Soil

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Soil

• What is it?
• Mineral matter
• SiO2
• Feldspars
• micas
• Clays
• High surface area
• High CEC
• Oxides and hydroxides
• Al (gibbsite), Fe (goethite, hematite), Ti, Si
• Phosphate minerals
• Evaporites (gypsum, halite)
• Carbonates (calcite, siderite, etc.)

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Soil

• What is it? (cont.)
• Organic matter
• High CEC
• Source of organic acids
• Organisms
• Microfauna
• mesofauna
• Air
• Water
• Contains ions, organic matter, silica in solution
• Weathering
• Organic processes
• Rainfall/snowmelt wet deposition
• Dry deposition

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Soil Moisture

• The volume of air and water in pore spaces is
  complementary as one increase, the other
  decreases. In poorly drained soils, all pore
  space may be occupied by water.
• In freely drained soils, water lost from large
  cavities and larger pores is called gravitational
  water.
• About two or more days after raining or
  irrigation, all gravitational water will be
  removed, and the soil is said to be at field
  capacity.
• In this condition, water remains in finer pores,
  known as capillary water, held by capillary
  attraction to the surface of mineral particles.
• Under drought conditions, even capillary water
  can be removed such that plants can no longer
  remain turgid. This point is called the wilting
  point, and is dependent on the soil properties.
• The amount of water that exists between the field
  capacity and the wilting point is called the
  available water capacity.
• Water held in soil above the oven-dry temperature
  of 105 is known as hygroscopic water and is
  generally not available for plant use, as this
  water is virtually part of the soil matrix
  itself.

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Soil Moisture
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Soil Moisture Regimes

• Aquic moisture regime (Hydric)
• The aquic (L. aqua, water) moisture regime is a
  reducing regime in a soil that is virtually free
  of dissolved oxygen because it is saturated by
  water.
• Aridic and torric
• (L. aridus, dry, and L. torridus, hot and dry)
  moisture regimes
• occur in areas of arid climates.
• little or no leaching in this moisture regime,
  and soluble salts accumulate in the soils if
  there is a source.
• Udic moisture regime
• (L. udus, humid)
• common to the soils of humid climates that have
  well distributed rainfall
• have enough rain in summer so that the amount of
  stored moisture plus rainfall is approximately
  equal to, or exceeds, the amount of
  evapotranspiration
• or have adequate winter rains to recharge the
  soils and cool, foggy summers, as in coastal
  areas.
• Water moves downward through the soils at some
  time in normal years.

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Soil Moisture

• Ustic moisture regime
• ustic (L. ustus, burnt implying dryness)
• intermediate between the aridic regime and the
  udic regime
• Xeric moisture regime
• xeric (Gr. xeros, dry)
• Mediterranean climates

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Organic Matter

• 0.5 to 5 in mineral soils
• Amorphous in nature
• Decomposition of plant and animal residue
• Major influence on physical/chemical properties
  of soils

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Organic Matter

• Nonhumic substances
• Compounds belonging to the well known classes of
  biochemistry
• amino acids
• carbohydrates
• lipids
• Humic substances
• Complex organic compounds with high-molecular
  weight
• Multiple rings, multiple polar substitutes
  (Phenols, carboxylic acids, aliphatic hydroxides)
• Many sites to chelate ions like Ca2, Fe2, Al3,
  etc.
• Yellow, brown to black substances formed by
  secondary synthesis reactions

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Organic Matter

• Sources
• Litter
• macroorganic matter that lies of the soil surface
• important in nutrient cycling in forest and
  natural grasslands
• Light fraction
• plant residues in various stages of decomposition
• labile portion
• source of nutrients for plants
• affected by pH, temp, texture, and moisture
• Microbial biomass
• soil bugs
• agent for plant residue decomposition
• labile pool of nutrients
• Water soluble organics
• soil solution
• low molecular weight alphatic and aromatic acids
• important in plant nutrition
• Soil Enzymes
• soil contains wide variety

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Organic Matter

• Formation of Humic and Fulvic acids not well
  understood
• Most recent theory follows synthesis from
  polyphenols

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Soils and Organisms
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Soil Organisms
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Soil Organisms Microflora

