1. 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 (Plate 22; after Page 112).

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.

5. Review: surface area higher for smaller clasts
286 cm2
1,536 cm2

6. Mineral Type vs. Clast Size

7. Properties 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 4&5 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

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

15. Granular peds

16. Plate-like structure

17. Angular blocky peds

18. Prismatic structure (prisms roughly 3-5 cm across)

19. Columnar peds

20. 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 ( 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 than clayey or silty soils
The latter are organized into more porous granules (intraped micropores)

24. Well-sorted 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!

25. 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

27. 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

29. Soil Strength

30. Total Porosity
Particle density approximately 2.65 Mg/m3 for silicate-dominated minerals.
Total porosity (%) = [(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.

31. Packing pores (between primary soil particles)
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

32. 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)

33. 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

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

35. Organic matter stabilizes aggregates

36. 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.

37. Flocculation Factors Affecting Aggregate Formation and Stability
Physical-chemical Processes
Biological 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.

38. 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.

40. 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)

41. 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

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

45. PUDDLED
WELL-AGGREGATED

49. Some simple homework:
Read: