SOIL ARCHITECTURAL
AND 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).
Factors affecting soil colour:
1. Organic content
- darkness and masking of oxidation effects
2. 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.
Soil Texture
Based on sand, silt and clay fraction (see earlier notes)
Effect of exposed surface area on other soil properties
1. Increases capacity to hold water
2. Nutrients and chemicals retained more effectively
3. 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.
Review: surface area higher for smaller clasts
384 cm2
1,536 cm2
Mineral Type vs. Clast Size
Properties of soils vs. clast size
Particle-size analyses in the laboratory
Pipette or hydrometer methods
1. 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)
a. Separating out the
sand fragments
b. Silt and clay
suspension
c. Weight of each
sand fragment is
determined
Hydrometer Method (Lab 2)
2. Place measured quantity of soil in a stirring cup and mix
with deionized water and a dispersing agent
[eg.(NaPO3)6]
3. Transfer to settling cylinder, add deionized water to a
measured level (eg. 1L) and record the temperature of the
suspension.
4. Insert plunger and mix by pulling plunger up with short
jerks. Record the start time with second accuracy.
5. Gently insert the hydrometer and record its reading after
a set time (eg. 40 seconds). Correct for temperature.
Repeat 4&5 three times or more to get a good average.
6. After 3 hrs (less in our case), take another reading with
the hydrometer.
7. Calculate % sand, silt, and clay, and determine the soil
textural class
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
p. 134-135
Granular peds
Plate-like structure
Angular blocky peds
Prismatic structure (prisms roughly 3-5 cm across)
Columnar peds
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)
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.
•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!
Well-graded
Uniform-graded
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
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
Total Porosity
Particle density approximately 2.65 Mg/m3 for silicatedominated 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.
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
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)
•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
Vertical distribution
of pore size in three
distinct soils
(a) Sandy loam
(b) Well-structured silt loam
(c) Poorly-structured silt loam
Organic matter stabilizes aggregates
Micropores <0.08 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.
Factors Affecting Aggregate Formation and
Stability
(i) Physical-chemical Processes
(ii) 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.
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.
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)
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
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
PUDDLED
WELL-AGGREGATED