• Nitrogen-fixing bacteria
• form symbiotic associations with the roots of
  legumes like clover and lupine, and trees such as
  alder and locust.
• The plant supplies simple carbon compounds to the
  bacteria, and the bacteria convert nitrogen (N2)
  from air into a form the plant host can use.
• When leaves or roots from the host plant
  decompose, soil nitrogen increases in the
  surrounding area.
• Nitrifying bacteria
• change ammonium (NH4) to nitrite (NO2-) then to
  nitrate (NO3-)
• Denitrifying bacteria
• convert nitrate to nitrogen (N2) or nitrous oxide
  (N2O) gas.
• Denitrifiers are anaerobic, meaning they are
  active where oxygen is absent, such as in
  saturated soils or inside soil aggregates.
• Actinomycetes
• bacteria that grow as hyphae like fungi
• responsible for the characteristically earthy
  smell of freshly turned, healthy soil.
• decompose a wide array of substrates, but are
  especially important in degrading recalcitrant
  (hard-to-decompose) compounds, such as chitin and
  cellulose,
• active at high pH levels.

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Soil Organisms Microflora

• saprophytic fungi
• convert dead organic material into fungal
  biomass, carbon dioxide (CO2), and small
  molecules, such as organic acids.
• generally use complex substrates, such as the
  cellulose and lignin, in wood,
• essential in decomposing the carbon ring
  structures in some pollutants.
• mycorrhizal fungi
• colonize plant roots.
• In exchange for carbon from the plant,
  mycorrhizal fungi help solubolize phosphorus and
  bring soil nutrients (phosphorus, nitrogen,
  micronutrients, and perhaps water) to the plant.

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Soil Organisms Microfauna

• Nematodes are non-segmented worms typically 1/500
  of an inch (50 µm) in diameter and 1/20 of an
  inch (1 mm) in length.
• important in mineralizing, or releasing,
  nutrients in plant-available forms.
• When nematodes eat bacteria or fungi, ammonium
  (NH4) is released because bacteria and fungi
  contain much more nitrogen than the nematodes
  require.
• Protozoa
• feed primarily on bacteria, but also eat other
  protozoa, soluble organic matter, and sometimes
  fungi.
• important role in mineralizing nutrients
• release the excess nitrogen in the form of
  ammonium (NH4).

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Soil Air

• Atmosphere penetrates into the soil through the
  pore space and fissures.
• Soil air differs from atmospheric air in that
• it is saturated with water vapor (near 100
  humidity), and
• carbon dioxide, a by-product of decomposition, is
  sometimes 5-10 times higher.
• When the pores are saturated with water, fresh
  oxygen can not diffuse into the soil, creating
  anaerobic conditions.
• Plant growth is inhibited, and chemical reduction
  may occur in soils (as opposed to oxidation).
• The by-products of the reduction of nitrates,
  manganese oxide, sulfate, and iron oxide cause
  fermentation that produces gases in the soil that
  are ultimately released to the atmosphere.
• Such gases include nitrogen (N2), nitrous oxide
  (NO2), hydrogen sulfide (H2S), carbon dioxide
  (CO2), carbon monoxide (CO), and methane (CH4).
• Disruption of natural soil processes, such as by
  deforestation and increased cultivation, releases
  carbon dioxide to the atmosphere.

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Soil Formation

• Initial state
• Relief (Topography)
• Parent material
• Influxes
• Climate
• Temperature
• Precipitation
• Controls weathering reaction and soil biota
• Organisms
• Vegetation
• Animals
• Bacteria and fungi
• Time

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Soil Systems

• Soil porosity
• Mediates chemical weathering moisture
  retention, gas exchange
• Controlled by both physical and chemical
  processes
• Physical transport
• Solid and dissolved matter is washed down through
  soils
• Capillary action and evaporation can lead to
  upward transport of dissolved material
• Frast action, root growth, tree falls, and
  burrowing can mix material in a soil profile
• Lateral porosity can be greater than vertical
• Chemical activities of many species (H2O, CO2,
  O2)can change tremendously on geologic and short
  term (minutes days) time scales

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Soil Chemical Processes

• Formation of soil acidity
• Oxidation of organic matter
• Creation of acetates, Biogeochemical processes
• Bacterial degradation
• Formation of fulvic and humic acids
• Nitrogen fixing
• Ion exchange
• Soils contain abundant leachable ions and
  molecules
• Reactions take place on thin films (double layer)
  and grain surfaces
• Clays and organic matter have high CEC

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Biogeochemical Processes

• detritusphere
• Association with plant litter
• Fungi decompose cellulose while taking oxygen in
  and respiring carbon dioxide.
• Inside anoxic corners of leaf structure, bacteria
  convert nitric oxides to nitrogen.
• driolosphere
• portion of soil volume influenced by secretions
  of earthworms 2-3 mm thickness - sites of
  enhanced C, N and P mineralization
• Allow rainwater to penetrate soil

