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SOIL SURVEY TECHNOLOGY OVERVIEW
Michael A. Wilson Research Soil Scientist USDA-NRCS National Soil Survey Center Lincoln, NE
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OBJECTIVES Useful tools for answering questions about soil properties
Soil Survey Office Labs Active C VNIR Portable X-ray Fluorescence Climate stations Isotic/Spodic Geophysical tools
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SSO Laboratories—Your lab
Field kits provide opportunity to collect limited data to answer specific questions Not a replacement for NSSC KSSL Lower accuracy/higher detection limits Less efficient Why use field kits? Check of certain properties as part of map unit evaluation or mapping Rapid turn around of data Prescreening pedons for SSL analysis
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Types of Analyses CaCO3 equivalent Gypsum content Active C
Particle size distribution - hydrometer Chemical properties pH EC Hach kit Base saturation Extractable cations Salts CaCO3 equivalent Gypsum content Active C Isotic / Spodic properties
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Field methods available to use are documented in this 2009 Manual
Field methods available to use are documented in this 2009 Manual
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Laboratory Safety Not a trivial matter Protect yourself and others
Use common sense OSHA citation
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Soil Survey Office Laboratory Safety Manual is available on-line
ftp://ftp-fc.sc.egov.usda.gov/NSSC/Lab_References/safety_guide.pdf
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MSDS Material Safety Data Sheet
Information regarding the proper procedures for handling, storing, and disposing of chemical substances Required to be read, kept, and stored in lab in organized manner ftp://ftp-fc.sc.egov.usda.gov/NSSC/Lab_References/chem_disposal.pdf
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Chemical Hygiene Plan Document you write for your lab
Policies, procedures, and responsibilities to protect employees from the health hazards of chemicals used OSHA’s Occupational Exposure to Hazardous Chemicals in Laboratories Standard details requirements of a CHP to protect persons from hazardous chemicals CHP has required elements
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Soil Survey Technologies to Discuss
Active C VNIR Portable X-ray Fluorescence Spodic Field Kit Climate Stations Geophysical tools
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Active Carbon Kit C. Stiles, et al., Validation Testing of a Portable Kit for Measuring an Active Soil Carbon Fraction. Soil Sci. Soc. Am. J. 75:2330–2340
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Active Carbon Analysis
Potassium Permanganate -- weak oxidant of organic C Active (labile) C is the biologically active fraction Soil Health/Soil Quality Indicator of active food web, microbial biomass, and nutrient cycling Has been shown to be related to tillage systems Early indicator of C degradation
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Active C:Total C Ratio Acitive C related to total C but not consistently
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Visible Near IfraRed Diffuse Reflectance Spectroscopy
Chemical bonds interact with Visible and IR energy Different bonds – different wavelengths Atomic Bond Energy Vibration Bending Rotation
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Visible Near IfraRed Spectroscopy
Amount of interaction proportional to quantity Measurement is spectrum Statistical prediction based on spectral library Soil minerals Si-O bonds Al-O bonds Bond energy varies with mineralogy Organic matter C-O bonds C-H bonds VNIR –Measures spectral signatures of materials defined by their reflectance as a function of wavelength. Signatures are due to electronic transitions of atoms and vibrational stretching and bending of structural groups of atoms that form molecules and crystals.
