ESTIMATION OF SOIL PROPERTIES AND WHEAT YIELDS ON COMPLEX ERODED HILLS USING GEOSTATISTICS AND THEMATIC MAPPER IMAGES

Better assessment of large scale patterns in soil fertility, erosion, and crop productivity is needed to address environmental concerns in the steeply rolling dryland farming region of eastern Washington. Spatial variability of organic carbon, soil phosphorus, and wheat yields measured on several 655 m long transects in complex eroded hills of the Palouse region of eastern Washington was studied using classical statistics and geostatistics. In addition, Landsat Thematic Mapper images of bare soil at the study site were used to estimate organic matter content in blocks having an area of 28.5 m x 28.5 m. Classical statistical analysis showed moderate coefficients of variation in soil properties (25-50%) and wheat yields (30%). Spherical semivariograms of soil properties and wheat yields showed strong spatial dependence, with a range of influence on the order of from 70 m to 145 m. The semivariogram for Landsat estimated organic matter was almost identical to that for organic matter measured in the surface transects. The cross-semivariogram between measured organic matter and wheat yields also exhibited strong spatial dependence, with a range of influence of about 120 m. Organic matter, soil phosphorus, and wheat yield were interpolated at unsampled locations using block kriging and block cokriging techniques. Excellent agreement was obtained between kriged wheat yields and wheat yields cokriged from Thematic Mapper estimates of organic matter. Satisfactory agreement was obtained between kriged soil phosphorus and cokriged soil phosphorus based upon remotely sensed organic matter. Results of this study provided strong evidence for nonrandom spatial patterns in soil properties and crop yields. These spatial patterns are associated with changes in surface organic matter content across the landscape resulting from extensive erosion and exposure of subsoil. Geostatistical techniques provide a powerful tool for interpolating ground measurements of soil properties and crop yield over large areas in combination with estimates of organic matter obtained from Thematic Mapper images.