7: Soil Composition and Characterization

a technique for surface characterization in which absorption of x-ray photons results in photoelectron emission by $\varepsilon= h \nu - Φ$, where
$\varepsilon$ is the electron kinetic energy
$\nu$ is the frequency of the incident radiation
$Φ$ is the work function (binding energy)

a technique for surface characterization in which the electron photoemission, leaving an empty state, eventually is occupied by another electron falling from upper levels, which results in energy emission (fluorescence) in the form of x radiation, although of lower energy than the absorbed photon, corresponding to the difference between the lower level (typically 1s) and the upper level (e.g., 3p for Mn(II)) of the electronic transition

an instrument that measures the intensity of the diffracted beam as a function of the angle θ (or the total angle 2θ), presenting the XRD diffractogram

a plot of a unique pattern of intensities as a function of the incidence angle

a qualitative and, usually, semiquantitative technique for studying crystalline solids based on the concepts of Bragg's law; the lattice spacing distances can be determined

1. water molecules that are either part of the crystal structure (water of hydration) or "wetting" the solid, known as "water of adsorption," their removal being termed dehydration and desorption respectively
2. the removal of –OH groups (or OH- ions) in which two such groups react to give a –O- atom (or an O2- ion) liberating a water molecule, termed "dehydroxylation"

the surface zone, including the zone of accumulation of organic material (usually the A horizons); can be modified by management practices such as ploughing and addition of fertilizers

optical, electronic, scanning probe

physical methods, positive adsorption methods, negative adsorption methods

thermal analysis in which the sample mass is continuously weighted as it is heated at a constant rate, and the mass, or its time (or temperature, which is equivalent) derivative, is recorded as a function of temperature. As temperature rises, reactions leading to gas evolution (e.g., water elimination) result in mass losses characteristic of the component involved and its contents in the sample

Si acts with a +4 oxidation state (strictly, it is not an ion); also if the surrounding atoms are equal, the polyhedron must be regular due to symmetry

layers underlying the topsoil (usually B and C horizons), which cannot usually be modified except by drainage

the sum of all natural and thermally altered biologically derived organic material found in the soil, or on the soil surface, irrespective of its source or stage of decomposition and whether it is living or dead, but excluding the aboveground portion of living plants

the layers of soils, often clearly distinguishable by their color and morphology, developed in pedogenesis, briefly:
A horizon--top layer, high amt organic matter
B horizon--mid layer, mainly partially "weathered" material
C horizon--bot layer, relatively undisturbed material

new minerals resulting from chemical weathering of primary minerals
-phyllosilicates (clay minerals)
-oxides (e.g., gibbsite [Al], hematite [Fe])
-limes (e.g., calcite, dolomite)
-evaporites (e.g., halite [NaCl], gypsum)

$R_R=\frac{r_+}{r_-}$, the quotient between the cation and the anion radii, respectively

a continuous mass of rock, usually too strong to dig with hand tools; constitutes the basement for the soil structure

original rock minerals, formed under volcanic conditions, under high temperatures and pressures
-oxides (e.g., quartz)
-silicates (e.g., pyroxenes, olivines)
-silicoaluminates (e.g., feldspars, micas)

the amount adsorbed of a given chemical species (either molecular or ionic) on an initially free surface is determined and, in principle, from the knowledge of the area covered by that species, the total solid area can be computed, $A_{S,ad}=Q_{ad} N_A a_{ad}$

$A^{\text{cryst}}_S=\frac{A_p}{\rho V_p}$
-classically, using XRD and electron microscopy to determine the shapes and dimensions of typical particles, but this is only reasonable for single components (e.g., kaolinite) and difficult at best for whole soil
-more recently, AFM measurements have been employed, but roughness is not taken into account in the equation

the long-lived emission of light that sometimes follows the absorption of light by certain atoms and molecules

the continuous process causing soil formation and evolution, including geological and climatological transformations, biological action, etc

dominated by organic minerals in various stages of decomposition that have accumulated either under water or in very wet areas--often referred to as peat layers

dominated by organic matter that has accumulated on the surface of the soil, subdivided by degree of organic material decomposition; these horizons are not common and are mostly restricted to moist or cool environments (e.g., alpine areas, swamps, rainforests)

