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Use of Near-Infrared Reflectance for Soil pH and Buffer pH Measurement

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Paper number  031045,  2003 ASAE Annual Meeting . (doi: 10.13031/2013.15388)
Authors:   D. L. Stangeland, M. D. Montross, T. S. Stombaugh, S. A. Shearer
Keywords:   Near-infrared, soil property, spectroscopy, reflectance, pH, precision agriculture, NIR, spectral analysis

In Kentucky, soil pH and buffer pH are both used in amending soils to a desired pH level. The goal of this project was to develop a global near-infrared spectroscopy calibration for prediction of soil pH and buffer pH, among other soil properties within Kentucky. A NIRSystems 6500 near-infrared spectrometer was used to measure the soil pH, buffer pH, phosphorous, potassium, calcium, magnesium, and zinc. Over 4,000 soil samples from counties within Kentucky were available for analysis. The soil samples were dried and ground to pass through a 2 mm sieve. Reflectance data along with wet chemistry data was used to develop the calibration model. The majority of the samples were taken from Shelby County near Louisville, KY. Other samples were taken from Calloway, Hardin, and Nelson counties. The samples represented different major land resource area (MLRA) locations across Kentucky. Approximately 400 samples from Shelby County were used to develop a calibration model and another 40 samples were used to validate the calibration. Soil samples from each of the other counties were scanned and saved for further calibration development and analysis. Good predictive results were achieved for both pH and buffer pH with r2 = 0.81 and RPD = 2.3. The best results were achieved for Ca with r2 of 0.86 and RPD of 2.7. Moderate results were attained for both P and K with r2 = 0.5 and RPD = 1.3. Predictions for both Mg and Zn attained poor results with r2 = 0.39 and RPD = 1.3.

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