Soils Notes

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
                                    Topsoil
                                    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
                                    Subsoil
                                    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
            Water
                        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)
            Air
                        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
            Atmosphere

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

Soil Formation
            Weathering
                        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
            Decomposition
                        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
           Climate
                        Precipitation:
                       Increased rainfall->soil leeching, erosion
                       Little rainfall- wind transportation, cemented soils
                        Temperature- affects decomposition and weathering, speeds chemical rxns
                        Paleoclimate
                        Influences vegetation
            Biota
                        Organic matter accumulation – thickness of O and A horizons
                        Organisms (ei, ants, worms, termites, gophers, fungi, microbes…) Micro and Macro
            Topography
                        Elevation
                        Slope
                                    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
            Order
                        Suborder
                                    Great groups
                                                Subgroups
                                                            Families
                                                                        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

            Alfisols
                        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
            Andisols
                        Volcanic ejecta
                        Weakly weathered soil
                        Volcanic glass
            Aridsols
                        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)
            Entisols
                        Litte or no profile development
                        Ochric epipedon
            Gelisols
                        Permafrost soils within 100 cm of soil surface
            Histosols
                        Dominantly organic
                        Bogs, moors, peat, and mucks
                        No permafrost
            Inceptisols
                        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
            Mollisols
                        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
            Oxisols
                        Tropical and subtropical
                        Oxic horizons, highly weathered
                        Nearly featureless soil without clearly marked horizons
                        Arbitrary horizon boundaries
            Spodosols
                        Acidic
                        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
            Ultisols
                        Very old soil
                        Heavly weathered
                        Argillic horizon -High clay content, low bases, translocated to silicate clays
                        Base saturation less than 35%
            Vertisols
                        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
Color
            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
Texture
            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
Porosity
            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