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March, 2007

  1. 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