Soil Physics 2010

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• Review session?
• Homework 5 due Wednesday, March 24
• No office hours this afternoon
• Exam II on Friday, April 2
• Remember I’ll be gone most of next week
Soil Physics 2010

Where were we?
Pore structure is about Transport, so it can be useful to examine other transportation networks.
Soil Physics 2010

Soil Structure (Pores)
Characteristics:
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Hierarchy of pore sizes
•
Hierarchy of attachment
•
Hierarchy of coordination
•
Preferential orientation
•
No empty regions
Soil Physics 2010

Soil Structure (Pores)
Characteristics:
•
Hierarchy of pore sizes: range of sizes. We think of pore structure as being mainly about the big pores
•
Hierarchy of attachment: big pores attach preferentially to other big pores
•
Hierarchy of coordination: big pores tend to have more connections than small pores
•
Preferential orientation: for this soil, mostly vertical
& horizontal
•
No empty regions: every part is connected
Soil Physics 2010

Why one structure versus another?
What does the structure do?
What structure(s) would work?
What wouldn’t?
Let’s take a teleological approach…
Soil Physics 2010

Teleology * of soil structure
Required:
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Ability to store lots of water, but also
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Ability to shed excess water
•
Water has access to all points (plants)
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Combination of large & small pores
Soil Physics 2010
* Teleology: a philosophy that explains a form or phenomenon by its purpose, not by how it occurs.

Constraints on a soil
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Mineralogy & particle size distribution
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Weather & climate
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Vegetation, other biological conditions
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Topography
•
Time
← soil must persist in time
(These are Hans Jenny’s 5 soil-forming factors)
Soil Physics 2010

Constraints on a soil (2)
For a given set of constraints – say, an Iowa-like climate:
Fast drainage:
Stream erosion.
No storage:
No plants
Balance of storage and transmission
Soil Physics 2010
Lots of runoff,
Lots of erosion:
No soil or plants
Porosity
Lots of storage, but runoff if storage is exceeded

Soil Structure thermodynamics
Soil structure develops (pores and particles both) over time.
How? Don’t the laws of thermodynamics require that the soil lose structure?
No. That applies to a closed system. Soil is an open system: energy constantly moves through it.
Some of that energy goes to building structure.
Heat (up & down)
Water (up & down)
Evaporation, condensation, freezing, etc.
Biological processes
Soil Physics 2010

But soil already has large & small pores, so what’s special about structure?
A currently fashionable theory,
Constructal Theory , explains structure in terms of how it evolves
(though it looks quite teleological) .
“For a finite-size flow system to persist in time, its configuration must evolve such that it provides easier and easier access to its currents.” (Bejan, 1996)
Soil Physics 2010

Constructal Theory (1)
Evolutionary tendencies of a flow system:
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Greater access
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Greater conductivity
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Freedom to morph
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Svelteness
Soil Physics 2010

Svelteness?
The ratio of area or volume served to the length of its transport network
Example: area of catchment / length of its streams
This gives mean distance to a stream
A system can’t be all big channels, or there wouldn’t be space for anything else.
A svelte system doesn’t have more or bigger channels than it needs.
Soil Physics 2010

Constructal Theory (2)
All flow systems are imperfect, so:
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Optimize how imperfections are distributed
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Usually this means that the highest resistance elements are at the smallest scale
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Hence the prevalence of tree-like structures in natural systems
Soil Physics 2010

How exactly does structure form?
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Agglomeration (clumping, aggregation)
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Removal (erosion, channeling, piping)
+
-
Soil Physics 2010

How exactly does structure form?
•
Agglomeration (clumping, aggregation)
•
Removal (erosion, channeling, piping)
High potential
Low potential
Low potential
Clay particles in the flow may be sucked onto walls, where they stick
Soil Physics 2010

Why does structure persist?
Or, how is a soil with structure better able to resist the forces acting on it, than a soil without structure?
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Higher K s
•
Better air escape during infiltration
•
Better aeration
→
→
→
Less runoff
Less runoff, better soil water recharge
More roots
Soil Physics 2010

Air entry = structure?
Air entry value:
The smallest pressure required to push air through a saturated soil
By the capillary equation, this corresponds to the biggest continuous pore system
Interestingly, the air entry value is a good predictor of K s
Soil Physics 2010

Our understanding of soil structure
Richard Feynman, the late Nobel Laureate in physics, was once asked by a Caltech faculty member to explain why spin one-half particles obey
Fermi Dirac statistics.
Rising to the challenge, he said, “I’ll prepare a freshman lecture on it.”
But a few days later he told the faculty member,
“You know, I couldn’t do it. I couldn't reduce it to the freshman level. That means we really don’t understand it.”
Soil Physics 2010