SOIL CHARACTERISTICS
SOIL FORMATION
Surficial deposits: wind blown (aeolian), water born (alluvial), weathering in
place
Soil pedogenesis: in place formation of soil horizons
Weathering of parent formation
Development of biological processes at ground surface
Carbonic acid from CO2 leads to dissolution of parent
Clay minerals from weathering of parent
Downward migration of material (dissolved or colloids) at the surface eluviation
Deposition (chemical or physical) in lower layer illuviation
PHYSICAL CHARACTERISTICS
Access: Pit: best; thin-walled tube sampler: good; Auger: O
Soil Horizons
Master Horizons and Layers
O Horizon: dominated by organic material
A Horizon (eluvial): Mineral horizon mixed with an accumulation of humic material;
or properties resulting from cultivation.
E Horizon: Mineral horizon in which the main feature is the loss of silicate clay, iron
aluminum, leaving a concentration of sand and silt particles of quartz or some
other resistant mineral.
B Horizon (illuvial): Materials leached from A Horizon deposited in B horizon. Clays
transported from A to B. Dominated by carbonates, gypsum, and/or silica.
Concentrations of sesquioxides; alterations that form silicate clay; formation
of granular, blocky, or prismatic structure. Formed below O, A, or E.
C Horizon: Parent material for A and B horizons.
R Horizon: Bedrock that is sufficiently coherent to make digging impractical.
Contact: Usually gradational, properties of overlying or underlying formation
superimposed on the other (AB), or distinct units of one horizon embedded
with in the other (A/B).
Fragipan: hard layer formed by the deposition of carbonate, silica, or iron oxides. Low
permeability, water ponding. May impeded contaminants if present.
Subordinate distinctions of horizons or layers. Depends on location. See Soil Survey.
SOIL TEXTURE CLASSES
Grain size
Ternary diagram with classes
Grain size determination
Sieve to measure coarser than silt
Hydrometer to measure finer than silt
Plot on Semi-log as Percent finer than vs grain size
Characterized by the diameter Dn, where n denotes the percent that
is smaller. D10: effective size of the soil
Manipulate data to mm or seive size Fraction finer than
1.0
100
0.5
85
0.1
50
0.05
20
0.001
10
Plot data on semi-log plot
Cu = D60/D10
coefficient of uniformity
Competely uniform materials: Iu=1; some sands Iu= 10; well
graded materials: Iu=100
Use particle size distribution to classify soil
Ternary diagram
Methods for determining grain size
see handout for details
Weigh total sample
Multiple sieves, weigh fractions
Take silt and finer fraction and use hydrometer.
Color
Color name: yellowish red…
Color notation: (Munsell)
Water state: Wet, moist, dry
Physical state: Broken, rubbed (between fingers if moist), crushed (dry)
Mottles: abundance, size, colors
Munsell color classification
Hue: red, yellow, green, blue, purple
Value: lightness
Chroma: strength
Individual charts of the same Hue (upper right),
Hues become progressively lighter from bottom to top (value increases),
Chroma increases in chroma from left to right.
Hue: give letter of base color along with number (0 to 10) indicating mix with
neighboring colors. R: red; Y: yellow;
Gley: bluish and greenish grey hues, typical of reduced iron, poor drainage
High values oxidizing, low reducing
low also indicative of organic material present
Color of iron oxides
Red and yellow colors in soil typically due to iron oxides and hydroxides.
General guidelines
7.5YR-2.5Y
Goethite
7.5R-5YR
Hematite
-FEOOH
FE2O3
5YR-7.5YR value>6 Lepidocrocite
5YR-7.5YR value<6
Ferrihydrite
Fe5HO8-4H20
Value and chroma decreases (color is darker) as grain size decreases.
Cementation may also decrease value and chroma
Structure
Soils commonly consist of discrete elements where the mineral or organic grains
are bonded into elements of of a typical shape and size, resulting in a structure. Some
soils, such as those consisting of dry clean sand, will be loose and structureless. Here the
structural element is the soil grains themselves and they are said to have a single-grained
structure. Other soils may be formed into large, tightly packed blocks, as is sometimes
the case with dried clay. Such soils are termed massive. Most soils lie somewhere
between these two extremes, where the soil grains are aggregated into quasi-stable small
blocks, termed “peds”. Flow of water or contaminants will occur preferentially along ped
boundaries.
STRUCTURE OF GRANULAR SOILS
Packing of grains: depends on distribution of grain sizes, coordination number
(how many grains are in contact)
Uniform (well sorted):
number (6),
cubic, greatest porosity (n=0.46), minimal coordination
tetrahedral, octahedral: least porosity (n=0.26), greatest coordination
number (12).
More than one grain size: depends on relative sizes and concentrations. Can get
high coordination numbers (30 or more) with porosities of 20 or less.
Natural materials: Angularity and elongation tend to make packing looser than
ideal based on spherical particles. Porosity generally between 25 and 50
percent.
STRUCTURE OF AGGREGATED SOILS
Describe structure by: Shape, Grade, Size
Grade: 0 structureless or massive; 1 or poorly defined peds; 2 moderate (well
formed peds but not distinct); 3 strong (durable peds, quite evident in place, will stand
displacement).
Shape: pl: platy; gr: granular; columnar; crumb; sbk: subangular blocky; pr:
prismatic; abk: angular blocky.
Size: See Munsell charts
MACROPORES
Cracks
1. Surface initiated, reversible cracks: Close after relatively slight
surficial wetting and have little influence on infiltration rates. Form
as a result of drying from the surface downward.
2. Surface initiated, irreversible cracks: remain open slightly after
rewetting. cracks from freeze-thaw action, perhaps wet/dry or other
processes (e.g. root growth).
3. Subsurface initiated, reversible cracks. Close after wetting, initiate in
subsurface. May not propagate through surface layer
4. Subsurface initiated, irreversible. Permanent cracks, joints
Insert wire to determine how deep crack penetrates
Roots: Size, depth, frequency, root diameter, distribution within ped (or
boundaries between peds).
Burrows: Ants, worms, rodents, etc. Describe size, depth of burrows.