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					Soil Aeration
Several processes affect the aeration status
of soil, beginning with the composition of
the gasses in the atmosphere
                Air              Soil Air

N2              78 %             78 %
O2              21 %             0 - 20 %
CO2             0.036 %          1 - 20 %*
*   Toxic when > 10 %
Differences …
 Notice the composition of the atmosphere
 and soil air are different, primarily in the
 amount of oxygen and carbon dioxide
 Nitrogen is essentially the same.
 Oxygen is used by organisms during
 respiration, releasing carbon dioxide
 The total amount of air in a soil is a
 function of soil porosity and water content
Review (Things you should know!)
    6 CO2 + 6 H2O Light C6H12O6 + 6 O2
    Organisms containing chlorophyll use carbon
    dioxide from the air and water from the soil in
    the presence of light to convert energy into
    Oxygen is released, usually above ground, with
    the exception of some soil algae at shallow
    depths, or in water.
Review (Things you should know!)
  Aerobic Respiration
   C6H12O6 + 6 O2       6 CO2 + 6 H2O + e-
    Photosynthesis in reverse
    Oxygen is consumed to metabolize
    carbohydrates in the cells of organisms,
    releasing carbon dioxide, water, and the energy
    required to sustain life.
    Oxygen must be supplied, CO2 must be
Review (Things you should know!)
  Anaerobic Respiration
    C6H12O6 6 3 CO2 + 3 CH4 + energy
    When oxygen is not available, some organisms
    are able to use other elements as terminal
    electron acceptors
    In order, the elements preferred as electron
    acceptors by organisms from aerobic to most
    anaerobic (fermentation) are
    Aerobic O>Fe>Mn> N> S>C Anaerobic
Review (Things you should know!)
  Anaerobic Respiration
    In the soil biology unit, organisms were
    classified as obligate aerobes, facultative, and
    obligate anaerobes.
    Obligate aerobes only function in the presence
    of oxygen
    Facultative organisms can use oxygen,
    nitrogen, or other elements
    Obligate anaerobes do not need O2
    Fermentation is an anaerobic process
Aeration is affected by
 Soil water content
   Dry soils: more air-filled pores, more O2
   Wet soils: more water-filled pores, thus less O2
   and poorer aeration
 Gaseous interchange
   Diffusion (dP/dx) – movement from areas of
   high concentration to areas of low
   Mass flow (breathing) – diurnal and seasonal
   movement due to temperature differences (hot
   air expands, cool air contracts)
Aeration is affected by (cont’d)
 Porosity: Total pore space, pore sizes, and
   Clay soils have the most total pore space, but
   often have poor aeration because air cannot
   quickly move through the small pores
   Sands typically have the best aeration, as the
   large pores allow water to move through and
   out of the pores. The water is replaced with air.
   Compacted soils have smaller pores, and
   generally have poorer aeration
Aeration is affected by (cont’d)
 Microbial activity
   Soil organisms use oxygen during
   decomposition of residues in the soil
   If the population of organisms is high, and the
   activity rate is also high, O2 may be used faster
   than it can be replaced through diffusion,
   especially if the soil is wet
   As the organisms respire, they release CO2,
   which accumulates, diminishing the aeration
Aeration is affected by (cont’d)
 Plant root density, number or mass of roots
 per volume of soil
   Plant roots need O2 to complete all necessary
   plant functions
   The availability of O2 to the plants is
   determined by the aeration status of the soil,
   and by the amount of the soil that is explored
   by the plant roots
   Plants with fibrous roots have high plant root
   densities, and are capable of reducing the
   aeration status when O2 is limited
Aeration is affected by (cont’d)
   The bulk density of most soil profiles increases
   with depth (due to more weight above,
   compressing the soil below)
   Higher bulk densities result in lower porosities,
   and often smaller pore sizes, both of which
   slow the rate of O2 supply
   Aeration typically decreases with soil depth,
   especially with the presence of shallow water
   Oxygen Diffusion Rate (ODR) is used to
   describe the resupply rate of O2 in the soil
Aeration is affected by (cont’d)
 Soil heterogeneity
   Soil structure creates different aeration
   conditions between aggregates than exists
   within individual aggregates
   Conditions change more rapidly between
   aggregates (on the outside) than within them
   Aggregates that exist near water tables will
   have lower internal O2 concentrations for
   longer time periods than the outside of that
   same aggregate.
