Section 18: The Geography of Soils

SLOs: soil characteristics, soil identification, soil classifications.


Lesson 1:

Instruction

Soil, made up of particles of varying size, contains both organic and inorganic matter.  Soil characteristics are used to classify soil types, but classification is a complex process.  First, a pedon, or vertical column of soil must be extracted.  Second, each layer, or soil horizon, within the sample must be analyzed.  The general characteristics of the pedon are as follows: the top layer is called humus (a mixture of organic materials), the middle layers can be a combination of the humus material from above and the parent soil material from below, while the bottom layer is composed of either loose or consolidated rock.  Because the organic materials tend to be distributed on the top horizons (layers), these top horizons can look darker in color than the ones below.  Soil color, texture, structure, consistence, porosity, moisture, chemistry, and pH levels are all useful for soil identification purposes.           

Photos and Images
 (soil pedon)     (an example pedon from Piru)

Quiz

Using the example pedon photograph, identify each soil horizon (O, A, E, B, C, and R).

 


Lesson 2:

Instruction

Soil texture identification is a necessary step in classifying soil layers.  The ratio of clay, sand, and silt are what matter.  Loam is a nearly equal combination of two or three textures of soil.  The soil texture triangle, below, shows the identification scheme.  Note that both natural and human factors can account for soil texture as well as other soil properties. 

Photos and Images

 
 (basic soil texture triangle)

Quiz

Sands are the largest soil particles, silts are the medium-sized soil particles, and clays are the smallest soil particles.  What sized particles would you expect to find in a loam sample of soil?

 


Lesson 3:

Instruction

Soil identification, or soil taxonomy, is broken down into orders.  These orders are: oxisols, aridisols, mollisols, alfisols, ultisols, spodosols, entisols, inceptisols, vertisols, histosols, andisols, and gelisols (along with rock lands, shifting sands, and ice fields).  Ventura County likely has a predominance of alfisol soils.  Alfisols are productive, fertile soils where crops can be grown.  There are five suborders of these soils, making alfisols the most wide-spread soil order on Earth.  Xeralfs, one of the five suborders, is connected with Mediterranean climates, like that of southern California.  Some of the soil characteristics of this suborder include moderately weathered forest or grassland soils, the presence of clay, and gray-brown soil colors.  However, because Ventura County has such a diversity of conditions, other types of soils are present in this region too.                   

Photos and Images
    (alfisol soils in the Los Padres National Forest)          (alfisol soils on agricultural lands near Fillmore) 

Quiz

Each of the above listed taxons has differing characteristics.  Using the Christopherson text introduced in the index pages or another soil identification source, list some of the characteristics of each type of soil.   


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2006 Jessica Douglas