Soils: Chapters 1&2
Introduction and Solids & Pores


Soils are crucial to life on earth


The many functions of soil can be grouped into six categories




Formation of Soils From Parent Materials


Physical and chemical weathering break down rocks and minerals

  • modifies or destroys their physical and chemical characteristics
  • carries away finer particles and soluble products
  • also synthesizes new products


Characteristics of Rocks and Minerals



Rock types:

Igneous Rocks
  • Magma (intrusive)
  • Lava (extrusive)


composed primarily of minerals of light-colored:

  •  quartz
  • muscovite
  • feldspars

and dark-colored

  • biotite
  • augite
  • hornblende








Potassium Feldspar
Sodium Feldspar
Biotite Mica


      Granite containing Potassium feldspar,
      plagioclase feldspar, quartz and biotite




In general, dark-colored minerals contain iron and magnesium and are more easily weathered

Therefore, dark-colored igneous rocks such as gabbro and basalt are more easily broken down than granite and other light-colored igneous rocks





Sedimentary Rocks




Conglomerate - rounded particles





Photo: Sandstone hills

Vermillion Cliffs, Utah/Arizona







Thinly bedded siltstone




Maskovce - West Carpathian flysh belt -  Paleogene claystone




Metamorphic Rocks


              Granite on left, Gneiss on right









Gradational Processes

Weathering: is the name given to processes that change the surface material of the Earth as a result of exposure to the effects of water and the atmosphere.

Erosion means the removal of material from one position on the surface of the Earth to a different position, again by the effects of water and the atmosphere.



Differential Weathering        Monument Valley, AZ
                                                  (resistant sandstone overlying shale)



Mechanical Weathering Processes

frost action

abrasion - refers to the breaking and grinding away of solid rock by collisions with moving particles.

pressure release - expansion of rock as they are brought close to the surface forming sheet joints.

biologic effects - plant roots, burrowing animals, mining.

temperature changes.


Chemical Weathering: changes the composition of rocks

  • Chemical Weathering Processes

reactions in the atmosphere:

  • (carbonic acid) - H2O + CO2 = H2CO3


Salty water can help the formation of salt crystals that can grow and dislodge sand particles. Also, slightly acidic water can dissolve the cementing material in sandstone.












Four Basic Processes of Soil Formation

  • Transformations
  • Translocations
  • Additions
  • Losses





Soil-Forming Processes

Consider: changes that might take place as a soil develops from a thick layer of loess parent material in a climate conducive to grass vegetation




Soil Horizons



Horizon Development

  • A-Horizon: organic-mineral mixture near the surface
  • B & C Horizons:






Soil Architecture & Physical Properties


Soil physical properties closely influence....


Soil scientists use the physical properties of the soil horizons in classifying soil profiles and in making determinations about soil suitability for agricultural and environmental projects

  • Soil texture
  • Soil structure
  • Soil color

Three major factors influence soil color:

  • organic matter content
  • water content
  • the presence and oxidation states of iron and manganese oxides










Soil Texture (details)

Knowledge of the proportions of different sized particles in a soil is critical for understanding soil behavior




  • feels gritty between the fingers
  • particles generally visible to the naked eye
  • may be rounded or angular depending on degree of weathering and abrasion





A thin section of a loamy soil.
Empty pores appear black.
Silicate minerals include
quartz (q), plagioclase (p)
and feldspar (k).















40x scanning electron micrographs of quartz sand (left) and feldspar (below)





  • large particles mean large pores that can't hold water against the pull of gravity and so drain rapidly and promote entry of air







Specific Surface Area: surface area for a given mass of particles




















  • in between sized particles
  • similar to sand in shape and mineral composition but microscopic in size
  • feels smooth or silk when rubbed between fingers
  • pores much smaller so retains more water




  • very small particles (less than 4 microns)



Influence of Surface Area on Other Soil Properties







Soil Texture Classes

Loams are a combination of sand, silt and clay particles.









Structure of Mineral Soils

Soil Structure relates to the arrangement of sand, silt, clay and organic particles in soils

Types of Soil Structure

  • Single Grained: soil is broken into individual particles that do not stick together.
    • is a loose consistency
    • commonly found in sandy soils


Most soils exhibit some type of aggregation and are composed of peds that can be characterized by their:

  • shape (or type)
  • size
  • distinctiveness (or grade)


Principal Soil Ped Shapes

  • Spheroidal

  • Platelike
  • Blocklike
  • Prismlike





Making a Soil Monolith

A monolith, or vertical slice from topsoil down to subsoil, preserves a soil’s colors and layered horizons in position. Scientists make monoliths of the important soils in their region and use them in teaching.


Step 1
In the field, scientists dig a pit about
6 feet deep, keeping one face a flat
vertical plane.
Donald Hurlbert, Smithsonian Institution









Step 2
Then, they press a mounting board
with a bed of nails into the pit face
and dig in about 6 inches around
the board’s edges.










Step 3
As they pull the board back, they use strips of cloth to secure the soil to the board.







Step 4
Back at the lab, the soil is “picked” away to leave about 1 or 2 inches of soil with a natural surface, and then coated with glue or acrylic floor wax to “fix” it to the board.