While a large amount of soil data is now available to end-users, questions remain regarding the validity of that data. This is particularly important when soils data is used in modeling various production, land use, and resource management scenarios. However, rather than conducting exhaustive field sampling, the Soil Survey Staff is now adopting and using several new technologies which produce quantifiable (lab-quality) data, in-situ. Two technologies that have been receiving the most attention are visible near infrared diffuse reflectance spectroscopy (VisNIR DRS) and portable x-ray fluorescence (PXRF) spectrometry. VisNIR DRS is a technique whereby both visible and near infrared light (350-2,500 nm) are emitted from a high intensity light source such as a contact probe or mug lamp. Some of the light is then reflected off of the soil surface, with wavelengths and intensities precisely measured by the spectrometer. Collected spectra are then processed using chemometric software (e.g. R (R Development Core Team, 2004), The Unscrambler (CAMO Software, 2013)) for quantitative interpretation on a parameter of interest. Examples of commonly employed analysis techniques include partial least squares regression, boosted regression trees, stepwise multiple linear regression, penalized splines, as well as clustering strategies such as linear discriminant analysis, support vector machines, and random forests. Notably, both raw reflectance patterns as well as the first derivative of reflectance can provide useful spectral signatures. Applied to soils, VisNIR DRS has been used to measure soil organic carbon (Morgan et al., 2009; Sarkhot et al., 2011), clay mineralogy (Brown et al., 2006), soil clay content (Waiser et al., 2007), soil moisture content (Zhu et al., 2010), and levels of hydrocarbon contamination (Chakraborty et al., 2010; 2012a; 2012b). PXRF is another tool for rapid, in-situ soils analysis. However, it fills a decidedly different niche than VisNIR DRS. While VisNIR DRS is sensitive for carbon analysis and soil moisture, PXRF is used for total elemental analysis and improves in accuracy as atomic mass increases. Thus, PXRF has increased accuracy and lower detection limits for heavier elements such as As, Cd, Hg, Cr, and Pb, which are of special importance to environmental quality. Also, PXRF can quantify a range of plant essential elements such as Ca, Mg, S, Zn, Cu, Mo, Fe, and Cl. For this technique, x-rays are generated from a Ta/Au x-ray tube and strike the soil surface. X-rays effectively eject an inner shell electron from the elements it strikes. Outer shell electrons then cascade down to fill the inner shell, but in doing so, give up energy, termed fluorescence. The wavelength and intensity of the fluorescent radiation allow for the identification of the element and its concentration. Scanning is rapid (~60-90 sec) and unlike VisNIR DRS, the data is directly reported without a need for advanced spectral post-processing. In 2007, the US Environmental Protection Agency (US-EPA) sanctioned the use of PXRF for elemental analyses in soils and sediments via Method 6200 (USEPA, 2007).
|Effective start/end date||05/1/13 → 08/31/13|
- National Institute of Food and Agriculture