Download Soils and the Environment

Soils and the Environment
Soil is one of the most important earth materials we
encounter each day, but the definition of soil is
difficult.
Soil Scientists (and most ordinary people):
• fine-grained, well-weathered earth material that is able to
support plant growth
• focus on the physical and chemical properties
Engineers:
• any earth material that can be removed without blasting
• focus on particle size and the amount of organic material
• engineering applications
GEOL g406 Environmental Geology
S. Hughes, 2003
Soils and the Environment
Environmental Geologists:
• must understand soil from many perspectives
• characteristics affect agriculture, engineering, hydrology,
natural hazards and other aspects of land use
• soil development and soil character is crucial to good land
use planning.
Read Table 3.1 (Soil Taxonomy)
• Understand the meaning of soil types, but do not memorize
all of them.
Read Table 3.2 (Unified Soil Classification)
• Learn the definition of each constituent that makes up soil.
GEOL g406 Environmental Geology
S. Hughes, 2003
Soil Development
Soil is an important part of the
geologic cycle and soil characteristics
are influenced by parent material,
climate, topography, weathering, and
the amount of time a particular soil
has had to develop. Unsurprisingly,
variations in climate, parent material,
type of weathering and amount of time
produce distinct soils that express
these variations.
As soil develops, weathering creates distinct layers in soil. We
call these layers soil horizons, and each soil horizon has
distinctive characteristics. Every soil has a soil profile, a list of
the horizons that describe a particular soil.
GEOL g406 Environmental Geology
S. Hughes, 2003
Soil Horizons
Materials in a Soil System:
Vertical and horizontal movements create a soil profile
made up of distinct layers parallel to the surface, which
are called soil horizons.
Organic top layer (O)
Zone of leaching (A and E)
Soil
Rock
GEOL g406 Environmental Geology
Zone of accumulation (B)
Weathered rock (C and R)
S. Hughes, 2003
Soil Horizons
O Mostly organic materials, decomposing leaves,
and twigs. Often dark brown color.
A Mineral and organic materials, light black to
brown. Leaching of clay, Fe and Ca.
~3m
E Light colored materials due to leaching of clay,
Ca, Mg, and Fe to lower horizons. Horizons
A and E make up the Zone of Leaching.
B Enriched in clay, Fe oxides, Silica, carbonate
and other material leached from above. This is
the Zone of Accumulation.
C Partially altered (weathered) parent material,
which is either rock or loose sediment.
R Unweathered (unaltered) parent material = rock.
S. Hughes, 2003
Soil Development
A soil’s profile depends on its age and its conditions of formation.
Soil profile is the primary criteria for soil classification.
Soils can be compared in terms of their relative development.
Weakly developed soil profiles are generally younger and may
have fewer horizons; well-developed soils are generally older and
have more horizons.
Chronosequences
Relative development of a series of soils allows their
arrangement in a soil chronosequence. A soil chronosequence
gives information about the history of the landscape. The relative
development of the soils in a chronosequence tells the
investigator about the climate and depositional history of the area.
GEOL g406 Environmental Geology
S. Hughes, 2003
Soil Taxonomy
Entisols - soils with little or no morphological development
Vertisols - clayey soils with high shrink/swell capacity
Inceptisols - soils with weakly developed subsurface horizons
Aridisols - CaCO3-containing soils of arid environments with
moderate to strong development
Mollisols - grassland soils with high base status
Andisols - soils formed in volcanic ash
Spodosols - acid soils with a subsurface accumulation of
metal-humus complexes
Alfisols - soils with a subsurface zone of silicate clay
accumulation and >35% base saturation
Ultisols - soils with a subsurface zone of silicate clay
accumulation and <35% base saturation
Oxisols - intensely weathered soils, tropical and subtropical
Histosols - organic soils (peak, bog, muck)
Gelisols - soils with permafrost within 2 m of the surface
S. Hughes, 2003
Soil Texture
Texture = relative proportion of sand, silt and clay.
Texture classes:
Coarse
sands, loamy sand and sandy loams with less than 18 %
clay, and more than 65 % sand.
Medium
sandy loams, loams, sandy clay loams, silt loams with less
than 35 % clay and less than 65 % sand; the sand fractions
may be as high as 82 % if a minimum of 18 % clay is
present.
Fine
clays, silty clays, sandy clays, clay loams and silty clay
loams with more than 35 % clay.
GEOL g406 Environmental Geology
S. Hughes, 2003
See Figure 3.2
in textbook
100 % CLAY
Clay
Clay (%)
Silt (%)
Clay loam
Sandy loam
Loam
Silt loam
Sand
Silt
100 % SAND
100 % SILT
Sand (%)
S. Hughes, 2003
Soil Classification
Soils are often referred to as being sandy or clayey, or
sometimes silty. Different countries use different standards to
define sand particle and silt particle sizes.
