Spatial Characteristics of Claypan Soil Properties in an Agricultural Field

Spatial variability in soil properties has long been observed within uniformly managed fields. Understanding the spatial characteristics of soil properties would be helpful in understanding soil–landscape relationships and in the development of site-specific management. The primary objective of this research was to quantify the spatial characteristics of claypan soil properties for a 4-ha agricultural field located in north-central Missouri. Soil samples were collected in 2002 at three depths (0–7.5, 7.5–15, and 15–30 cm) on a 30-m grid. Samples were analyzed for physical, chemical, and microbiological properties. Handheld and mobile apparent soil-profile electrical conductivity (ECa) values were also obtained. Spatial dependence was not identified for many properties. Yet, at a separation distance of |40 m, clay and silt content, cation exchange capacity, and Bray-1 P were spatially autocorrelated for the 15to 30-cm depth. Soil ECa showed a similar spatial autocorrelation. Spatial variations in most soil properties were better estimated by cross-semivariance analysis with ECa as a secondary variable than by simple semivariance analysis. Clay content was lowest and mostly homogeneous at the 0to 7.5-cm depth (mean 5 170 g kg, SD 5 2.0), and highest and most variable at the 15to 30-cm depth (mean5 410 g kg, SD5 15.8). Thus, the spatial characteristics of soil texture and related soil properties varied greatly by depth and landscape position, probably the result of an uneven distribution of topsoil caused by tillage-accelerated water erosion. We conclude that characterizing the variation in the depth of the claypan horizon is a helpful step in describing other properties for