Outline
• Announcements
• Where were we?
• Soil Structure (particles)
Soil Physics 2010
Announcements
• Reminder: Homework due Feb. 8
• Reminder: Exam Feb. 12
• Example exam is now posted.
Don’t
panic! I covered material in a different order that
year, and the class was not dual-listed at the 400level.
• Note: after homework is handed in, I re-post
the file with the answers
Soil Physics 2010
Where were we?
Soil strength
Soil structure
Soil Physics 2010
Specifically, most soil structure
exists because of cohesion
Soil structure
Everyone agrees that soil structure is important,
but no one knows how to define it or measure it.
Soil structure has defeated more soil scientists
than (probably) any other topic.
The state of the art in studying soil structure
has barely advanced in 50 years.
Soil Physics 2010
Classification of structure: Granular
Platy
Blocky
Crumb
Granular
Columnar
Prismatic
Angular
Subangular
…
Soil Physics 2010
Classification of structure: Blocky
Soil Physics 2010
Classification of structure: Platy
Soil Physics 2010
Classification of structure: Prismatic
Soil Physics 2010
Classification of structure: Columnar
Soil Physics 2010
What is soil structure?
• Structure: the arrangement of parts
• Not just physical locations:
• Relationship between a particle and
its neighborhood
Geometry
• Connections: bonds, glue, loadbearing links
Topology
• A blueprint is about more than the
location of each brick!
Soil Physics 2010
Same as in earlier lecture: what is
required to describe a porespace.
Why soil structure (particles)?
Figure & Ground
Particles & Pores
Soil Physics 2010
Figure and Ground
Dual networks
Voronoi & Delaunay
Dual networks
Triangular & honeycomb
Soil Physics 2010
Duals in 3D
Soil Physics 2010
Space between barley
grains. Grains were
continuous; the porespace
(dual) is also.
Structure implies not random
This might be a preferential arrangement
Based on chance, you
shouldn’t find lots of this:
Clay should hang out with
the other particles, too.
Why do clay quasicrystals
(and other non-random structures)
form?
Soil Physics 2010
Drivers of structure (particles)
Gravity: if it can’t stand, it will fall
Stability: if it’s not stable, it will soon change
Water, heat, roots: different ways
energy disturbs the soil, shaking it
into a more stable configuration
Climate, life, parent material
Soil Physics 2010
Hierarchical structure
Structures are built from smaller structures:
clay platelets → quasicrystals
quasicrystals → clay skins & bridges
…
microaggregates → crumbs
crumbs → aggregates
aggregates → peds
flocs, tactoids, cutans …
Soil Physics 2010
Fragmentation systems
N r   r
d f
This is characteristic of
fragmentation systems.
It implies that larger pieces
are easier to break than
smaller pieces.
Soil Physics 2010
log[N(r)]
N(r)
When an aggregate is dropped, there is
usually a power-law distribution of pieces:
slope = –df
r
log(r)
Causes / consequences of hierarchical structure
• Small structures tend to be denser
than large structures
• Small structures are more stable
than large structures
• Bonds within and between small
structures are stronger than bonds
within and between large structures
• Spaces (pores) between large
structures are bigger than those
between small structures
In soil, these structures are called aggregates
Soil Physics 2010
Bonds in soil structure
Chemical bonds:
covalent
hydrogen
Flocculation
Physical bonds:
Cementation
Van der Waals
Aggregation
surface energy
Cohesion / Adhesion
Biological:
hyphae
root exudates
worm casts
other yucky gooey stuff
Soil Physics 2010
Aggregate properties
• Size, shape, distribution
• Strength versus physical forces
• Strength versus chemical forces
Wet sieving
Soil Physics 2010
Fragmentation
Dry sieving
Rupture
Granular structure
Eventually, physicists try to treat everything as spheres…
Soil Physics 2010
Granular structure
… or something close, like M&Ms™
Soil Physics 2010
Sphere packing
Soil Physics 2010
Sphere packing
Soil Physics 2010
The main point of this sphere packing:
A (fairly) predictable pore size distribution
results from randomly packing particles of a
known size distribution
We call these “textural pores”
Structure generally produces pores that
would not occur by random packing
We call these “structural pores”
Structure emerges from the particles and
pores competing for stable arrangements
Soil Physics 2010