SOIL ARCHITECTURAL AND PHYSICAL PROPERTIES

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SOIL ARCHITECTURALAND PHYSICAL PROPERTIES
  Soil Colour Valuable clues to the nature of
soil properties and conditions.   Munsell Colour
Charts Hue (colour) Chroma (intensity) Value
(brightness) Value and chroma are assessed from
each hue page (p. 122).
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• Factors affecting soil colour
• Organic content
• - darkness and masking of oxidation effects
• Moisture level (darker when wet)
• 3. Presence and oxidation state of Fe and Mn
  oxides
• - Oxidized - iron oxides - red
• - Reduced - greys and blues when iron reduced
  (gley)
• Well-drained soils have more oxidized
  conditions.
• Calcite gives whitish colour in semi-arid
  regions.

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• Soil Texture
• Based on sand, silt and clay fraction (see
  earlier notes)
•  
• Effect of exposed surface area on other soil
  properties
• Increases capacity to hold water
• Nutrients and chemicals retained more effectively
• Release of nutrients from weatherable minerals
  faster
• 4. Electromagnetic charges. Increases stickiness
  and aggregation.
• Not a lot of clay/organics are required to
  impart these features
• Best soils are usually those with relatively
  equal proportions of the different soil texture
  classes.

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Review surface area higher for smaller clasts
384 cm2
1,536 cm2
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Mineral Type vs. Clast Size
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Properties of soils vs. clast size
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•  
• Particle-size analyses in the laboratory
• Pipette or hydrometer methods
• Treat soil (eg. with H2O2) to remove organic
  matter
• Pipette Method
• 2. Separate out the coarse fragments (gravel,
  coarse sand and fine sand). Silt and clay
  fragments are washed into a sedimentation
  cylinder.
• 3. Silt and clay suspension is stirred and
  allowed to settle
• 4. Clay fraction assessed using pipette at given
  depth determined by Stokes Law (d is particle
  diameter)
• V kd2
• t h/(d2k)

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• Separating out the
• sand fragments
• Silt and clay
• suspension
• Weight of each
• sand fragment is
• determined

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• Hydrometer Method (Lab 2)
• Place measured quantity of soil in a stirring cup
  and mix with deionized water and a dispersing
  agent eg.(NaPO3)6
• Transfer to settling cylinder, add deionized
  water to a
• measured level (eg. 1L) and record the
  temperature of the
• suspension.
• Insert plunger and mix by pulling plunger up with
  short jerks. Record the start time with second
  accuracy.
• Gently insert the hydrometer and record its
  reading after
• a set time (eg. 40 seconds). Correct for
  temperature.
• Repeat 45 three times or more to get a good
  average.
• After 3 hrs (less in our case), take another
  reading with the hydrometer.
• Calculate sand, silt, and clay, and determine
  the soil textural class

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• Structure of Mineral Soils
• - aggregates or peds
• - affects water movement, heat transfer, aeration
  and porosity
• affected by human action (logging, grazing,
  tillage, drainage, manuring, compaction and
  liming)
• 1. Spheroidal (granular or crumb)
• - most common in A Horizons
• 2. Plate-like
• - most common in E Horizons
• - due to compaction or inherited from parent
  material
• 3. Block-like
• - common in B Horizons of humid regions
• 4. Prism-like
• - common in B Horizons of arid and semi-arid
  regions

p. 134
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p. 134-135
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Granular peds
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Plate-like structure
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Angular blocky peds
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Prismatic structure (prisms roughly 3-5 cm across)
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Columnar peds
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Analysis of structure in the field 1. Type of
peds 2. Relative size of peds (fine, medium,
coarse) 3. Distinctness or development of peds
(weak, moderate, strong) Difficult to assess
when the soil is wet
Soil Particle Density Dp Mass per unit volume
of soil solids Measured in Mg/m3 Particle
density is not affected by pore space, because it
does not take them into account. Mineral soils
mainly in the 2.60 to 2.75 Mg/m3 range Up to 3.00
Mg/m3 if minerals very dense (eg. magnetite,
hornblende) Organic matter has a much lower
particle density (0.90-1.30 Mg/m3)
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• Soil Bulk Density
• Db Mass per unit volume of dry soil
• Soil corers used to obtain known volume without
  disturbance
• Soils are then dried and weighed
• Db includes both solids and pores
• Bulk density is affected by soil porosity
• Highly porous soils have a low bulk density
• Sandy soils have a higher bulk density (larger
  pores, but lower porosity overall)
• than silty or clayey soils.

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• Well-sorted (poorly-graded) soils generally have
  lower bulk density
• Well-graded soils generally have higher bulk
  density
• Tightly-packed soils have higher bulk density
• A typical, dry medium-textured soil weighs 1250
  Kg/m3 or 1.25 Mg/m3
• Careful with your pick-up truck!

