Lecture 3

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Chapter 2

Soil Formation

 

Why does one soil look different than another? What geological forces act upon soils and what are the effects of the action? The famous geologist, Hans Jenny, answered these and other questions in his description of five factors responsible for soil formation.

  1. Rocks and minerals change by the processes of weathering. Typically weathering causes mineral materials to disintegrate into smaller parts. The elements released as products of weathering may form new, secondary minerals. Weathering products that are loose or unconsolidated are called soil. Weathering can be accomplished by one or a combination of physical and chemical processes.

    Physical weathering is most pronounced in cold and dry climates. Physical processes include effects of temperature. Most notable is the force exerted by the expansion of water as it freezes. Another physical process is abrasion caused by bombardment of minerals by materials suspended in wind and flowing water. Finally, plant roots established in the crack of a rocks often exert a force strong enough to cleave the rock.

    Chemical weathering dominates in warm or moist climates. Worldwide, chemical weathering processes tend to be more important in soil formation than physical forces. The chemical processes are described in the textbook. The processes include: oxidation and reduction (of great importance for iron-containing minerals), carbonation (dissolution of minerals in water that has been made acidic by carbon dioxide), hydrolysis (when water splits into hydrogen and hydroxide, and one or both components participate directly in the chemical process), and hydration (when water is incorporated into the crystal structure of a mineral, changing mineral properties). Minerals differ greatly in their resistance to weathering (see Table 2-1 in the textbook).

  2. Soil Formation means the development of a particular soil in a particular place. Weathering produces unconsolidated mineral material that one would call soil. However the formation of any given unique soil includes process that continue to operate long after this unconsolidated material is formed. The products of weathering are the materials in which soils form. Soil materials are often transported vertically within the soil profile (see Figure 2-2 in the textbook). Materials can be added (such as humus from plants or sediments added by wind or water) or can be lost (such as by erosion or by leaching through the soil profile). The unconsolidated materials in which soils form is called parent material. The term, soil formation, implies the formation of soil horizons and other features in the parent material.

  3. Parent material arrives on location through various routes and mechanisms. The parent material in a landscape may have been transported to the location (see Figure 2-9 in the textbook). Residual soils formed in place from weathering of the underlying bedrock. Residual soils are common in the great plains of the United States. Residual landforms include mesas, plateaus and plains.

    Stream deposits are very common worldwide. Such deposits are called alluvium or alluvial. Examples of alluvial soils are: flood plains (as along the Mississippi River), alluvial fans (as in the piedmont region of the southeastern states), lacustrine deposits (dispersed material deposited as a stream entered a lake), and marine deposits (flocculated or aggregated material deposited as a stream entered the sea). In some instances marine deposits form deltas.

    Wind deposits are called eolian materials. Only small particles are readily carried by wind. Large sands are too heavy to be carried by most winds. Clays also rarely blow because they form aggregates with other clay particles producing large clods or peds. Silts and fine sands are examples of material readily carried by wind. Silts are small mineral fragments, barely visible to the naked eye. (A typical silt particle is about as wide as the thickness of a sheet of paper.) Wild-blown silt deposits are important in agriculture for the ease with which they are cultivated. Such deposits are called loess. Sand dunes and landscapes made of ash are also eolian.

    Glaciers have profoundly affected the landscape of the northern states. These deposits from moving sheets of ice are called glacial materials or glacial till. During past 1.5 million years (Pleistocene--modern times) the world has experienced four ice ages. The most recent was the Wisconsin ice age, ending 12,000 years ago. Much of the soil in the midwest came from Canadian glaciers. Glaciers leave landforms such as: lateral moraines, terminal moraines, ground moraines, and outwash plains.

    Gravity deposits are colluvial material or colluvium. Such material results from mass-wasting, mud flows, or the gradual movement of individual particles down a slope. These deposits are found on a relatively small-scale such as mountain valleys, and are usually coarse materials (see Figure 2-9 in the textbook again).

  4. According to the classic work by Hans Jenny, five soil-forming factors account for the differences in soils. The five factors are: climate, relief, organisms, parent material, and time. The acronym "CROPT" may help you remember this.

  5. Parent Material is the only factor that can be considered inherited as opposed to acquired. The effect of parent material on a soil include such feature as soil texture, pH and mineral constituents.

  6. Climate is often considered the most powerful soil-forming factor. Climate is expressed as both temperature effects and rainfall effects. Temperature controls rates of chemical reactions. Many reactions proceed more quickly as temperature increases. Warm-region soils are therefore normally more developed or more mature than are cool-region soils. Rainfall affects leaching, pH and soil aeration. In addition to direct effects of climate, climate also profoundly affects vegetation which in turn also affects soil formation.

  7. Organisms (biota) affect and are affected by soil formation. Man is perhaps now the most influential of all organisms. He affects the soil by such activities as: plowing, irrigating, mining, clearing, disposing and leveling. The effects of large animals other than man on the land are minor. The effects of vegetation on soil formation are very profound. Different soils form in a grassland than under a forest. Much of this difference is due to the rapid nutrient cycling in grasslands. Vegetation effects extent of cover, thereby influencing runoff and erosion. Vegetation type and amount directly influences the type and amount of organic matter accumulation on the soil, and thereby influences such soil chemical properties as pH and nutrient supply. Finally, vegetation is the food source for most microorganisms so the vegetation exerts a strong influence on soil microbial populations.

  8. Relief (topography) modifies the effects of other factors. Relief modifies climate by affecting the smoothness of the surface and also the angle at which the soil surface orients towards the sun. A convoluted surface dilutes solar energy over more surface area than does a smooth surface. In the northern hemisphere a north-facing slope will be cooler than a south-facing slope. Relief also affects the amount of rainfall that infiltrates a given parcel of soil. A steep slope will encourage runoff. A soil in a sloping location will experience less effective rainfall than that which one would measure in a rain gauge. Likewise, a low area may receive run-on water beyond the actual rainfall. Also, relief influences erosion. Soil horizons form from the top downwards. If the topsoil readily erodes away as it forms, the soil formation processes appear to have halted.

  9. Soil formation is a function of time. Soil development is a process, not an event. Soils change over time. Clays are secondary minerals. They form in the soil, then change forms, and all the while they are moving downward with leaching rainwater. Similarly, organic matter forms on the surface as it moves downward with rainwater. It declines after reaching a maximum as old soils loose their ability to produce vegetation fast enough to keep up with decomposition. These are highly dynamic processes. Soils as viewed today are just snapshots in time. Soils looked different in the past and will look different in the future.

Students are encouraged to look up the following vocabulary words in the textbook glossary and to browse the following web site.

Vocabulary

sand
silt
clay
pH
illuvial horizon
loess
landform
residual material
soil genesis
weathering

 

Web site

An elder statesman/soil scientist/artist/philospher opines about soil. URL: www.soils.wisc.edu/soils/poets/hole1.htm

 

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