Utilization of the boundary line approach in the development of soil nutrient norms for soybean production

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Abstract
Boundary Unes, which previously have been employed for development of plant diagnostic norms for DRIS, were applied to soils for critical value Improvement. Soil test values and yields for soybeans (Glycine max L.) were collected for Alabama, Georgia, and South Carolina experiment station plots and farmers’ production fields. The sampling population was divided on the basis of surface soil texture and yield level in order to formulate soil critical value norms for both fine‐textured (clayey) and coarse‐textured (sandy) Ultisols in the southeastern United States. Boundary Une approach (BLA) norms for the fine‐ and coarse‐textured soils were similar to those norms employed by state‐operated soil testing laboratories for Piedmont and Coastal Plain soils, respectively. Two Coastal Plain soils having different surface soil textures, viz. clayey versus sandy, gave critical values by the BLA corresponding to the norms for fine‐ and coarse‐textured soils. Greater precision in the development of soil critical values requires greater mineralogical and textural delineation of soils, a process for which the BLA is well‐suited.