   The condition may be reversed in normally dry
Aeration is affected by (cont’d)
 Soil heterogeneity examples
   Gleyed soil conditions below constant water
   table depths (usually values >5, chromas #2)
   Gleyed soil matrix with concentrations of
   oxidized Fe and Mn in horizons that are usually
   below the water table but have periods above it
   Oxidized soil matrix with depletions of Fe and
   Mn in horizons that spend most of their time
   above a water table but have periods below it
   Collectively, these last two conditions have
   been called mottling
Aeration is affected by (cont’d)
 Soil heterogeneity examples
   Vertically stratified soils affect water
   infiltration and hydraulic conductivity, which in
   turn affect the aeration status
   Vertically stratified soils are often (but not
   always) associated with alluvial depositions,
   and have soil layers with strongly contrasting
   particle sizes adjacent to one another, e.g., a
   clay over a fine sand, or a silt under a coarse
   Stratified soils can facilitate artificial water
   tables, thus decreasing aeration in a soil layer
Aeration is affected by (cont’d)
 Seasonal differences
   Fluctuating ground water tables result in
   different aeration status, resulting in periods of
   oxidation when the horizon is above the water
   table, and periods of reduction when the
   horizon is below the water table
   Fluctuating ground water tables are often the
   result of climates with distinct wet and dry
   Warm seasons can result in higher O2 usage by
   plants and microbes than cool seasons, thereby
   affecting aeration
 Oxygen Diffusion Rate (ODR)
   This measures the air movement in soil to
   establish the time required for O2 to be
 Reduction/Oxidation (Redox) Potential
   Tendency of a system to oxidize or reduce
   elements, measured with a platinum electrode
   Primary elements subject to redox reactions in
   the soil include: Fe, Mn, N, S, C
Redox Status Affects:
 Element and nutrient solubility
 Nutrient Availability
 Element and nutrient toxicities
 Soil color
   Mottling: Concentrations and Depletions
 Low redox potentials are associated with
 poor aeration (anaerobic, or reducing,
Poor Aeration
   Plant growth, especially root growth
   Nutrient uptake (absorption) – Active nutrient
   uptake requires energy, obtained by plant roots
   through respiration. When O2 is limited, the
   roots cannot respire to obtain the energy to
   move the nutrient across the cell membrane
   Water uptake – Most plants require O2 in the
   root zone. When O2 is limited, some plants wilt
   when water is standing all around them
 Increases toxicities
Aeration Management
 Soil structure may be manipulated to
 improve aeration status
   Aerifying soils opens large poors to facilitate
   oxygen diffusion into the topsoil
   Addition of organic matter may improve
   aggregation, improving aeration
   Addition of sand increases large pores to
   facilitate water and air movement
Aeration Management
 Soil structure may be manipulated to
 improve aeration status
   Gypsum added to sodic soils improves
   aggregation as Ca replaces Na on the soil
   colloids. Gypsum will not improve aeration in
   soils not affected by sodium!
   Tillage and special row/planting configurations
   may be used in soils with high clay content to
   improve aeration, e.g., wide beds and deep
Aeration Management
 Plant selection is important in soils affected
 by poor aeration
   As with microbes, plants have preferences for
   soil aeration status, and have mechanisms to
   cope with the unique environments
   Obligate Hydrophytes – plants that must live in
   water (or with saturation in the root zone)
   Facultative Hydrophytes – plants that can live
   in aerobic or anaerobic conditions
   Many hydrophytes have special cells in the
   stems that transport air (O2) to the roots
Wetland Soils
 Wetland soils are unique ecosystems that
 help purify natural waste products, filter
 nutrients from water, offer wildlife habitat,
 provide aesthetic value and other benefits
 Wetlands are classified (delineated) using
 three characteristics
   Vegetation: Presence of hydrophytes (obligate
   or facultative)
   Hydrology: Presence of a water table
   Soils: Gleyed conditions
Wetland Soils
 All three conditions must be present in the
 area for some duration each year for the
 area to be classified as a wetland.
 The USDA-NRCS provides lists of
 hydrophytic plants by region for use in
 delineating wetlands.
 The USDA-NRCS developed a list of
 Hydric Soils in the USA. Typically hydric
 soils occur in low positions on the
Wetland Soils
 The hydrology (presence of water) in a
 wetland can have two primary sources:
 overland flow (episaturation) or shallow
 groundwater tables (endosaturation)
 Playas in the Texas High Plains are
 classified as hydric soils due to the
 frequency and duration with which they are
 often inundated as they receive runoff from
 the local watershed during periods of above
 normal precipitation or intense storms
 Problems with wetland histic soils
Histic horizons develop when water and/or
cold temperatures limit decomposition of plant
These soils are sometimes drained to remove
free water from the soil profile so they can be
used for agricultural, urban, or industrial
This process reintroduces oxygen, resulting in
 Problems with wetland histic soils
Decomposition results in a reduction of soil
volume, or subsidence
Additionally, once organic horizons are dry,
they become flammable (CSI: Miami featured
this characteristic on two episodes in the
Organic materials are light, and are subject to
wind erosion