Two definitions for soil:

• the upper layer of the earth that may be dug or plowed and in which plants grow (Websters).
• unconsolidated material at the earth's surface.

What are the initial materials soils are formed from?

• minerals: inorganic, crystalline, naturally occurring
• types of?
  • silicates: most common type, (SiO4)-4 the common ionic group, includes quartz, feldspars, micas.
  • oxides: form in presence of oxygen, by weathering. Hematite as nature's pigment. Common in some soils because communication with the atmosphere produces an oxidizing environment. Other soils reducing.
  • salts: precipitate out of water.
  • sulfides: form often in oxygen poor environments.
  • carbonates: soluble so relatively mobile.
• clays: a special mineral group important in soils.
  • submicroscopic, and very variable and complex.
  • usually dominated by sheet silicates that provides them with unique traits (shrink-swell behavior, adsorption capabilities, low strength).
• organics.
• rock fragments: some common rock types (represent source material for sediment or soil):
  • igneous: granite, basalt, andesite.
  • sedimentary: sandstone, shale, limestone.
  • metamorphic: schist, gneiss, slate.

What important factors control soil formation? Students should be able to expand a bit on each of the below.

• climatic factors?
• source factors?
• time factors?
• slope factors?
• biologic factors?

What processes occur during soil formation and what approaches could you take to study these processes? Rephrasing the first question - how does rock turn into soil.

• Types of approaches to take in studying:
  • literature search (but what if not studied before?).
  • experimental approach (but limiting time factor).
  • theoretical approach (but theory may be limited).
  • field studies: spatial variation can be a function of history.
• Field studies indicate great diversity, but general trend of loss of soluble elements (see below). Effects diminish from the surface downward.
• Below is a table taken from Goldich (1938) that shows the chemical analysis of a quartz-feldspar-biotite gneiss and variably weathered versions of it. The inference is that this is a temporal progression, but this is not necessarily the case (think of other factors that control the degree of weathering).

  	fresh rock	least weathered	moderate weathering	most weathered
  SiO2	71.54	68.09	70.30	55.07
  Al2O3	14.62	17.31	18.24	26.14
  Fe2O3	.69	3.86	1.55	3.72
  Fe O	1.64	.36	.22	2.53
  MgO	.77	.46	.21	.33
  CaO	2.08	.06	.10	.16
  Na2O	3.84	.12	.09	.05
  K2O	3.92	3.48	2.47	.14

• Below is a table from the same source showing some of the mineralogic changes.

  	fresh rock	least weathered	moderate weathering	most weathered
  quartz	30	40	43	25
  K-feldspar	19	18	13	1
  plagioclase	40	1	1	nondetectable
  biotite	7	trace	trace	trace
  hornblende	1	none	none	trace
  oxides	1.5	5	2	6
  kaolinite	none	36	40	66

• Physical weathering processes:
  • rock jointing and fracturing: most common type of structure.
  • abrasion by wind, water or ice carrying particles.
• Chemical weathering processes:
  • solution.
  • silicate hydrolysis.
  • oxidation.

View of weathering pattern in a shallow Tertiary intrusive from the foot of the Chisos Mountains in Big Bend, Texas. Note the rectangular fracture pattern that outline weather blocks. The rock was originally all uniform in color and the banding represents oscillatory iron staining and reflects weathering that worked from the fracture margins into the interior of the blocks. The middle of the blocks are less weathered than the fracture margins. This is the same geometry and process that gives rise to spheroidal boulders when the more weathered material is eroded away.

View of desert soil near Moab, Utah. The reddish material is wind blown fine red sand. The red coloration is due to oxidation of the small bit of iron in the grains. Of distinct interest here is the dark, bumpy covering. To the touch it has a leathery feel, and under the microscope it is a complex community of unicellular plants and organisms called a cryptobiotic crust. This community thrives under the harsh arid desert conditions. It also serves as an important element in soil formation, by trapping sediment, moisture and seeds. The germination of seeds and growth of larger desert plants often depends upon this substrate being developed. Unfortunately the crust doesn't thrive underfoot, and new efforts are being made to educate people about the importance of this cryptobiotic crust to the desert soils and ecosytem.