Particle sizes
Gravel, Cobbles, and Boulders
particles greater than 2 mm diameter
Coarse and medium sand
particles from 2 mm to 0.2 mm diameter
Fine and very fine sand
particles from 0.2 mm to 0.074 mm diameter
Silt
particles from 0.074 mm to 0.004 mm diameter
Clay
particles less than 0.004 mm diameter
S. Hughes, 2003
Soil Classification
WELL SORTED
GEOL g406 Environmental Geology
WELL GRADED
S. Hughes, 2003
Gravels
Sands
>50 % larger
than 0.074 mm
GW = well-graded gravel
Clean
(<5 % fines) GP = poorly graded gravel
Dirty
GM = silty gravel
(>12 % fines) GC = clayey gravel
SW = well-graded sand
Clean
(<5 % fines) SP = poorly graded sand
Dirty
SM = silty sand
(>12 % fines) SC = clayey sand
Silts
ML = silt
Non-plastic MH = micaceous silt
OL = organic silt
>50 % smaller
CL = silty clay
than 0.074 mm
CH = high plastic clay
Plastic
OH = organic clay
PT = peat and muck
Mostly Organics
Clays
FINE-GRAINED
COARSE-GRAINED
Unified Soil Classification System
Water in Soils
Types of water:
Water on Earth is known by different terms, depending
on where it is and where it came from.
• Meteoric water = water in circulation.
• Connate water = "fossil" water, often saline.
• Juvenile water = water from the interior of the earth.
• Surface water = water in rivers, lakes, oceans and so on.
• Subsurface water = groundwater, connate water, soil,
capillary water.
• Groundwater exists in the zone of saturation, and may be
fresh or saline.
GEOL g406 Environmental Geology
S. Hughes, 2003
S. Hughes, 2003
Water in Soils
Moisture Content of soil is calculated as follows:
W = weight, so that:
[(Wwet - Wdry)/Wdry] x 100 = H2O content (%)
Moisture content affects the engineering properties and
stability of soils. A soil that is stable in dry conditions may
become unable to support the structures built on it when
saturated with water.
Be sure to read the sections of your text describing the
engineering properties of soil.
GEOL g406 Environmental Geology
S. Hughes, 2003
Adhesion and Cohesion
GEOL g406 Environmental Geology
S. Hughes, 2003
Engineering Properties of Soils
Plasticity
• related to the water content
Liquid Limit (LL)
• water content above which the soil behaves like a liquid
Plastic Limit (PL)
• water content below which the soil is no longer plastic
Plasticity Index (PI),
PI = LL - PL
• range of water contents that make the soil behave as a
plastic material
Low PI (5 %): small change in water content, soil changes from
solid to liquid
High PI (> 35%): potential to expand and contract on wetting
and drying
S. Hughes, 2003
Engineering Properties of Soils
Expansive Soils
• high content of swelling clay (montmorillonite)
• soils swell when water is incorporated between clay plates
• shrinking occurs when soil is dried
• damage to building and road foundations
Study Table 3.3 in textbook to understand more about soil
descriptions and their significant properties.
Study the Universal Soil Loss Equation (erosion) :
A = RKLSCP
GEOL g406 Environmental Geology
S. Hughes, 2003
Universal Soil Loss Equation
A = RKLSCP
A = long-term average annual soil loss for the site
R = long-term rainfall runoff erosion factor
K = soil erodibility index
L = hillslope/length factor
S = hillslope/gradient factor
C = soil cover factor
P = erosion-control practice factor
Used to predict the impact of sediment loss on
local streams and other resources and to develop
management strategies for minimizing impact.
GEOL g406 Environmental Geology
S. Hughes, 2003
Water in Soils
FOREST
Precipitation
Evapotranspiration
Interception
Soil
Water infiltrates and
runs through soil
Rock
GEOL g406 Environmental Geology
S. Hughes, 2003
Water in Soils
CLEARCUT
Precipitation
Soil compaction
Little Interception
and Evapotranspiration
Increased surface runoff
and sediment; weaker soil
Rock
GEOL g406 Environmental Geology
More sediment
in channel
S. Hughes, 2003
Water in Soils
Farming
Precipitation
Less Interception
and Evapotranspiration
Soil
Increased surface runoff
and soil erosion from
freshly plowed land
Rock
GEOL g406 Environmental Geology
Increased sediment
in channel
S. Hughes, 2003
Water in Soils
URBANIZATION
Precipitation
Soil
Large increase in runoff
from urban surfaces and
storm sewers
Rock
GEOL g406 Environmental Geology
S. Hughes, 2003
Effect of Land Use on Sediment Yield,
eastern U.S. Piedmont Region.
GEOL g406 Environmental Geology
S. Hughes, 2003
Soils and the Environment
Key Terms to Review:
• weathering
• soil horizons
• soil profile development
• soil chronosequence
• soil fertility
• unified soil classification
• soil strength
• soil sensitivity
• liquefaction
GEOL g406 Environmental Geology
• compressibility
• erodibility
• permeability
• corrosion potential
• shrink-swell potential
• expansive soils
• soil pollution
• desertification
• water table
• soil plasticity index
S. Hughes, 2003