Soil Physics 2010

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# Soil Physics 2010 - PowerPoint PPT Presentation

Outline. Quiz 2 answers Finishing up particle sizes Particle size distributions Clay. Soil Physics 2010. Quiz 2 answers. Water rises higher in a capillary tube with a (larger / smaller ) interior diameter. Water is (more / less ) viscous when it is hot.

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## Soil Physics 2010

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Presentation Transcript

Outline

• Finishing up particle sizes
• Particle size distributions
• Clay

Soil Physics 2010

Water rises higher in a capillary tube with a (larger / smaller) interior diameter.

Water is (more / less) viscous when it is hot.

A mineral salt (e.g., NaCl) will dissolve more readily in a liquid with a (low / high) dielectric permittivity.

Soil Physics 2010

Measuring soil particle sizes: Sedimentation

Assumptions:

Particles are smooth spheres

Particles fall slowly (laminar flow)

All particles have the same density

Dilute: particles don’t affect each other

Fluid is otherwise at rest

Terminal velocity is reached instantly

Soil Physics 2010

Measuring soil particle sizes: Sedimentation

90%

70%

40%

After Matthews, 1991

What is the “equivalent sphere” in Stokes settling?

Settling velocity, % of sphere

Oblate

(lentil)

What are the effects of roughness & shape?

Volume?

Surface area?

Projected area?

Longest transect?

Largest inscribed sphere?

Smallest circumscribed sphere?

Largest circle inscribed in projection?

Smallest circle circumscribing projection?

…?

Prolate

(rice)

Soil Physics 2010

Measuring soil particle sizes: Laser diffraction

Small particle

Large particle

Soil Physics 2010

Measuring soil particle sizes: Laser diffraction

Fast

No calibration

Wide size range

Good repeatability

Expensive equipment

Soil Physics 2010

Measuring soil particle sizes: Image analysis

Volume?

Surface area?

Projected area?

Longest transect?

Feret diameter?

Sauter mean diameter?

Largest inscribed sphere?

Smallest circumscribed sphere?

Largest circle inscribed in projection?

Smallest circle circumscribing projection?

…?

Soil Physics 2010

Particle size distributions

Sand

Clay

Silt

Soil Physics 2010

Particle size distributions

Break it in half* and bend it over…

Clay

Sand

Silt

* about halfway on a log scale

Soil Physics 2010

Particle size distributions

Clay

…and you have the “texture triangle”

Quick and easy, but not really a distribution

Silt

Sand

Soil Physics 2010

Pedology

Fert/Chem

Hydrology

Calculated by:

Mass?

Volume?

Number?

Do we assume some mathematical model?

What use is this distribution?

Soil Physics 2010

Distributions?

?!?

Soil Physics 2010

Cumulative distributions

% depends on bin size

Soil Physics 2010

Cumulative distributions

Variable bin size?!

equal spacing in linear space

equal spacing in log space

Cumulative graph stays the same

Soil Physics 2010

Displaying particle size distributions

?

?

Cumulative distribution must be

Continuous & Monotonic

Sieving

Stokes settling

Usually shown as:

Log x-axis (size)

Linear y-axis (S%)

Soil Physics 2010

More particle size distributions

Abrupt

10-2

100

10-4

10-1

101

10-3

Soil Physics 2010

Clay

• Kaolin:
• “china clay” from Chinese Kao-ling, transliteration of the name of a mountain in Jiangxi, China (near which it was originally dug up), from Chinese gao “high” + ling “mountain, hill.”

Soil Physics 2010

Large surface area

Soil strength & structure

Porosity & permeability

Small

< 2 mm

Secondary mineral

platy shape

Soil Physics 2010