Performing Department
School of Plant, Environmental, and Soil Sciences
Non Technical Summary
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).
Animal Health Component
50%
Research Effort Categories
Basic
40%
Applied
50%
Developmental
10%
Goals / Objectives
1. To assess the physical and chemical properties of soils as a validation of existing/historical data. 2. To document temporal shifts in soil properties in response to management practices and shifting land use patterns. 3. To utilize new technologies such as visible and near infrared diffuse reflectance spectroscopy, portable x-ray fluorescence, and others in quantifying soil parameters. 4. To produce high resolution soil maps of areas of interest using interpolation techniques, field/lab data, orthophotography, and georeferencing.
Project Methods
Labwork will consist of standard physicochemical soils analysis to include modified particle size analysis (sand, silt, clay) (Gee and Bauder, 1986), pH (Soil Survey Staff, 2004), salinity (Soil Survey Staff, 2004), organic carbon (Nelson and Summers, 1996), total C/N, (Nelson and Summers, 1996) and cation exchange capacity (Soil Survey Staff, 1954). Separately, select samples will be subjected to ICP analysis (Soltanpour et al., 1996) following semi-total digestion per EPA method 3050B (US-EPA, 1996) for verification of elemental concentrations and x-ray diffraction analysis (Wittig and Allardice, 1986) for clay mineralogy determination. Lab analyses will be used as auxiliary data for predication and validation data for PXRF and VisNIR DRS. It is noteworthy that PXRF elemental analyses occasionally depart from ICP elemental data. This is chiefly due to incomplete digestions associated with ICP sample preparation. Ideally HF would be used to facilitate total digestion, but given the extremely caustic nature of HF, other reagents (HNO3, H2SO4) are often used (US-EPA, 2001). Spatial variability in soils across Louisiana will be examined. Sampling density in each area will reflect variability in soil type initially indicated by SSURGO soils data and field scouting with deference to sampling strategies employed by Weindorf and Zhu (2010) and Pennock (2004). Examples of sampling techniques could include grid sampling, Latin hypercube, or transect sampling, as appropriate (e.g. Chu et al., 2010; Rover and Kaiser, 1999). For grid sampling, blocks will range in size from one to five acres, but will be consistently sized throughout the area of interest. Compositing of samples within each block will be used to combine subsampling points. Coordinates of sampling points will be recorded via handheld global positioning system (GPS) receivers. For some applications, soil cores of ~1m will be collected using a hydraulic probe. Cores will be split in half to expose surfaces unaffected by the probe. Cores can be subdivided with depth as appropriate to each study (e.g. based on morphological horizons or fixed depth increments). Spatial interpolation of soil properties will be conducted using ArcGIS software. Specifically, inverse distance weighting and various types of kriging interpolation will be conducted as these are common techniques applied to soils (e.g. Marchetti et al., 2012; Li et al., 2010). In areas with obvious soil variability, complete pedon descriptions will be made. Pedons will be excavated to a depth of 1-1.5 m and described in the field. Parameters of description will include horizonation with master horizons and appropriate subordinates, soil texture, color, moist consistence, structure, structural grade, redox features, rooting depth and percentage, and boundary characteristics (Schoeneberger et al., 2002). Landform, parent material, slope position, etc. will also be described (Schoeneberger et al., 2002). Field classification of soils will be initially set using Keys to Soil Taxonomy 11th Ed. (Soil Survey Staff, 2010) and confirmed based on the results of lab data.