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Soils Exam 1 Notes

March 5, 2007 by siera104

To Prepare for Exam 1

What is soil?
Why is soil important?
Soil Horizons 
Soil Components 
Soil Interaction 
Soil Quality 
Soil Forming Factors

Soil Taxonomy 
Surface Horizon (Epipedon) diagnostics
Soil Moisture Regime 
Soil Temperature Regime 
Soil Order 
Soil Properties

What is soil?
Soil is the naturally occurring, unconsolidated or loose covering of broken rock particles and decaying organic matter (humus) on the surface of the earth, capable of supporting life.[1

Why is soil important?
Medium for Plant Growth
Supports roots, plant’s foundation
Aerates plant roots
Moisture for roots
Moderates root temperature
Environment is free from plant toxins (phytotoxins)
Provides most essential elements for growth (13/18)
Regulates Water Supplies
Most water travels through the soil
Soil absorbs and stores water
Water would evaporate much faster without soil
Recycle of Raw organic material
Habitat for Soil organisms
Used a Building and Engineering Medium
Soil Interacts with the other sphere, water air organisms (Pedosphere)

Soil Horizons
Master Horizons (O,A,E,B)
Subdivisions Lower letter after master letter – Ap
O– organic material added to surface,
Absent in grasslands
Present in Forests
A- top most layers dominated by mineral material
Darkened by accumulation of organic matter
Plow Layer (Ap)
Most plant roots found in the A horizon
Translocates organics
Coarse due to loss of fine to depth
E – a leeched layer (optional in soil profiles)
Eluvial horizon
Intensely weathered and leached
Linked to acidic conditions
No organic Matter
Light in Color
Common in forests
Generally harder minerals that are less soluble (like quartz)
B- very little organic matter
Parent material no longer discernable
Silicate clays, iron and aluminum oxides, gypsum, CaCO3, accumulated here
May be formed in place through weathering
C- unconsolidated material mainly from parent material
Least weathered part of profile
____________End of Soil__________________________
R- Regolith or bedrock
Little to no weathering
Parent material of soil
Underlying geology

Soil Components
Mineral (inorganic)
Little pieces of rock
Gravel 2 mm up to 64 mm
Sand 2 mm to .
Silt 0.0625 mm down to 0.004
Clay smaller than 0.002 mm
Small Clays = colloids
Kinds of clays -> very important
Soil Texture is determined by the proportion of sand silt and clay

soil texture

Organic Matter
1-6% of dry weight
Influences all soil properties and uses
Including living biomass (critters, roots)
Humus – well decomposed portion
Resistant to further microbial attack
Half of soil volume of pores
Pores filled with water or air
Some soils hold water well, moderately or poorly
Soil water is not pure = soil solution
Contains ions
Water determines pH of soil (usually between 5 – 8)
Soil can’t contain air when comply saturated with water
Depleted of O2 -> rich in CO2
Highly variable (temporally and spatially)
Greater CO2 than atmosphere

Soil Interaction
Plant Roots

Soil Quality
Ability to perform ecological functions
Water filtration
Plant medium
Physical chemical biological properties

Soil Formation
Mineral content
Physical Weathering Processes:
Temperature- exfoliation, thermal expansion, frost wedging of rocks
Abrasion from wind, water, and ice
Plants and Animals- from roots and burrows
Biogeochemical weathering processes
Increased temperatures speeds up reactions
Hydration – water molecules bind to minerals
Hydrolysis- water splits into H+ and OH- replaces cations in mineral structures
Carbonation- making carbonic acid
Oxidation and Reduction
Organic Content
Temperature- increase -> decomposition increases

Soil Forming Factors (5) – affects soil properties
Parent Material
Weathering of underlying rock
Either formed in place or transported by wind water or ice
Colluvial Debris- Poorly sorted -> transported by gravity
Alluvial deposits- Stream transportation -> floodplains, alluvial fans, deltas
Glacial deposits- transported by glaciers- very poorly sorted
Organic deposits- in bogs peat = remains of plants
Moss peat- sphagnum
Herbaceous peat- sedges and reeds
Woody Peat- tree and shrub remain
Sedentary peat – aquatic plants and animals
Increased rainfall->soil leeching, erosion
Little rainfall- wind transportation, cemented soils
Temperature- affects decomposition and weathering, speeds chemical rxns
Influences vegetation
Organic matter accumulation – thickness of O and A horizons
Organisms (ei, ants, worms, termites, gophers, fungi, microbes…) Micro and Macro
Steep- faster water velocities -> erosion / poorly developed profiles
Not so steep slopes- soil thickens and accumulates

Time increases soil profile develops and thickens
Soil Formation Processes (4)
Transformations- chemically or physically altered
Translocations- movement of material laterally or vertically
Additions- organic matter, salts
Losses- leeching, erosion, dissolution, decomposition, gas losses, soil respiration

Soil Taxonomy
Great groups
Series- classes of soil, similar to species
Polypedon (soil individual)- group of similar pedons
Pedon- smallest sampling unit which displays the full range of properties of a soil (1-10 m2)
Surface Horizon (Epipedon) diagnostics
Mollic – dark color, high organic matter, soft grasslands
Umbric- similar to mollic, lower in base (metal) saturation, indicator of higher rainfall
Ochre- lighter in color, thinner than mollic and umbric, may be hard when dry (possibly because CO3)
Melanic- black, high organic matter and rich in minerals developed from volcanic ash, light and fluffy.
Histic- 20-60 cm thick organic layer over mineral soil, typical of wetlands with peat

