Soil Physics 2010

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 400-level.

• Note: after homework is handed in, I re-post the file with the answers

Soil Physics 2010

Where were we?

Soil strength

Soil structure

Specifically, most soil structure exists because of cohesion

Soil Physics 2010

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.

Classification of structure:

PlatyBlockyCrumbGranularColumnarPrismaticAngularSubangular…

Granular

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

Soil Physics 2010

What is soil structure?

• Structure: the arrangement of parts

• Not just physical locations:

• Relationship between a particle and its neighborhood

• Connections: bonds, glue, load-bearing links

• A blueprint is about more than the location of each brick!

Geometry

Topology

Same as in earlier lecture: what is required to describe a porespace.

Soil Physics 2010

Figure & Ground

Why soil structure (particles)?

Particles & Pores

Figure and Ground

Dual networksTriangular & honeycomb

Dual networksVoronoi & Delaunay

Soil Physics 2010

Soil Physics 2010

Duals in 3D Space between barley grains. Grains were continuous; the porespace (dual) is also.

Soil Physics 2010

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

Climate, life, parent material

Water, heat, roots: different ways energy disturbs the soil, shaking it into a more stable configuration

Soil Physics 2010

Hierarchical structure

clay platelets → quasicrystals

quasicrystals → clay skins & bridges

…

microaggregates → crumbs

crumbs → aggregates

aggregates → peds

flocs, tactoids, cutans …

Structures are built from smaller structures:

Soil Physics 2010

Fragmentation systems

When an aggregate is dropped, there is usually a power-law distribution of pieces:

fdrrN

This is characteristic of fragmentation systems.

It implies that larger pieces are easier to break than smaller pieces.

rN

(r)

log(r)log[N

(r)]

slope = –df

Soil Physics 2010

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: covalenthydrogen

Physical bonds:Van der Waalssurface energy

Biological:hyphaeroot exudatesworm castsother yucky gooey stuff

FlocculationCementationAggregationCohesion / Adhesion

Soil Physics 2010

Aggregate properties

• Size, shape, distribution

• Strength versus physical forces

• Strength versus chemical forces

FragmentationDry sievingRupture

Wet sieving

Soil Physics 2010

Granular structure

Eventually, physicists try to treat everything as spheres…

Granular structure

… or something close, like M&Ms™

Soil Physics 2010

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