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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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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 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 Soil Physics 2010
Measuring soil particle sizes: Laser diffraction Soil Physics 2010
Measuring soil particle sizes: Laser diffraction Fast No calibration Wide size range Good repeatability Expensive equipment 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 Start with a continuum of sizes… 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
About size distributions 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 Gradual Abrupt Poorly graded 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
Key things about clay Large surface area Soil strength & structure Porosity & permeability Small < 2 mm Secondary mineral platy shape Soil Physics 2010
