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Soil Physics 2010
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.
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
9
2 2grv ws
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
Volume?Surface area?Projected area?Longest transect?Largest inscribed sphere?Smallest circumscribed sphere?Largest circle inscribed in projection?Smallest circle circumscribing projection?…?
What is the “equivalent sphere” in Stokes settling?
Measuring soil particle sizes: Sedimentation
90%
70%
40%
After Matthews, 1991
Settling velocity, % of sphere
Oblate(lentil)
Prolate(rice)
What are the effects of roughness & shape?
Measuring soil particle sizes: Laser diffraction
Small particle Large particle
Soil Physics 2010
Measuring soil particle sizes: Laser diffraction
Soil Physics 2010
Measuring soil particle sizes: Laser diffraction
Soil Physics 2010
Measuring soil particle sizes: Laser diffraction
FastNo calibrationWide size rangeGood repeatabilityExpensive equipment
Soil Physics 2010
Equivalent radius?
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 Silt Clay
Start with a continuum of sizes…
Soil Physics 2010
Particle size distributions
Break it in half * and bend it over…
Sand Silt
Clay
* about halfway on a log scale
Soil Physics 2010
Particle size distributions
…and you have the “texture triangle”
Quick and easy, but not really a
distribution
Sand
Silt
Clay
About size distributions
Soil Physics 2010
Calculated by: Mass?
Volume? Number?
Do we assume some mathematical model?
What use is this distribution?PedologyFert/ChemHydrology
Soil Physics 2010
Distributions?
?!?
Cumulative distributions
0.0
0.2
0.4
0.6
0.8
1.0
0 2 4 6 8
Cumul., 0.5
0.5
Cumul., 1.0
1
Soil Physics 2010
% depends on bin size
Soil Physics 2010
0
0.005
0.01
0.015
0.02
0.025
0.0
0.2
0.4
0.6
0.8
1.0
1 10 100 1000 10000
Series1
Series2
0.0001
0.001
0.01
0.0
0.2
0.4
0.6
0.8
1.0
1 10 100 1000 10000
Series1
Series2
0
0.001
0.002
0.003
0.004
0.005
0.0
0.2
0.4
0.6
0.8
1.0
1 10 100 1000 10000
Series1
Series2
0.000001
0.00001
0.0001
0.001
0.01
0.1
0.0
0.2
0.4
0.6
0.8
1.0
1 10 100 1000 10000
Series1
Series2
equal spacing in linear space≠
equal spacing in log space
Cumulative graph stays the same
Cumulative distributions
Variable bin size?!
Displaying particle size distributionsCumulative distribution must be
Continuous & Monotonic
Usually shown as:Log x-axis (size)
Linear y-axis (%)
?
Soil Physics 2010
SievingSt
okes
set
tling ?
10110010-110-210-310-4
More particle size distributions
Soil Physics 2010
GradualAbruptPoorly graded
Clay
Soil Physics 2010
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
Key things about clay
Small< 2 m
Secondary mineralplaty shape
Large surface area
Soil strength & structurePorosity & permeability
