Soil Physics 2010

Outline

• Announcements

• Structure (pores)

Soil Physics 2010

• 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

Announcements

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:

•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

What does the structure do?

Why one structure versus another?

What structure(s) would work? What wouldn’t?

Let’s take a teleological approach…

Soil Physics 2010

Teleology* of soil structure

Required:• Ability to store lots of water, but also• Ability to shed excess water

• Water has access to all points (plants)

• Combination of large & small pores

* Teleology: a philosophy that explains a form or phenomenon by its purpose, not by how it occurs.

Soil Physics 2010

• Mineralogy & particle size distribution

• Weather & climate

• Vegetation, other biological conditions

• Topography

• Time

Constraints on a soil

← soil must persist in time

(These are Hans Jenny’s 5 soil-forming factors)

Soil Physics 2010

Constraints on a soil (2)

Porosity

Per

mea

bil

ity

For a given set of constraints – say, an Iowa-like climate:

Lots of runoff,Lots of erosion:No soil or plants

Lots of storage, but runoff if

storage is exceeded

Balance of storage and transmission

Fast drainage:Stream erosion.

No storage: No plants

Soil Physics 2010

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:

• Greater access

• Greater conductivity

• Freedom to morph

• 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 streamA 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:

• Optimize how imperfections are distributed

• Usually this means that the highest resistance elements are at the smallest scale

• Hence the prevalence of tree-like structures in natural systems

Soil Physics 2010

How exactly does structure form?

• Agglomeration (clumping, aggregation)

• 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?

• Higher Ks

• 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 Ks

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.”