What are the soil constituents that result from these processes?

• mineral and rock fragments.
• clays.
• precipitates: oxides, salts and sulfates.
• organics.
• water and ice.
• air, in pore space.

What structures (order) exists in soils?

• Clear that soils are zoned, from the surface down.
• Soil exposed along bluffs of Elkhorn River just west of Omaha. Letters A, B, and C show approximate positions of layers described before. The soil is developed in material mainly derived from the wind blown loess (material with columnar fractures at base of cliff). A is evident by the dark coloration produced by the collection of organics. B is evident by orange and white staining and C is the transition zone to unaltered material below.
• soils with clays also have unique fracture system.

How can soils be classified?

• agricultural, scientific, engineering - i.e. many ways.
• climatic, by process. Some examples.
  • oxisols: tropical environments, intense chemical weathering, oxide accumulation, laterites old term for one type.
  • aridisols: arid environments so get evaporation out of soil, accumulation of calcium carbonate a key.
  • cryosols: freezing important part of process, permafrost, patterned ground one result.

Soil erosion:

• what are natural rates of erosion? how would you measure?
• what are mechanisms of erosion?
  • sheet wash.
  • gullying: headward retreat, fractal pattern
  • wind erosion.
• human activity the promotes soil erosion?
  • logging, especially clear cutting.
  • improper farming.
  • construction.
  • off road vehicles, intense foot traffic.
• effect of plumes of sediment on aquatic ecosystems.
• Dust Bowl Era - early 1930s. Dust in the wind was topsoil from the field. Cause?
• Soil Conservation Service, born April, 1935.
• possible solutions to soil erosion?
  • maintenance of plant cover.
  • minimum tillage, contour plowage, minimize grazing.
  • windbreaks.
  • gully sediment traps (prevent headward retreat).
  • contour land.

Soil salinization:

• cycles of irrigation and some numbers to play with.
  • salinity of seawater: circa 35,000 ppm
  • salinity of input irrigation waters 350 ppm.
  • >2000 to 3000 ppm toxicity level for many crops
  • critical question: if the climate is arid enough that water evaporates out of soil where does the salt go?
• extent of concern?
  • San Joaquin valley in California, Pakistan, "half of the world's irrigated land .... suffers from reduction of crop yields due to salinization."
• possible solutions:
  • desalinize irrigation water (very expensive).
  • direct trickle irrigation (water usage reduction).
  • tile fields: can flush salts through soil, run off waters high in Selenium in California, expensive.
  • salt resistant crops (will only delay problem).

Nutrient overloading: runoff associated with soil erosion includes dissolved and suspended load material that serve as nutrients for algal growth. This is especially true for urban areas, and for fields were agrichemicals are used. When these standing bodies of water the nutrients can cause algal blooms, oxygen depletion and associated problems. The Mississippi provides enough nutrients to cause algal blooms and what are known as anoxic events in the nearshore Gulf Coast area. One is this is a major disturbance in the Nitrogen cycle.

Engineering concerns

• differential compaction (settling).
  • good local examples on campus: library, parking garage.
  • are sediments naturally compacted or not: soil tests are critical.
  • organic rich soils an extreme of 20' to 1' compaction (contributes to subsidence problems).
  • muds can easily compact from several feet to 1'. Sand compacts a little.
• swelling clays.
  • $2-7 billion annual damages in U.S. (Krohn & Slassna, 1980)
  • Dallas area alone $10 million a year.
  • common where volcanic ash a major component in soils or where have marine clays (Denver and Houston).
• soil creep.
• piping.
• drainage characteristics.
• permafrost.

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