Spatial variability of soybean quality data as a function of field topography: I. Spatial data analysis

Analysis and quantitative characterization of the spatial variability of soybean [Glycine max (L.) Merr.] protein and oil concentrations is an important task for site-specific soybean management. The objectives of this study were to spatially characterize the variability of soybean protein and oil concentrations in five fields of central Illinois, and to determine the influence of field topographical features, such as elevation range, terrain slope, and terrain curvature, on soybean protein and oil concentrations and their distributions within the studied fields. More than 200 samples for soybean quality analysis were collected from each of the three studied fields in fall of 1998, and from each of the two fields in fall of 1999. Dense elevation measurements were taken on each of the fields and topographical features, such as slope and curvature, were derived from the elevation data using the geographic information system (GIS). Both protein and oil concentrations were spatially correlated in all the studied fields as indicated by variograms. Field topography strongly affected soybean quality; however, its influence depended on the weather conditions during the growing seasons. Higher protein was observed at higher elevation sites, as well as at sites with higher slopes and convex curvatures during growing seasons with sufficient or excessive precipitation, while lower proteins were observed at such topographical conditions when the growing season was dry. Ranges of spatial cross-correlation between protein concentrations and elevation were related to the ranges and changes in shape of the elevation variograms, suggesting that spatial variability of field elevation can be used as an indicator of the spatial distribution of high protein or oil soybeans.