Soils 205- General Soils

Lecture 22 Soil Formation

I. 

    a.  Parent material (p) when considering parent material we classify by both rock type and the type of deposit.

        i.  Rock types:

            1.  Igneous: formed from molten magma 

            2.  Sedimentary- compacted or cemented weathering products of other rocks

                examples:  sandstone, shale

            3.  Metamorphic- igneous or sedimentary rocks that have been subjected to intense heat and pressure

                examples:  schist, gneiss, marble, slate

        ii.  influences of rock type

            1.  Soil texture

            Granite  -->  coarse-grained, sandy soils

            Basalt --> fine-grained, clayey soils

            2.  Soil fertility

            Limestone --> Ca and Mg rich soils = fertile

            Sandstone -->  quartz rich soils = less fertile

        iii.  Types of surficial deposits- parent material may either be residual or transported

            1.  Residual

            2.  Organic deposits- build up over time and give rise to organic soils      

  2.  Transported parent materials

            a.  Colluvium- transported by gravity

            b.  alluvium- transported by rivers.  Alluvium tends to be well-sorted.  The particle size depends on the energy of the water; alluvial soils tend to be very productive.

        c.  lacustrine- deposited by slow moving, low energy water (lake deposits).

 The next group of parent materials are transported by ice.  There have been four continental glaciers which have affected North America.  Thus extensive areas have parent materials that have been transported by ice.

        d.  glacial till- poorly sorted material deposited in contact with the ice, either below it or on the sides of it.

        e.  glacial outwash- deposited by melt water; more like alluvium

        The last group of parent materials are those deposited by wind.  Wind deposited materials are called eolian.

     f.  Dune sand- medium to fine grain sand.  These materials are well sorted and coarse textured.

      g.  Loess- materials consisting of coarse silts and very fine sands which are actually suspended in the air during transportation.

        h.  Volcanic ash- soils formed from volcanic ash are generally light and porous.  They may cover relatively large areas as the ash is transported in the wind.

  b.  Climate (cl)- Climate considers the amount and timing of precipitation and the temperature of an area

    i.  precipitation- water is required for weathering and for the transport of soil constituents.

    ii.  temperature- influences the rate of chemical, physical, and biological processes

    c.  Organisms (o):  Includes both plants, microbes, humans, etc.

        i.  Influence of organisms

           1.  Organic matter accumulation

            example- Grassland versus forest

        ii.  Nutrient cycling

examples-

    coniferous tree litter- lower base cation content = more acid soils

    deciduous tree litter- higher base cation content = more basic soils

    earthworms- increase nutrient cycling by ingesting mineral and organic particles

    iii.  Bioturbation- mixing by gophers, earthworms, etc.

    d.  Relief (topography)- position on the landscape that a soil occupies defined by:

        1.  percent slope

        2.  aspect- the direction a slope faces strongly influences microclimate.  North aspects will be moister and cooler than south aspects = different vegetation and horizonation.

        3.  position on slope- influences erosion and deposition

    e.  Time (t)

II.  Soil forming Processes

a.  additions

b.  Removals

c.  Transfers

d.  Transformations