a powerful technique for investigating the structure of chemical compounds, in which an atomic nucleus is exposed to high-frequency radiation in a magnetic field (on the order of 1-20 T [tesla])
the frequency of a nucleus is measured relative to that of a standard compound (typically tetramethylsilane)
the resonant frequency is then given as the difference, or "chemical shift" (δ), between these two frequencies, expressed as parts per million

substances recognizable as known biochemical compounds, such as amino acids, carbohydrates, fats, organic acids, resins and waxes; these are usually under degredation conditions and are thus relatively short lived

a method of elemental analysis based on the production of radioisotopes of the element of interest (this is the activation step), which is done by neutron capture (hence the name)

a charged solid surface is placed in suspension in an electrolyte solution, where the ions of like charge are repelled (desorbed) from the surface, producing a region depleted of such ions; this effect is measured by placing the solution in two chambers separated by a dialysis membrane permeable to water and ions but not to the colloid

$A=\log{\frac{I_0}{I_T}}=\varepsilon bc$, where
$A$ is the absorbance at a given wavelength
$\varepsilon$ is the molar absorptivity at that wavelength
$b$ is the optical path in the sample
$c$ is the absorbent concentration

-identify soil constituents
-elucidate structural features of inorganic and organic components
-study the adsorption of ions and small molecules onto soil colloids through techniques such as attenuated total reflection IR spectroscopy

the splitting of a chemical bond with water, resulting in the addition of H and OH to the products

a collection of refractory, relatively high-molecular-weight, dark-colored substances formed from the debris of plants and other biological organisms by degredation and secondary synthesis reactions; these constitute the major fraction of SOM, up to 80%

spectroscopy in which an interference pattern from a Michelson-type interferometer is processed using Fourier transform analysis

a family of flow-driven separation techniques that provide a continuous separation of particles whose sizes range from about 1 nm to 50 μm
an external field applies over the particles a force perpendicular to the separation channel, forcing them against a wall; a nonuniform distribution results, due to the different particle sizes and properties, and thus they are separated by the laminar flow in the channel

oxidation (especially organic matter), crystal formation

dehydration
desorption
dehydroxylation
decomposition
phase change

thermal analysis in which a controlled rate heating is applied simultaneously to a sample and to a reference material, and the differences in temperature are recorded; exothermic is $\Delta T \uparrow$, endothermic is $\Delta T \downarrow$

similar to DTA, but here the measured quantity is the amount of energy (heat, q) required to keep the sample and the reference at the same temperature, and the difference between them, δq, is determined and plotted as a function of temperature

a soil layer below A and B horizons, which differ in general character and are not C horizons. They cannot be reliably described as buried soils but have a contrasting pedological organization to the overlying horizons

the maximum number of neighbors that can be accommodated around a given central atom (a cation in this case)--essentially due to the radius of the central cation

borderline 4 and 6; usually 6, but can replace Si in a tetrahedron, thus giving isomorphic substitution

Al3+, Fe3+, Mg2+, Ti4+, Fe2+, Mn2+

weathering in which the minerals in a rock undergo slow dissolution and reprecipitation reactions

the complexing and removal of metal ions, helping to drive hydrolysis reactions

-composed of consolidated or unconsolidated materials
-these materials are usually partially weathered and geological features are often evident
-can be dug by hand when moist

$2d \sin{\theta}=n \lambda$
where λ is the radiation wavelength, d the lattice spacing, θ the incidence angle and n an integer giving the order of interference

-concentration of clay, Fe or Al oxides or organic material
-different structure/consistency or stronger colors than horizons above or below

a type of scanning probe microscopy, in which a nanometer-sized tip is attached to a cantilever, which scans the surface, moved by piezoelectric actuators (not shown); a laser beam reflected by the cantilever is received by a position-sensitive detector, allowing the detection of deflection under surface-tip forces

one or more surface mineral horizons (A1, A2, A3) with some accumulation of organic materials (less than that of O horizons); usually darker than underlying (due to increased organic matter), but may be lighter colored (due to lower content of clay)

weathering of feldspar, where the reactants and products, respectively, are: orthoclase feldspar, carbonic acid, water -> dissolved kaolinite, dissolved silica, dissolved potassium ions, dissolved bicarbonate ions

x-ray absorption near-edge structure
OR near-edge x-ray absorption fine structure (NXAFS?)

extended x-ray absorption fine structure