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Well-graded
Uniform-graded
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• High bulk density indicates
• Poor environment for root growth
• Reduced aeration
• Reduced water infiltration and drainage
• Human Practices Increasing Bulk Density
• Vehicular traffic and frequent pedestrian traffic
• major impact on forest soils, which have low bulk
  density
• Tillage
• Loosens soil initially, but depletes organic
  matter, resulting in
• higher bulk density

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Effect of Soil Compaction on Root
Growth 1. Resistance to penetration (roots must
push the particles aside and enlarge the pore
to grow if pore is too small) Exacerbated by
dryness due to increased soil strength. 2. Poor
aeration 3. Slow movement of nutrients and
water 4. Build-up of toxic gases and root
exudates Roots penetrate moist sandy soils most
easily for a given bulk density
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Soil Strength
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• Total Porosity
• Particle density approximately 2.65 Mg/m3 for
  silicate-dominated minerals.
•  
• Total porosity () 100 - (Db/Dp) x 100
•  
• Porosity varies
•  
• 25 in compacted subsoils
• 60 or more in well-aggregated, undisturbed soils
  with high organic matter content
• 80 in organic soils (peat)
• Cultivation reduces pore space, organic matter
  content and granulation
• Cropping reduces macropore space.

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Pore Type and Shape   Packing pores (between
primary soil particles) Interped pores (shape
depends on ped/granules) Biopores (often long,
narrow and branched some are spherical)
PACKING PORES
BIOPORES
INTERPED PORES
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• Macropores vs. micropores
•  
• Macropores 0.08mm to 0.5cm
• Allow ready drainage of water and air movement.
• Penetrable by smallest roots and a multitude of
  organisms.
• Spaces between sand grains are macropores
• This is why sandy soils have low total porosity
  but rapid drainage (hydraulic conductivity)

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• Interped pores are macropores found between peds
  and granules.
• Biopores are macropores produced by roots,
  earthworms and other organisms
• Biopores are very important for root growth and
  infiltration in clayey soils.
• Vertical Pore-Size Distribution
• Macropores most prevalent near the surface
• Micropores usually dominate at depth
• Why?
• 1. Small aggregates are more stable than larger
  ones
• 2. More organic material near surface

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• Vertical distribution
• of pore size in three
• distinct soils
• Sandy loam
• Well-structured silt loam
• Poorly-structured silt loam

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Organic matter stabilizes aggregates
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• Micropores lt0.08 mm
• Too small to permit air movement
• Water movement slow (usually filled with water)
• A high porosity soil can still have slow gas and
  water movement if dominated by micropores.
• Water generally unavailable to plants (held too
  tightly)
• Reduces root growth and aerobic microbial
  activity
• Decomposition by bacteria very slow to near-zero
  in smallest pores.

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• Factors Affecting Aggregate Formation and
• Stability
• Physical-chemical Processes
• Biological Processes
• Physical-chemical Processes
• of Aggregation
• Flocculation
• clumps of clay develop, called floccules
• Two clay platelets come close
• together the cations of the layer
• between them are attracted to
• the negative charges on each
• platelet.

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• Clay floccules and charged organic colloids form
  bridges that bind to each other and to fine silt
• Clay domain platelets are stuck together due to
  Ca2, Fe2, Al3 and humus.
• This results in well-structured soils.
• Na has a weaker attraction to negative charges
  on clays, so clays repel one another and remain
  dispersed.
• This results in poorly-structured soils.

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• Shrinking and swelling
• Upon drying, water is removed from
• within the clays, so the clay domains
• move closer together
• Shrinkage results, with cracks along planes of
• weakness (therefore, peds form)

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• Biological Proceses affecting Aggregation
•  
• (1) Earthworms and termites (burrowing and
  moulding)
• Move soil, ingest it, and produce pellets or
  casts
• Plant roots also move soil particles
•  
• (2) Roots and fungal hyphae (stickiness)
• Exude sticky polysaccharides
• Soil particles and microaggregates bound into
  larger agglomerations called macroaggregates
• Mycorrhizae secrete a very gooey substance called
  glomalin
• N.B. Hyphae are tubular filaments making up the
  fungus
•  
• (3) Organic glues produced by microoganisms
• Bacteria also produce sticky polysaccharides in
  decomposed plant residues
• The glues resist dissolution by water

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• Effect of Tillage on Aggregation
• Short term
• Improvement in aggregation if done on moderately
• dry soil
• Breaks up large clods, loosening soil and
  increasing porosity
• Incorporates organic matter into the soil
• Long term
• Loss of aggregation
• Enhanced oxidation of organic material reduces
  aggregation
• Loss of macroporosity occurs if tillage is
  carried out in a wet soil (puddled)
• Effect less pronounced where Fe Al oxides
  plentiful

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WELL-AGGREGATED
PUDDLED
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