Measurement of pH, pH Buffering Capacity, and Other Soil Properties with NIR Reflectance Spectroscopy.
David Kissel1, Colin Christy2, S. Shaaban1, Paul Vendrell1, and Miguel Cabrera3. (1) Univ of Georgia, 2400 College Station Rd, Athens, GA 30602, (2) Veris Technologies, 601 N Broadway, Salina, KS 67401, (3) Univ of Geogia, Dept of Crop and Soil Sciences, 3111 Miller Plant Sciences Building, Athens, GA 30602
Soil properties such as pH and pH buffering capacity are highly spatially variable in crop production fields. Creating lime application maps is therefore cost prohibitive using traditional grid sampling. Measurement with an appropriate sensor would therefore be useful if it can be done with relatively high accuracy and low cost. Our objectives were to determine if Near Infrared Reflectance Spectroscopy (NIR) could be used to estimate soil pH and pH buffering with a laboratory grade instrument and to determine if accurate sensing in the field might be possible with a reduced wavelength instrument. Soil samples were taken from five fields in South Georgia. Sampling locations were selected visually to cover the range of soil organic C and clay concentrations. The soil pH was measured in 0.01 M CaCl2, and soil pH buffering capacity by titration. The soils were scanned from 400 to 2500 nm (Si and PbS detector) using ISI quarter cups and transport module in a computer controlled NIRS system, model 6500 scanning monochrometer (Foss-NIR Systems, Silver Spring, MD), with data collected every 2 nm and using the Win ISI II, version 1.5 software. NIR reflectance measurements, R, were transformed to log 1/R and subsequently first derivatives were calculated. When all data was used for the calibration of pH and pH buffering capacity, the resulting predicted pH and pH buffering capacities were highly related to the values measured in the laboratory (R2 = 0.97 for pH and 0.77 for pH buffering capacity). When two thirds of the data were used for calibration to predict the remaining one third, the regression of predicted vs lab measured values yielded R2 values the ranged from 0.91 to 0.95 for pH and from 0.53 to 0.69 for pH buffering capacity. These results are promising for use of NIR technology in the field.