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

Soils and the Environment

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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.

Soils and the Environment

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.

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Soil DevelopmentSoil 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. Wecall 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.

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Soil HorizonsMaterials in a Soil System:Vertical and horizontal movements create a soil profilemade up of distinct layers parallel to the surface, which are called soil horizons.

Organic top layer (O)Zone of leaching (A and E)

Zone of accumulation (B)

Weathered rock (C and R)

Soil

Rock

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Soil HorizonsO Mostly organic materials, decomposing leaves,

and twigs. Often dark brown color.

A Mineral and organic materials, light black tobrown. Leaching of clay, Fe and Ca.

E Light colored materials due to leaching of clay,Ca, Mg, and Fe to lower horizons. HorizonsA and E make up the Zone of Leaching.

B Enriched in clay, Fe oxides, Silica, carbonateand other material leached from above. This isthe Zone of Accumulation.

C Partially altered (weathered) parent material,which is either rock or loose sediment.

R Unweathered (unaltered) parent material = rock.

~3m

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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.

ChronosequencesRelative 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.

Soil Development

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Soil TaxonomyEntisols - soils with little or no morphological developmentVertisols - clayey soils with high shrink/swell capacityInceptisols - soils with weakly developed subsurface horizonsAridisols - CaCO3-containing soils of arid environments with

moderate to strong development Mollisols - grassland soils with high base statusAndisols - soils formed in volcanic ashSpodosols - acid soils with a subsurface accumulation of

metal-humus complexesAlfisols - soils with a subsurface zone of silicate clay

accumulation and >35% base saturationUltisols - soils with a subsurface zone of silicate clay

accumulation and <35% base saturationOxisols - intensely weathered soils, tropical and subtropicalHistosols - organic soils (peak, bog, muck)Gelisols - soils with permafrost within 2 m of the surface

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Soil TextureTexture = 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.

Mediumsandy 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.

Fineclays, silty clays, sandy clays, clay loams and silty clay loams with more than 35 % clay.

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Clay (%) Silt (%)

Sand (%)

100 % CLAY

100 % SILT100 % SAND

Clay

Sand Silt

Clay loam

LoamSandy loam Silt loam

See Figure 3.2in textbook

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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 sizesGravel, 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 diameterSilt

particles from 0.074 mm to 0.004 mm diameterClay

particles less than 0.004 mm diameter

Soil Classification

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Soil Classification

WELL SORTED WELL GRADED

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Unified Soil Classification SystemFI

NE-

GR

AIN

EDC

OA

RSE

-GR

AIN

ED

>50 % largerthan 0.074 mm

>50 % smallerthan 0.074 mm

Cla

ys

Silt

s

San

ds

G

rave

ls GW = well-graded gravelGP = poorly graded gravelGM = silty gravelGC = clayey gravelSW = well-graded sandSP = poorly graded sandSM = silty sandSC = clayey sandML = siltMH = micaceous siltOL = organic siltCL = silty clayCH = high plastic clayOH = organic clayPT = peat and muckMostly Organics

Clean(<5 % fines)

Dirty(>12 % fines)

Clean(<5 % fines)

Dirty(>12 % fines)

Non-plastic

Plastic

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.

Water in Soils

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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.

Water in Soils

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Adhesion and Cohesion

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Engineering Properties of SoilsPlasticity

• 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

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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

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Universal Soil Loss EquationA = RKLSCPA = long-term average annual soil loss for the siteR = long-term rainfall runoff erosion factorK = soil erodibility indexL = hillslope/length factorS = hillslope/gradient factorC = soil cover factorP = 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.

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Water in SoilsFOREST

Precipitation

Interception

Evapotranspiration

Soil

Rock

Water infiltrates andruns through soil

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Water in SoilsCLEARCUT

Precipitation

Little Interceptionand EvapotranspirationSoil compaction

Rock

Increased surface runoffand sediment; weaker soil

More sedimentin channel

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Water in SoilsFarming

Precipitation

Less Interceptionand EvapotranspirationSoil

Rock

Increased surface runoffand soil erosion fromfreshly plowed land

Increased sedimentin channel

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Water in SoilsURBANIZATION Precipitation

Soil

Rock

Large increase in runofffrom urban surfaces andstorm sewers

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Effect of Land Use on Sediment Yield,eastern U.S. Piedmont Region.

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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

• compressibility• erodibility• permeability• corrosion potential• shrink-swell potential• expansive soils• soil pollution• desertification• water table• soil plasticity index

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