ARS | Publication request: Spectral Reflectance Estimates of Surface Soil Physical and Chemical Properties

Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: May 16, 2008
Publication Date: June 30, 2008
Citation: La, W., Sudduth, K.A., Chung, S., Kim, H. 2008. Spectral Reflectance Estimates of Surface Soil Physical and Chemical Properties. In: ASABE Annual International Meeting Technical Papers. American Society of Agricultural and Biological Engineers Annual International Meeting. June 29-July 2, 2008, Providence, Rhode Island. Paper No. 084242.

Interpretive Summary: Measuring the variation in soil properties within fields is an important component of precision agriculture. For many soil properties, it is difficult to obtain enough data to accurately characterize their spatial variation, due to the cost of traditional sampling and laboratory analysis. Sensors that can estimate soil properties without the need for sampling are a promising alternative. One technology that has received considerable attention in this regard is optical reflectance sensing in the visible and near infrared wavelength bands. Another technology for which prototype systems have been developed is rapid electrochemical analysis using ion-selective electrodes (ISE). In a previous study, we developed an ISE system and evaluated its performance using soil samples from Missouri and Illinois. Our goal in this study was to apply reflectance sensing to the same samples and evaluate the results in comparison to the ISE results. We found that optical reflectance sensing worked well to estimate a number of soil physical properties, but did not provide good results for chemical properties – soil pH, phosphorus (P) and potassium (K). However, when we combined reflectance sensing data with data from our prototype ISE system, we obtained very good estimates of P and K. This study shows that combining the outputs of multiple sensors, sometimes called “sensor fusion”, has potential for improving soil chemical property estimates, and should be investigated further. If proven, this combination approach has the potential to benefit producers by providing them with a rapid, accurate method of quantifying variation in soil chemical properties, as needed for fertility management in precision agriculture.

Technical Abstract: Optical diffuse reflectance sensing in visible and near-infrared wavelength ranges is one approach to rapidly quantify soil properties for site-specific management. The objectives of this study were (1) to determine the accuracy of the reflectance approach for estimating physical and chemical properties of selected Missouri and Illinois surface soils, and (2) to compare the accuracies of soil P and K estimates from reflectance sensing, a prototype ion-selective electrode (ISE) system, and a combination of both reflectance and ISE sensing. Diffuse reflectance spectra of air-dried, sieved samples were obtained in the laboratory. Calibrations relating spectra to soil properties determined by standard methods were developed using partial least squares (PLS) regression. Good estimates (R2 = 0.83 to 0.92) were obtained using spectral data for soil texture fractions, organic matter, and CEC. Estimates of pH, P, and K were not good (R2 < 0.7), and P and K estimates were considerably worse than achieved by ISE. Including both spectral and ISE information in P and K calibrations provided very good results (R2 = 0.93), which were a considerable improvement over ISE data alone. Further investigation of this combined approach is warranted.