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: Platy Blocky Crumb Granular Columnar Prismatic Angular Subangular … 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

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 networks Triangular & honeycomb Dual networks Voronoi & Delaunay 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: This is characteristic of fragmentation systems. It implies that larger pieces are easier to break than smaller pieces. r N(r)N(r) log(r) log[N(r)] slope = – d f

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: covalent hydrogen Physical bonds: Van der Waals surface energy Biological: hyphae root exudates worm casts other yucky gooey stuff Flocculation Cementation Aggregation Cohesion / Adhesion

Soil Physics 2010 Aggregate properties Size, shape, distribution Strength versus physical forces Strength versus chemical forces Fragmentation Dry sieving Rupture 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

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