Title: SOIL ARCHITECTURAL AND PHYSICAL PROPERTIES
1SOIL 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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3- 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.
4- 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.
5Review surface area higher for smaller clasts
384 cm2
1,536 cm2
6Mineral Type vs. Clast Size
7Properties of soils vs. clast size
8-
- 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)
9- Separating out the
- sand fragments
- Silt and clay
- suspension
- Weight of each
- sand fragment is
- determined
10- 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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12- 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
13p. 134-135
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15Granular peds
16Plate-like structure
17Angular blocky peds
18Prismatic structure (prisms roughly 3-5 cm across)
19Columnar peds
20Analysis 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)
21- 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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24- 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!
25Well-graded
Uniform-graded
26- 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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28Effect 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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30Soil Strength
31- 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.
32Pore 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
33- 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)
34- 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
35- Vertical distribution
- of pore size in three
- distinct soils
- Sandy loam
- Well-structured silt loam
- Poorly-structured silt loam
36Organic matter stabilizes aggregates
37- 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.
38- 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.
39- 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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41- 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)
42- 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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44- 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
45WELL-AGGREGATED
PUDDLED
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