Geostatistical Analysis of Soil Chemical Properties of Large Land Areas. I. Semi‐variograms

Abstract Soil properties are usually sampled on some grid or pattern which presumes to represent the unsampled neighborhood. Spatial dependence is a measure of the extent a soil sample represents the unsampled neighborhood. Geostatistics provides procedures to measure spatial dependence. Eighty pedons (Andepts) on the Island of Hawaii were sampled and spatial dependence was measured with semi‐variograms. Areas of soil with similar properties (zones of influence) were delineated by the “range” of the semi‐variogram. Areas of similarity were much greater for soil properties of the 0‐ to 15‐cm depth than of the 30‐ to 45‐cm depth. The ranges of the semi‐variograms for soil pH, Ca, Mg, K, Si, and P sorbed at 0.02 mg P/L were 32 to 42 km while the range of the variogram of rainfall had a similar value of 32 km. Semi‐variograms for Ca, Mg, K, and P based on 30 to 45‐cm‐depth samples demonstrated much greater variability and had smaller zones of influence (Ca, Mg, and K) or were extremely variable (P). Silicon in saturation extract, however, had the same zone of influence in the subsoil as in the topsoil. These results suggest that rainfall has imposed a degree of uniformity on the surface soil properties not apparent in the subsoil. Subsoil properties were highly variable, as might be expected, resulting from great variation over short distances in age and weathering of volcanic ash. The results suggest that soil chemical properties may be spatially dependent and that the spatial dependence in this example represents the imprint of soil‐forming processes and perhaps management. Zones of influence as delineated by semi‐variograms of soil properties may suggest groupings for soil management or soil classification.