Soil Texture Characterization Using Apparent Electrical Conductivity through Kriging and Cokriging Analysis.

Understanding of soil texture spatial variability is important due to its influence on a large number of soil physical and chemical properties and to apply resources such as seeds, fertilizers and pesticides site specifically. Apparent electrical conductivity, which is a relatively low cost measurement, can be used as a secondary variable to estimate clay content at a fine spatial resolution. In this study we characterize soil clay content variability at the surface layer (0-20 cm) in a silty loam soil located in Princeton, Kentucky. Soil texture was measured at 96 locations with a distance of 50x50 m and was determined using the pipette method. Electrical Conductivity was measured using a contact sensor Veris 3150 at a shallow (≈30 cm) and deep (≈100 cm) depth. Data was analyzed using semivariograms, crossvariograms, ordinary kriging and cokriging (using electrical conductivity at a shallow depth). We developed scenarios using subsamples of 48, 24, and 12 data points per 27 ha from measured clay content. We use a cross validation to validate our data and to evaluate the different sampling scenarios based on the estimation of unused measured samples (48, 72 and 84 data points). By using cokriging we reduced the error (RMSE: 3.16) compared with ordinary kriging (RMSE: 4.08) for all 96 points. Cokriging RMSE error was in a range of 3-4 in all the scenarios for crossvalidation and for estimating unused sampling points, being constantly better than ordinary kriging. Our clay map using cokriging maintained a satisfactory precision even when we reduce our sampling points, a result that leads to the conclusion that electrical conductivity proves to represent and estimate the variation of clay content in the field, while the number of sampling points for soil texture or another related variable can be kept relatively low with 0.5 point per ha.

Key words: Soil texture, Electrical conductivity, Geostatistics, Soil Physics.