields were mapped to characterize the horizontal variability in apparent electrical conductivity down to 1.5 m soil depth using an electromagnetic induction meter (EM38 device). The data obtained were geo-referenced by dGPS. The other approach consisted of measuring the vertical variability in electrical conductivity along transects using a multi-electrode apparatus for electrical resistivity tomography (GeoTom RES/IP device) down to 2 m depth. Geophysical field work was complemented by soil analyses. The results showed that despite 40 years of different fertilization regimes, treatments had no significant effects on the apparent electrical conductivity. Instead, the comparison of sites revealed high and low conductivity soils, with gradual differences explained by soil texture. A significant, linear relationship found between apparent electrical conductivity and soil clay content explained 80\% of the variability measured. In terms of soil depth, both low and high electrical conductivity values were measured. Abrupt changes in electrical conductivity within a field revealed the presence of 'deviating areas'. Higher values corresponded well with layers with a high clay content, while local inclusions of coarse-textured materials caused a high variability in conductivity in some fields. The geophysical methods tested provided useful information on the variability in soil texture at the experimental sites. The use of spatial EC variability as a co-variable in statistical analysis could be a complementary tool in the evaluation of experimental results.},
  language  = {en}
}
@article{LueckRuehlmann2013,
  author    = {L{\"u}ck, Erika and R{\"u}hlmann, J{\"o}rg},
  title     = {Resistivity mapping with GEOPHILUS ELECTRICUS - Information about lateral and vertical soil heterogeneity},
  series = {Geoderma : an international journal of soil science},
  volume    = {199},
  journal   = {Geoderma : an international journal of soil science},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {0016-7061},
  doi       = {10.1016/j.geoderma.2012.11.009},
  pages     = {2 -- 11},
  year      = {2013},
  abstract  = {GEOPHILUS ELECTRICUS (nickname GEOPHILUS) is a novel system for mapping the complex electrical bulk resistivity of soils. Rolling electrodes simultaneously measure amplitude and phase data at frequencies ranging from 1 mHz to 1 kHz. The sensor's design and technical specifications allow for measuring these parameters at five depths of up to ca. 1.5 m. Data inversion techniques can be employed to determine resistivity models instead of apparent values and to image soil layers and their geometry with depth. When used in combination with a global positioning system (GPS) and a suitable cross-country vehicle, it is possible to map about 100 ha/day (assuming 1 data point is recorded per second and the line spacing is 18 m). The applicability of the GEOPHILUS system has been demonstrated on several sites, where soils show variations in texture, stratification, and thus electrical characteristics. The data quality has been studied by comparison with 'static' electrodes, by repeated measurements, and by comparison with other mobile conductivity mapping devices (VERIS3100 and EM38). The high quality of the conductivity data produced by the GEOPHILUS system is evident and demonstrated by the overall consistency of the individual maps, and in the clear stratification also confirmed by independent data. The GEOPHILUS system measures complex values of electrical resistivity in terms of amplitude and phase. Whereas electrical conductivity data (amplitude) are well established in soil science, the interpretation of phase data is a topic of current research. Whether phase data are able to provide additional information depends on the site-specific settings. Here, we present examples, where phase data provide complementary information on man-made structures such as metal pipes and soil compaction.},
  language  = {en}
}