SOIL ARCHITECTURALAND PHYSICAL PROPERTIES

 

Soil ColourValuable clues to the nature of soil properties and conditions. Munsell Colour ChartsHue (colour)Chroma (intensity)Value (brightness)

Value and chroma are assessed from each hue page (Plate 22; after Page 112).

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 TextureBased 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

286 cm2 1,536 cm2

Mineral Type vs. Clast Size

Properties of soils vs. clast size

 Particle-size analyses in the laboratoryPipette 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 settle4. Clay fraction assessed using pipette at given depth

determined by Stokes Law (d is particle diameter)

V= kd2

t = h/(d2k)

a. Separating out thesand fragments

b. Silt and claysuspension

c. Weight of eachsand fragment isdetermined

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 aftera 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

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 peds2. 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 solidsMeasured in Mg/m3

Particle density is not affected by pore space, because it does nottake them into account.

Mineral soils mainly in the 2.60 to 2.75 Mg/m3 rangeUp 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 than clayey or silty soils•The latter are organized into more porous granules (intraped micropores)

•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!

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

TillageLoosens soil initially, but depletes organic matter, resulting inhigher 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 aeration3. Slow movement of nutrients and water4. Build-up of toxic gases and root exudates

Roots penetrate moist sandy soils most easily for a given bulk density

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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.

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

INTERPED PORES

BIOPORES

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 macroporesThis 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 ones2. More organic material near surface

Vertical distributionof pore size in threedistinct 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 andStability

(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 glomalinN.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 moderatelydry 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

Some simple homework:

Read:http://www.physicalgeography.net/fundamentals/10v.html