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Visible to Near Infrared Diffuse Reflectance Spectroscopy
VNIR-DRS Measures reflectance at a wide range of wavelengths nm Rapid: 100 readings per second Interaction with sample composition and matrix produces diffuse reflectance Spectral Radiometer Fiber Optic Light Source VNIR measurements are taken by illuminating the sample with a broad spectrum halogen light source. Interaction of the light with the sample composition and matrix produces diffuse reflectance. A fiber optic gathers light and pipes it to the spectral radiometer which measures the reflectance across a wide range of wavelengths. Many readings are gathered and averaged for a high signal to noise ratio. Diffuse Reflectance Soil
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Data Collection Portable VNIR spectrometers Spectra measurements
1 per MO Spectra measurements ~1 min/depth Predictive models developed at SSL Organic and inorganic C Other properties as appropriate
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Benefits of VNIR Little or no sample prep
Field analysis of moist soils may be possible Possible to collect spectra in the field Spectra can be used to predict multiple properties Useful for rapid collection of data on many samples Traditional lab methods more accurate for a limited number of samples
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Clay Content Measured Clay (%) Measured Clay (%) Calibration Test Data
Estimated Clay (%) Estimated Clay (%)
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Cation Exchange (NH4OAc)
Measured CEC (meq 100g-1) Measured CEC (meq 100g-1) Calibration Test Data Estimated CEC (meq 100g-1) Estimated CEC (meq 100g-1)
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Soil Organic Carbon (Leco)
PLSR R2 = 0.82 RMSE = 0.15 RPD = 2.4
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Portable X-ray Fluorescence Spectroscopy
Different Manufacturers Bruker Tracer IV-SD Oxford X-MET 5000 Olympus Innov-X Delta RXF Thermo Scientific Niton XL2 GOLDD QSX Quickshot Field portable X-ray fluorescence (FPXRF)
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XRF VNIR VNIR= Visible and near infared reflectance; 350 to 2500 nm or 3.5 X 10-7 to 2.5 X 10-6 m VNIR -Spectral signatures of materials are defined by their reflectance, or absorbance, as a function of wavelength. Under controlled conditions, the signatures are due to electronic transitions of atoms and vibrational stretching and bending of structural groups of atoms that form molecules and crystals. The fundamental vibrations of most soil materials can be found in the mid-infrared region, with overtones and combinations found in the near-infrared region. For example, the fundamental features related to various components of soil organic matter generally occur in the mid-to thermal-infrared range (2.5–25 Am), but their overtones (at one half, one third, one fourth etc. of the wavelength of the fundamental feature) occur in the near-infrared (0.7–2.5 Am) region. Soil minerals such as different clay types have very distinct spectral signatures in the short-wave infrared region because of strong absorption of the overtones of SO4 2, CO3 2 and OH and combinations of fundamental features of, for example, H2O and CO2 (Hunt, 1982; Clark, 1999). X-ray fluorescence (XRF) is used to detect and measure the concentration of elements in substances. Fluorescence - phenomena of absorbing incoming radiation and re-radiating it as lower-energy radiation.
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Energy source: X-ray tube or radioisotope
Key Point: Atoms fluoresce at specific energies when excited by X-rays. Energy source: X-ray tube or radioisotope X-ray fluorescence (XRF) Subsequently, electrons of higher energy orbitals fall into the lower orbitals. A fluorescent photon with a characteristic energy is released. An inner shell vacancy is created (by an incident X-ray photon or other phenomena) leaving an electron hole in the inner shell. An outer shell electron falls to fill the inner shell vacancy as the atom relaxes to the ground state. This process gives off photons with energy in the X-ray region of the electromagnetic spectrum equivalent to the energy difference between the two shells. X-ray fluorescence (XRF) is the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis and chemical analysis, Underlying physics Physics of X-ray fluorescence, in a schematic representation. When materials are exposed to short-wavelength X-rays or to gamma rays, ionisation of their component atoms may take place. Ionisation consists of the ejection of one or more electrons from the atom, and may take place if the atom is exposed to radiation with an energy greater than its ionisation potential. X-rays and gamma rays can be energetic enough to expel tightly held electrons from the inner orbitals of the atom. The removal of an electron in this way renders the electronic structure of the atom unstable, and electrons in higher orbitals "fall" into the lower orbital to fill the hole left behind. In falling, energy is released in the form of a photon, the energy of which is equal to the energy difference of the two orbitals involved. Thus, the material emits radiation, which has energy characteristic of the atoms present. The term fluorescence is applied to phenomena in which the absorption of radiation of a specific energy results in the re-emission of radiation of a different energy (generally lower). Characteristic radiation Each element has electronic orbitals of characteristic energy. Following removal of an inner electron by an energetic photon provided by a primary radiation source, an electron from an outer shell drops into its place. There are a limited number of ways in which this can happen, as shown in figure 1. The main transitions are given names: an L→K transition is traditionally called Kα, an M→K transition is called Kβ, an M→L transition is called Lα, and so on. Each of these transitions yields a fluorescent photon with a characteristic energy equal to the difference in energy of the initial and final orbital. The wavelength of this fluorescent radiation can be calculated from Planck's Law: The fluorescent radiation can be analysed either by sorting the energies of the photons (energy-dispersive analysis) or by separating the wavelengths of the radiation (wavelength-dispersive analysis). Once sorted, the intensity of each characteristic radiation is directly related to the amount of each element in the material. This is the basis of a powerful technique in analytical chemistry. Figure 2 shows the typical form of the sharp fluorescent spectral lines obtained in the wavelength-dispersive method (see Moseley's law). [edit] Dispersion In energy dispersive analysis, the fluorescent X-rays emitted by the material sample are directed into a solid-state detector which produces a "continuous" distribution of pulses, the voltages of which are proportional to the incoming photon energies. This signal is processed by a multichannel analyser (MCA) which produces an accumulating digital spectrum that can be processed to obtain analytical data. In wavelength dispersive analysis, the fluorescent X-rays emitted by the material sample are directed into a diffraction grating monochromator. The diffraction grating used is usually a single crystal. By varying the angle of incidence and take-off on the crystal, a single X-ray wavelength can be selected. The wavelength obtained is given by the Bragg Equation: where d is the spacing of atomic layers parallel to the crystal surface. In principle, the lightest element that can be analysed is beryllium (Z = 4), but due to instrumental limitations and low X-ray yields for the light elements, it is often difficult to quantify elements lighter than sodium (Z = 11), unless background corrections and very comprehensive inter-element corrections are made When exposed to x-rays, electrons are ejected from inner orbitals
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Facts of Portable XRF Excitation source is either:
X-ray tube with target of Ag (silver), Ta (tantalum), Au (gold), Rh (rhodium), W (Tungston) Radioisotope (e.g., Fe-55, Co-57, Cd-109) Analysis covers area of 1 cm2 to a depth of approximately 2mm Minimum set up and calibration required (Factory Calibrated with alloy chip) Proper training is essential--User must understand the dangers of x-rays and fundamentals of radiation protection While wavelength dispersive XRF has been the mainstay of laboratory instrumentation, energy dispersive XRF (EDXRF) is the technique of choice for field instrumentation primarily due to the ease of use and portability of EDXRF equipment. Thermo Niton has a high voltage Xray tube (Ag anode; 45 kV maximum); Same thing with Bruker; QuickShot= X-Ray Source: 40 kV mini W-Target X-Ray Tube There is no observable difference between x-rays and gamma rays. The fundamental difference between x-rays and gamma rays is their origin. X-rays originate from the acceleration of electrons while gamma rays originate from the radioactive decay of the atomic nuclei. .3 Gamma-ray sources (Reference) Stable atoms have nuclei with a specific combination of neutrons and protons. Any other combination results in unstable nuclei that will eventually disintegrate, releasing energy in the form of charged particles and gamma rays. Gamma rays originate from the disintegration of the nuclei of atoms due to radioactivity. Radioactivity: A property exhibited by certain elements, the atomic nuclei of which spontaneously disintegrate and gradually transmute the original element into stable isotopes of that element or into another element. The process causes the emission of energetic particles and gamma rays. Gamma-ray sources used in XRF analyzers have energies from 5 keV to 90 keV. Note: Emitted gamma rays do not have a continuous spectrum but instead have discreet energies determined by the isotope undergoing disintegration. Quickshot XRF energy dispersive systems have many components but there are a few major critical parts in the process. First, an x-ray source known as an x-ray tube (generally 50-60kV, W) emits an x-ray beam into the sample being analyzed. After this beam excites and displaces electrons the resulting energy that is characteristic to the element is emitted as a wavelength and collected by another major component, the detector. The type of detector tube varies in each model of Quickshot XRF and each has different benefits that suite a particular application need. The handheld analyzer system (QSX-HH) utilizes micro-components but the same basic principle applies. Proportional Counter Detector Tube: One of the earliest detection systems, this style does provide an excellent solution for coating thickness measurements when combined with the proper software. Because of its low cost it has been adapted in gold testing equipment (like the QSX-79T) in recent years, but while it provides reasonable results on gold, it is fairly limited compared to the other analyzers available for gold and precious metals analysis. Si-PIN Detection System: While previous detection systems that reached the resolutions that Si-PIN detectors reach required Liquid Nitrogen (LN2) to cool the detector, Si-PIN technology does not require LN2. The resolution allows an analyzer using Si-PIN to separate elemental peaks with greater definition and the instrument will offer greater accuracy over a broader range of elements. It is much more economical than the next detection system developed (SDD, see below) and Si-PIN provides excellent results for precious metals identification; so it is featured in the QSX-295T. Silicon Drift Detector (SDD): One of the newer options in the Quickshot XRF line-up of instruments, this technology was developed in the early 2000’s and provides the benefit of detecting lighter elements than Si-PIN systems. For most applications this is not important (and can not justify the cost increase) but the technology has its place in laboratory settings and will be featured in a new handheld analyzer from QSX. X-ray tubes have higher radiation output, no intrinsic lifetime limit, produce constant output over their lifetime, and do not have the disposal problems of radioactive sources but are just now appearing in FPXRF instruments.