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Biogeochemical Processes

• Porosphere
• between soil particles, aggregates, and roots
• aggregatusphere
• aggregates form when combination of microbial and
  plant mucigels, fungal hyphae and small rootlets
  bind multiple particles
• bacteria attach to sand grains by fine fibrillae
  extending from their cell walls and by
  extracellular mucilaginous polysaccharides
  (mucigels)
• more intensively colonized by organisms and are
  more active sites metabolically than soil as a
  whole. C, N, S and P, sugars of microbial origin
  gt than soil generally
• nitrates (NO3-), ammonia (NH4), carbon dioxide
  (CO2), nitric oxides
• Rhizosphere
• Adjacent to roots and rootlets
• Area of extremely high metabolic activity and
  chemical process

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Soil Composition
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Soil Color

• Soil colors attributed to soil attributes

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Soil Horizons

• Soils may be divided into several horizons
• Manifestation of pedogenetic process and parent
  material
• Controlled by water movement
• Interaction between meteoric inputs and
  vegetative canopy
• Water transports ions, radicals, gases, particles
• Contributes to weathering of parent material

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Soil Horizons

• There are 4 processes involved in horizon
  differentiation
• Additions to the soil
• water as precipitation, condensation, or runoff
• O2 and CO2 from the atmosphere
• N, Cl, and S from the atmosphere and
  precipitation
• organic matter from biotic activities
• material from sediments
• energy from the sun
• Losses from the soil
• water by evapotranspiration
• N by denitrification
• C as CO2 from oxidation of O.M.
• soil by erosion
• energy by radiation
• water and material in solution or suspension

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Soil Horizons

• Translocations
• clay, organic matter, iron oxides, and chemicals
  by water
• nutrients circulated by plants
• soluble salts in water
• soils by animals
• Transformations
• decomposition of organic matter
• reduced particle size by weathering
• mineral transformations (primary to secondary)
• clay and organic matter reactions

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Master Horizons 

• designated by capital letters, such as O, A, E,
  B, C, and R.
• O horizons
• They are dominated by organic material.
• Some O layers consist of undecomposed or
  partially decomposed litter, such as leaves,
  twigs, moss, and lichens, that has been
  decomposed on the surface they may be on the top
  of either mineral or organic soils.
• Other O layers, are organic materials that were
  deposited in saturated environments and have
  undergone decomposition.
• The mineral fraction of these layers is small and
  generally less than half the weight of the total
  mass.
• In the case of organic soils (peat, muck) they
  may compose the entire soil profile.
• Organic rich horizons which are formed by the
  translocation of organic matter within the
  mineral material are not designated as O
  horizons.

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Master Horizons

• A horizons
• Mineral horizons that formed at the surface or
  below an O layer,
• exhibit obliteration of all or much of the
  original rock or depositional structure (in the
  case of transported materials).
• show one or more of the following
• An accumulation of humified organic matter
  intimately mixed with the mineral fraction and
  not dominated by characteristic properties of the
  E or B horizons or,
• Properties resulting from cultivation, pasturing
  or other similar kinds of disturbance.

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Master Horizons

• E horizons
• Eluviation layer
• Mineral horizons in which the main feature is
  loss of silicate clay, iron, aluminum, or some
  combination of these, leaving a concentration of
  sand and silt particles and lighter colors.
• The horizons exhibit obliteration of all of much
  of the original rock structure.

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Master Horizons

• B horizons
• Illuviation layer
• Horizons in which the dominant feature(s) is one
  or more of the following
• An illuvial concentration of silicate clay, iron,
  aluminium, carbonates, gypsum, or humus
• A residual concentration of oxides or silicate
  clays, alone or mixed, that has formed by means
  other than solution and removal of carbonates or
  more soluble salts
• Coatings of oxides adequate to give darker,
  stronger, or redder colors than overlying and
  underlying horizons but without apparent
  illuviation of iron
• An alteration of material from its original
  condition that obliterates original rock
  structure, that form silicate clay, liberates
  oxides, or both, and that forms a granular,
  blocky, or prismatic structure

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Master Horizons

• C horizons
• Mineral horizons that are little altered by soil
  forming processes.
• They lack properties of O, A, E, or B horizons.
• The designation C is also used for saprolite,
  sediments, or bedrock not hard enough to qualify
  for R.
• The material designated as C may be like or
  unlike the material form the A, E, and B horizons
  are thought to have formed.

