The diagnostic features used to differentiate soils in USDA Soil Taxonomy are primarily macromorphological Soils can also be studied at smaller scales Colony of bacteria ~1 micron

Coarse fragments Silt Clay Sand Is the whole greater than the sum of its parts ?

Soil micromorphology - the study of undisturbed soil samples with the help of microscopic techniques, in order to identify small scale features and interpret how they formed.

The soil fabric Granular crumb structure Compacted soil How are the primary particles arranged in real soils ?

Textural porosity Pores resulting from random arrangement of soil particles

Textural porosity + Structural porosity Total porosity

Vughs

Cracks

Channels and Chambers

Lots of tillage pores Few biopores Depth (cm) Origin of pores in a soil with annual moldboard plow/disk tillage

Fewer tillage pores Lots of biopores Depth (cm) Origin of pores in a soil with minimum tillage How were the Dutch soil scientists able to differentiate between tillage pores and biopores? Analysis of thin sections !!

Relationships between coarse and fine materials in soil fabrics

Distributions of coarse and fine materials

Mostly sand particles - very few fine materials

Bridging

External coatings

Microaggregates between sand particles

Coatings and microaggregates

Sand embedded in fine matrix

How much sand do you need to add to a clayey soil to make it stop acting like a clay? Massive amounts of sand are needed because sand needs to become the dominant component of the soil structure

< 20% sand > 50% clay >80% sand <10% clay Clay dominated soil matrix with embedded sand particles Sand dominated soil matrix with clay coatings

Dramatic soil textural modification is a standard part of golf course construction but is cost prohibitive (and impractical) in most other situations Play can resume 10 minutes after rain stops

Structural development at the microscale

Angular blocky structure

Platy structure

Granular crumb structure

Tall Grass Prairie the dominant native ecosystem in IL Clay particles

Multi-valent cations such as Ca +2, Al +3 and Fe +3 are important binding agents at this scale. High levels of monovalent cations such as Na + and K + cause clay domains to disperse. Domains of clay particles

Microbial glues are important binding agents at this scale Humic substances are also important binding agents at this scale 35,000 x Microaggregates (< 0.25 mm)

Macroaggregates (>0.25 mm) root micro- aggregate Networks of roots and fungal hyphae are important stabilizers of macroaggregates

Dry soil at the end of an extended dry period Soil processes can be studied at the microscale

Rain water rapidly infiltrates the surface horizon Eluviation/Illuviation at the microscale

Some clay particles disperse in water filled pores Eluviation/Illuviation at the microscale

Suspended clay moves downward through macropores to deeper, drier horizons Eluviation/Illuviation at the microscale

Capillary action moves water outward into micropores Eluviation/Illuviation at the microscale

A thin “skin” of oriented clay particles begins to accumulate on the walls of macropores Eluviation/Illuviation at the microscale

Oriented clay particles coat the walls of dry macropores Eluviation/Illuviation at the microscale

The process repeats itself over and over… causing illuvial clay skins to slowly increase in thickness Many years go by…

Oriented clay particles also accumulate around aggregates

Clay particles also orient around microorganisms.

Coating

Infilling

Quasi-coating

Fragments of illuvial features

Clay skins Illuvial features at the macro-scale

Essential factors for significant eluviation/illuviation of clay  Alternating periods of intense rain and drought  Sufficiently high clay content in surface horizons  Sufficient macroporosity for downward transport  Relatively stable land surface

Most of the images in this presentation were obtained from an invaluable web resource on soil micromorphology maintained by Dr. C. Dorronsoro, Department of Pedology, University of Grenada, Spain