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Advantages of Portable XRF
Rapid field analysis and ease of use. Portable and easily transported to job sites; can check on an airplane as baggage or carry-on Delivers qualitative and/or quantitative multi-element analysis; simultaneous determination Can be used on many different samples - solids, powders, liquids, etc. Little or no sample preparation required versatility of this instrument. In one application, the customer requires that a limited number of samples be tested with a high degree of accuracy. For this case, samples are finely ground and placed in sample cups for testing. Results are obtained with testing times of 20 to 30 seconds and a detection limit of 15 ppm. In the second application the customer requires that a very large number of samples be rapidly tested to profile the site and find “hot spots.” In this case, a lower level of accuracy is acceptable and the customer chooses to perform rapid, in-situ tests followed by limited laboratory confirmation.
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Very difficult to detect or quantify lighter elements e. g
Very difficult to detect or quantify lighter elements e.g., Si, Al, Li, Be, P, B, C Elements quantified by XRF Niton- Up to 30 elements (only); from Mg to U; varies by application Bruker Li-Ion battery; 6 hour operating time per battery . X-ray tube with Ag target; max voltage 40 kV Bruker Radiation safety It’s worth noting that Tracer contains zero radioactive material, which means much easier licensing requirements, safe transportation, no disposal restrictions1 and no need for a wipe test every six months. For extra security, the system also comes in a lockable case and is password protected. A sample sensor checks that the sample is correctly in place before X-rays are generated and a cover is supplied to minimize X-ray exposure when measuring small parts. DELTA Premium Combining a large area, high performance Silicon Drift Detector (SDD) and a 4W optimized x-ray tube, the DELTA Premium with the tantalum (Ta) or gold (Au) tube configuration is the ideal solution for ultra quick, analytically demanding applications with superior detection limits for the elements phosphorus (P) through uranium (U). The DELTA-50 Premium is a 4W, 50kV x-ray tube (Ta or Au); this configuration offers optimum sensitivity for Cd, Ag, and Sb as well as Ba and the rare earth elements (REE). The DELTA Premium with the rhodium (Rh) tube configuration is ideal for superior detection limits for light elements magnesium (Mg), aluminum (Al) and silicon (Si) as well as excellent detection limits for P through U. The DELTA Premium models for Exploration Soil Analysis are the DP-4000EX series and for Mining Analysis are the DP-6000 series. The DELTA-50 Premium model is referred to as DP-4050EX. Radiation safety tips for using XRF analyzers Do not allow anyone other than a certified XRF operator to operate the analyzer. Be aware of the direction that the x-rays travel. Avoid placing any part of your body (especially the eyes or hands) near the x-ray port to adjust the instrument during operation. Allow no one closer than 1 meter during operation of the analyzer. Never hold sample up to the analyzer by hand; hold the instrument up to sample Never defeat safety devices on analyzers such as sensors, switches, etc. Wear appropriate dosimetry as required by the regulatory authorities During transport to and from the field, store the instrument in a cool, dry location. The XRF operator is responsible for the security of the analyzer. Never leave the analyzer unattended when in use. When in use, the analyzer should be in the operator's possession at all times (i.e. either in direct sight or a secure area). The key should not be left in an unattended analyzer. Always store the instrument in a secure location when not in use; storage of the key in a separate location is recommended in order to avoid unauthorized usage.
