Geographic Operating Range Evaluation of a NIR Soil Sensor

A previously developed prototype soil sensor which used near infrared (NIR) reflectance techniques was tested in the laboratory. The test set of soils included 30 Illinois soils and another 33 soils collected from across the continental U.S., all prepared at soil moisture tensions ranging from 1.5 MPa (wilting point) to 0.033 MPa (field capacity). As the geographic range represented by the included samples was increased, the predictions of organic carbon, cation exchange capacity, and soil moisture became less accurate. Calibrations obtained with soils from several states of the lower Corn Belt (Illinois, Missouri, Indiana, and Ohio) were only slightly less predictive of soil organic carbon than calibrations developed for the Illinois soils alone. Extension of the geographic range westward or northward from this area reduced the organic carbon prediction accuracy to an unacceptable level.