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Master Horizons

• R Horizon
• Consolidated bedrock (hard bedrock), such as
  granite, basalt, quarzite, sandstone, or
  limestone.
• Small cracks, partially or totally filled with
  soil material and occupied by roots, are
  frequently present in the R layers.  

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Soil Horizons

• Soils may be divided into several horizons based
  on composition and chemical process

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Soil Taxonomy

• There are 12 general soil orders
• Based on soil characteristics
• Loosely correlated with latitude (climate zones)

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• Profile 1 is a hydric soil and the wettest of
  the soils in this photo series. It shows two
  hydric soil characteristics, a thickened organic
  layer, explained below, and a gray matrix
  explained under the second profile. Profile 1
  occurs at the lowest point in the wetland and is
  inundated for extended periods of time. The
  ponded water fills the soil pores preventing air
  from entering the soil. A few days of soil
  saturation is usually sufficient for soil
  microbes to exhaust the supply of dissolved
  oxygen in the soil water. The lack of oxygen
  slaws the process of microbial decomposition
  causing partially decomposed organic matter to
  accumulate above the mineral layers of soil
  creating the thickened 0 and A layers apparent in
  this soil profile. The thick organic layer at the
  soil surface indicates a hydric soil.
• Profile 2, also a hydric soil, is somewhat
  higher in the landscape and although still
  subject to fluctuating water rabies and lengthy
  periods of saturation, is not inundated for long
  periods. Therefore, the soil shows the gray
  matrix color in the Band C layers, but not the
  thickened organic surface. Without oxygen,
  microbes must utilize iron compounds to obtain
  energy from organic matter. In the process, iron
  compounds are converted from insoluble to soluble
  and flushed out leaving the gray background
  color. As iron precipitates in the aerated zones,
  additional soluble iron migrates from anaerobic
  sites unitl they become so depleted of iron that
  the gray color predominates. The term matrix is
  used to describe conditions in the dominant
  volume of soil within a layer. A soil layer is
  considered to be anaerobic when the sail matrix
  is dominated by gray depletion color. Layers B
  and C show the gray matrix indicating that these
  layers are saturated for long periods. The dull
  gray matrix extending to the soil surface
  indicates a hydric soil.
• Profile 3 is also a hydrie soil and although
  saturated far shorter periods of time, still
  shows the strong gray matrix color all the way to
  the soil surface. Fluctuating water levels in the
  soil allow air to fill the larger pores as the
  water level lowers and then trap the air as the
  water level rises again. When the iron dissolved
  in the water encounters a zone of trapped air, it
  farms a strong, red-brown colored precipitate.
  The precipitated colors are known as
  concentrations or mottles and are evident here in
  the AB and B layers. The gray matrix with bright
  mottles extending to very near the soil surface
  indicates a hydric soil.

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• Profile 4 is significantly higher in the
  landscape and though it weakly displays wetness
  characteristics it is not a hydric soil. In both
  the A and B layers the matrix has medium colors
  associated with the original evenly distributed
  oxidized iron coatings on sand grains. Small
  areas of iron depletion and concentration exist,
  however the segregation process and attendant
  gray color is not dominant in any layer. This
  soil is probably saturated to the surface for
  only very short periods of time and is,
  therefore, not a hydric soil.
• Profile 5 is not a hydric soil and shows
  evidence of saturation only in the deeper layers
  of the soil. As stated previously, iron
  depletions and concentrations are evidence of
  iron segregation due to saturation. The AB layer
  is saturated and anaerobic so infrequently that
  gray depletions are almost nonexistent. The
  deeper B layer shows faint depletions indicating
  that the soil is saturated only at depth and only
  for relatively short periods. Since the sail is
  seldom saturated to the surface and only
  saturated at depth for short periods of time, it
  is not a hydric soil.
• Profile 6 occupying the highest landscape
  position in this series, shows essentially no
  evidence of saturation and is not a hydrie soil.
  In this soil.the vertical redistribution of iron
  into horizons is associated with water
  percolating down through the soil profile. As a
  result, iron is evenly distributed within each of
  the soil layers. Saturation is either absent or
  restricted to such brief periods that the soil
  dues not develop anaerobic conditions, thus
  preventing iron segregation and development of
  the gray matrix within the soil horizons. The
  absence of inundation also prevents the
  development of organic accumulations on the
  surface.