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Limitation of Portable XRF
Comparison of Detection Limits for ICP and PXRD SSL InnovX Analyte Acid Digestion; ICP 3-beam; Ta/Au tube mg kg Al 31 5000 Ca 9 25 Fe 3 5 K 34 40 Mg 10000 Mn 1 4 Na 28 No detection P 29 600 As 0.0001 2 Cd 7 Cr 0.0062 8 Cu 0.0002 6 Hg Ni 0.0009 15 Pb Zn 0.0006 PXRF is viewed by EPA as a screening method
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Multiple applications in soils survey for element specific measurements
Geographic profiling of elemental constituents in profiles Target elemental concentration in calcic, gypsic, spodic, placic horizons Mapping/spatial variability of contaminants across landscape Assist in selection of sites for sample collection
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Spodic Field Kit Humic Fulvic Color—indicates translocation and accumulation of organic complexes in spodic horizons. (Holmgren and Holzhey, 1984). KOH extraction of Al – Comparable to acid oxalate extractable Al If KOH-Al >2%, P retention is generally 100% and likely andic soil properties. In Spodosols, a level of 0.7% KOH-Al has been found to indicate the presence of a spodic horizon if the ratio of Al in the spodic material to that in the E horizon is >2.
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Climate Stations and Data Loggers
Deb Harms Data loggers – Compact, battery-powered device to records measurements at set intervals over a period of time. Record temperature, relative humidity, differential pressure, light intensity, water level, soil moisture, rainfall, wind speed and direction.
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Climate Station Applications to Soil Survey
To define the line between soil climate regimes Supporting data to determine landscape and soil hydrology Determine soil temperature vs air temperature off-sets for the region Once the off-set is determined, other types of weather data can be used to estimate soil temperatures in other locations.
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Soil Climate Stations Measure soil temperature and soil moisture by volume Can include air temperature sensors and tipping-bucket rain gauge The stations are self contained and battery powered charged with a solar panel Store the data up to one year and then need to be manually downloaded. Possible to purchase cell phone system so the sites can be downloaded remotely. The sensors are smart sensors (SDI-12) and can be programmed to collect information throughout the day.
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Wells/Piezometers
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Geophysical Methods Jim Doolittle (PA) Wes Tuttle (NC)
Several states have equipment and operators Ground-Penetrating Radar (GPR) Electromagnetic Induction (EMI)
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Ground Penetrating Radar Principal Soil Survey Uses
Determine the presence, depth, and lateral extent of subsurface horizons, stratigraphic and lithologic layers. Improve interpretations by providing estimates of soil map unit composition. Characterize spatial and temporal variations in soil properties.
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Use of GPR for soil survey investigations is limited by the medium.
In the conterminous USA, only 22 % of the soils are considered well suited to GPR. About 36 % of the soils are considered poorly suited to GPR,
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GPR Application Characterization of Peatlands
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GPR Application-- Soil Depth Determinations
Rock fragments limit the effectiveness of conventional soil survey tools. GPR Application-- Soil Depth Determinations One of the most effective uses of GPR has been to chart bedrock depths and determine the taxonomic composition of soil map units based on soil-depth criteria.
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GPR Application Characterizing Subaqueous Soils
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Subaqueous Soils GPR can provide data on fresh water depths, subbottom topographies, and sediment types.
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GPR Application Preferential Flow Paths
The movement of water through soil is not uniform and is strongly influenced by soil layering. Discontinuities provide narrow flow paths that account for a significant proportion of the water and contaminants moved thru the soil.
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GPR Application Depth and Continuity of Water Restricting Layers
Cisne - Fine, smectitic, mesic Mollic Albaqualfs Darley - Fine, kaolinitic, thermic Typic Hapludults
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Electromagnetic Induction (EMI)
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The electrical conductivity of soils is influenced by:
the type and concentration of ions in solution. the amount and type of clays in the soil matrix. the volumetric water content. the temperature and phase of the soil water. THE APPARENT CONDUCTIVITY OF SOILS WILL INCREASE WITH INCREASING: Water Clay Soluble salt
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EMI Application High-Intensity Soil Surveys
Identification and delineation of small inclusions of dissimilar soils within generalized soil polygons.
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EMI Application Characterization of spatial soil variability
EMI can provide a level of resolution not practical with traditional soil survey tools and methods
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Identifying soil-hydrologic-landscape units
EMI Application Identifying soil-hydrologic-landscape units Menfro – Fine-silty, mixed, superactive, mesic Typic Hapludalfs
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EMI Application Spatiotemporal variations in ECa associated with changes in soil moisture.
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EMI Application Characterizing and mapping anthropogenic soils
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EMI Application Identifying the presence and extent of soil contaminants
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Management of saline soils
EMI Application Management of saline soils
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CONCLUSIONS Methods and equipment are available to assist in understanding properties of soils and landscapes Document your plan: why needed, type of data to be measured, your plans to use If you have a question, ask
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