Soil Moisture Regimes
Aquic – saturated  most of the time
Udic- enough moisture to meet plant needs
Ustic- some pl;ants viable moisture, long dry periods
Aridic and torric- soil dry for more than half of the growing season, moist for <90 consecutive days
Xeric- seasonal cool moist winters, warm dry summers, Mediterranean climates, long droughts in summer

Soil Temperature Regimes
Pergelic- Temp < 0o C, permafrost or ice
Cryic and frigid- 0-8 o C, great plains of use, spring wheat dominate crop
Mesic- 8-15 o C, Midwest, and Great Plains, corn and winter wheat
Thermic- 15-22o C, coastal plains, cotton central CA valley
Hyperthermic- >22 o C, Southern CA, HI, citrus

Soil Orders

Argillic (P rich), Kandic (K rich), Natric (sodium rich) horizon
Exchangable Base Metals
Base Saturation of 35% or greater
Ochric or umbric epipedon
May also have petrocalic horizon or a fragipan or a durapan
Volcanic ejecta
Weakly weathered soil
Volcanic glass
Soils are too dry for most plants
Aridic moisture regime
Ochric or  anthropic epipedon
Soil dry for half of the growing season (90 consecutive days)
Litte or no profile development
Ochric epipedon
Permafrost soils within 100 cm of soil surface
Dominantly organic
Bogs, moors, peat, and mucks
No permafrost
Young soils
B horizon may be hard to see
Poor hoizonation
Humid to sub humid regions
No illuvial horizon but argillic, nitric, kandic, spodic, and oxic horizons are excluded
Dark soft grassland soils
Base rich due to organic matter
High turn-over of organic matter (fast decomposition of grasses)
Agrillic, calici or natric horizon
Tropical and subtropical
Oxic horizons, highly weathered
Nearly featureless soil without clearly marked horizons
Arbitrary horizon boundaries
Sandy – high quartz content (quartz is reistant to chemical and physical weathering)
Low bases – does not buffer acids
Heavily leeched E horizon
Mixture of organic matter and aluminum with or without iron eluvial horizon
Very old soil
Heavly weathered
Argillic horizon -High clay content, low bases, translocated to silicate clays
Base saturation less than 35%
High content of expanding clays
Deep wide cracks
When soil is moistened after long dry period the clays swell and “turn” the soil over time
Soil Properties
Used to classify and understand
Musell colors used
Hue- color
Chroma- saturation
Value- light to dark
Influenced by:
Organic matter- darken soils have high Organic Matter
Moisture- wet = darker dry = ligher
Influences O2 concentration
Oxidation of  Minerals like Fe Mg
Warm, dry soils: well oxidized – bright red or yellow
Wet soils: blue/ gray or glaying (indicator of wetland) low chroma due to low oxygen levels- reduction conditions
Impacts soil behavior and management
Soil components
Gravel 2 mm up to 64 mm
Sand 2 mm to .
Usually quartz
Low plant nutrients
Fast drainage
Droughty soil
Low compaction
Silt 0.0625 mm down to 0.004
Feels like flour
Quartz dominate
Can release plant nutrients depending on non-quartz minerals
Small pores
Clay smaller than 0.002 mm
Huge surface area
Large capacity to hold water
Partials don’t settle out of water
Shaped like flakes or plates
Tiny pore- poor for water and air movement
Minerals impact clay properties
Small Clays = colloids
Kinds of clays -> very important
Soil Texture is determined by the proportion of sand silt and clay (Soil texture triangle, only need to know % of 2 components)
Soil Structure
Arrangement of primary soil particles – peds or aggregates
Characterized by Shape, size, and distinctness (grade) of ped
Spherodial (granular) – characteristic of A horizons. Subject to wide and rapid changes, related to soil organic matter
Platelike (platy)- common in E horizons – but may occur at any level of profile. Often inherited from parent material of soil or caused by compaction
Blocklike common in B horizons particularly in humid regions. May occur in A
Angular blocky- less weathering
Subangular blocky- more weathering
Prismlike- found in B horizons, most commin in arid and sub arid regions
Prismatic and Columlar
Particle Density
Mass * Volume = Density g/ cm3
Just particles – not the pores
Essentaloly the same a specific gravity
Usally use the same as quartz, except for soils dominated by organic matter and have a drastically different density
Bulk Density Db
Mass * Unit of dry soil
Measurement includes pore space
Structure of soil is important
Meausre using intact core
Bulk Density impacted by texture- more pore space the lower the bulk density
Pore space increases – bulk density decreases
Depth in profile increases with depth
Used in determining land use
Ex- skid trails, compaction for construction, agruculural uses
Lower the bulk density- higher poristy
Generally about 50%
Compacted subsoils 25%
High organic matter 60%
Macro pores >0.08mm
Allows air and water movement
Interped pores- between peds
Bio pores
Micro pores <0.08mm
Water filled usually
Too small to permit movement
Consistence- resistance to mechanical stress
Attraction amount soil particles
Attraction between soil particles and water
Depends on moisture content
Engineering uses
Consistency – resistance to penetration by an object
Blunt end of a pencil test
Cohesive soils- clay content >15%
Strength decreases a lot with pores fill with water
Mudslide potential
Settlement and compression
-soils compacted prior to building
Proctor test – optimum moisture for compaction
Compression consolidation test tells how much volume will be lost under different stresses
Sandy soils resists compression
Clays and organic soils have compressibility
Expansive soil




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