Ch 2: Soil Properties & Weathering

aggregate

within a soil horizon, one of the cohesive, discrete blocks or chunks of soil that make up the horizon and its internal structure. soil materials bound together weakly to strongly in a 3D unit

aggregate stability

ability of a soil aggregate (ped) to hold together during wetting, drying, or shaking. poorly aggregated samples tend to disintegrate in water & fall apart w/vibration

bioturbation

physical mixing or churning of soil by organisms

subangular (blocky) structure

faces of blocks are mixture of rounded & plane faces

bulk density

density of a soil sample overall; including the mass and volume of pore spaces, mineral matter and organic matter

chemical weathering

process by which the chemical composition, chem structure or state of matter of soil, minerals or rocks is altered. EX: hydrolysis, dissolution, redox, and some ion exchange reactions

chroma

saturation or intensity of a hue

clay or clay sized particles

<0.002mm in diameter; any mineral found within this textural class of particles and is not necessarily restricted to phyllosilicate minerals

columnar structure

elongated grains that grow across the layers in the direction roughly parallel to the build direction

consistence

the soils ability to resist deformations or ruptures and its degree of cohesion and adhesion

diagensis

weathering; generic, all-encompassing term to describe any chemical (or physical) alteration of a geologic material after deposition. ex: weathering by groundwater, heat and pressure deep within earth’s crust. can also include pedogenesis

differential heating

uneven distribution of heat across earth’s surface (equator)

differential weathering

difference in change of different kind of rocks exposed to the same environment

Freeze-Thaw

rocks that are porous allow water into cracks, it expands when frozen and then thaws exacerbating cracks in the rocks

granular structure

soil particles arranged in small, loosely packed, roughly spherical aggregates, resembling crumbs

hardpan

any subsurface soil horizon that has a high bulk density or a high degree of induration; may be natural or anthropogenic. ex: duripans, fragipans, gypsic horizons, petrocalcic horizons, plowpans etc

hue

color of a soil, measure of how red, yellow, grey, or green a soil is

hydrolysis

splitting of some substance or molecule by adding waterh

hydrometer

device that measures fluid density. used to determine particle size of a known mass of soil sample in the lab, based on settling time for sand & silt-sized particles in a column of fluid of known height, diameter & temperature

ion exchance

reversible process by which cations and anions are exchanged btwn solid and liquid phases

insolation

Inbound solar radiation; how much heat is received from sunlight by a given area of the Earth over a given time period

loam

a soil with proportionately equal parts of sand, silt and clay

Oxidation-Reduction

either the gain or loss of electrons in a specific element

particle density

density of the sold (mineral) matter of a soil ONLY

permeability

degree of interconnectedness of pore spaces and (to a lesser degree) the ease with which a fluid can migrate through soil pores

physical weathering

reduction in particle size & increase in surface area as a result in abrasion, transport, concussion, heating, cooling, pressure or other physical processes

plasticity

index of capacity of a soil to remain cohesive under shear and other types of deformational strain

porosity

total volume of empty space in a soil (or rock or sediment)pr

prismatic structure

longer natural breaks vertically rather than horizontally, usually found in B horizons

salt weathering

physical weathering most common in arid climates; involves the growth and expansion of salt crystals within cracks in rocks

sand

particles btwn 2mm and 0.05mm in diameter

shrink-swell

the process of volumetric change in groups of soil due to excess or lack of moisture

silt

particles btwn 0.05 and 0.002 mm in diameter

spall

to break apart in layers, from the outside in, ex: rock spalling during a fire or after prolonged differential insolation

stickiness

an indexed measure of how cohesive soil is to itself when wet; stickiness is largely imparted by clay minerals and greatly decomposed organic compounds (measure of soil consistence)

structure

a measure and description of the degree of aggregation of a soil sample (soil horizon); the macroscopic-scale “architecture” of a soil

texture

percentage of sand, silt, and clay in a given soil sample

value

lightness or darkness of a hue

volumetric

pertaining to volume

wet-dry cycle

act of adsorbing water and then desorbing water (the latter due to drainage or evaporation), often (w/clay minerals) accompanied by expansion and contraction