PHYSICAL GEOGRAPHY
LECTURE 16: SOILS 

To be very obvious, fertile soils are the key to human adaptation to the Earth. We have sought out the most fertile regions in the world, devegetated them, and transplanted vegetation that we value and like to consume. But, keep in mind that this does not mean that regions with infertile soils are devoid of vegetation. You will need the following maps for our discussion of soils and their relationships to vegetation patterns.

Soil map; text p. 538-539 (better map in the atlas, p.24-5)

Climate map; Atlas p. 10-11

Vegetation map; Atlas p. 26-27

Biomes; text p. 596-597

 

Soils are determined by parent materials, vegetation and the climates in which they exist.

If you recall discussion of regolith in chapter 13, you will remember that regolith is the term we give parent material that has been weathered. As you will note from figure13-5 (page 379), regolith consists of weathered bedrock near the surface including the soil layer. In the soil layer you will find; decayed parent materials, decaying plant material, decaying animal matter (manure) along with vegetation. Roots from plants work their ways into the bedrock, breaking down parent material (biological weathering), and stabilize the regolith along slopes.

Climate is the most significant factor in soil fertility. The region’s seasonality of precipitation and temperature stimulate the movement of water through the regolith. There are three processes which create soil fertility. If you skipped Chapter 10 in Christopherson, be sure to read this prior to the final exam.

Eluviation is the downward movement of particles (decaying vegetation and animal waste) which occurs as water percolates into the soil from the surface. Clearly this is related to the amount of precipitation a region receives. Humans can also stimulate eluviation by irrigation. As water percolates downward, minerals from the plant and animal waste is added to the soil layer. The eluviation process is important to plants since this brings nutrients to the root zone. 

The greater amount of precipitation an area receives the greater the soil depth.  Therefore; which areas would you predict to have the deepest soils?  the shallowest?  

Illuviation is the upward movement of minerals from parent materials which occurs as plants extract water from the soil. This is also called evapotranspiration by geographers and is related to the region’s temperature. As temperature increases during summer months, plants transpire more water to the atmosphere. To avoid wilting, roots of the plants draw water from moisture stored within the soil or aquifers beneath the soil layer. As this occurs, minerals from weathered parent material are also brought to the root zone and to the soil surface.

Which areas would you predict as having the greatest illuviation?  the least?

 

In soil science, humification is the term used to explain the downward movement of plant and animal waste into the soil layer through precipitation.  Humus is necessary for sustaining soil fertility on a long term basis.  Humus is the term for accumulated plant litter (leaves, decaying fruit, pollens...), animal waste (manures...), and decomposers (worms, earwigs...).  The downward movement of humus creates a deep soil layer as seen in mollisols (p.542) or alfisols (p.546), but not found in clay based soils, such as Spodosols (p. 548) which limit water percolation. If humus remains along the surface (as in spodosols or vertisols), the depth of the soil layer will be limited; limiting soil fertility.  

These three processes are central to soil fertility. We find that the most fertile soils in the world (mollisols and alfisols) are located in regions with seasonal precipitation and temperature. This creates a balance between eluviation and illuviation. As illustrated on page 538-539, mollisols (shown in green) are located in mid-latitude regions (30-50 degrees latitude) which have seasonal climates. Mollisols are found in North America, a stretch across Europe and Asia, the Pampas region in Argentina, the tip of South America, and parts of Central America, including Mexico. Alfisols (shown in yellow), the second most fertile soils are located in mid-latitude regions with less precipitation than Mollisol regions. These soils are found in California, the midwest of North America, Europe, Asia, India, Africa, Australia, and scattered sections of Central and South America.

As you will see from the inset map on page 538, the United States has a high amount of naturally fertile soils.
If you examine the location of fertile soils, you may notice that these are also the wealthiest countries in the world, any coincidence?

 

PROCESSES CREATING SOIL INFERTILITY: Following the description of each process, fill-in areas where such soils are located using p. 538-539. Aside from Mollisols and Alfisols, many soils are infertile due to the climatic factors found in their region. Infertile soils are simply located in regions where eluviation and illuviation are not balanced and do not allow for humification.

  1. Laterization: this process occurs in areas with abundant precipitation causing too much eluviation. This causes the nutrients from the soil surface to be leached from the soil layers. Oxisols are found in such locations. As you will see on page p.540, this creates very deep soils, yet does not allow the soil to retain these nutrients, creating very infertile soils. These soils look "rusted" reflecting the oxidation of iron minerals in the soil, and may be best utilized as a building material! (see picture p. 541)


    2. LOCATION:

    - climate type:

    - vegetation:

     

     

  2. Salinization – in areas with too much illuviation and limited precipitation (such as deserts), salts from parent materials are brought to the surface. Plants in arid climates have very high rates of potential transpiration (water demand). Dominant soils affected by salinization are aridisols. Plants which adapt to such conditions have long tap roots able to acquire water from subsurface storage. As salts are brought to the surface, the soils appear white as seen on page 541. Most plants cannot tolerate much salt, creating a poor habitat for most vegetation. Irrigation on marginal soils, such as alfisols, can stimulate illuviation in these areas, leaving soils vulnerable to salinization. Irrigation may stimulate percolation (eluviation), yet transplanting varieties not adapted to high temperatures (broad leaves) causes greater amounts of evapotranspiration, or greater demand for water from such plants. Farmers can then stimulate salinization by choosing to plant crops, such as tobacco or cotton (adapted to areas with 40-60 inches of precipitation each year, see p. 236), in semi-arid regions, like the Central Valley.


    3. LOCATION:

    - climate type:

    - vegetation:

     

     

  3. Calcification – also caused by too much illuviation. Arid conditions cause calcium deposits deep in soil layers to harden which limits the movement of nutrients (eluviation and illuviation). This creates what we call "hard pan" soils, found mostly in aridisols and alfisols (see page 541). The hard pan is shown in this picture as the yellow layer running through the middle of the soil sample.  Similar to salinization, humans can stimulate calcification through flood irrigation as well as compacting the soil with heavy machinery, or overgrazing of animals (too many head of steer per acre). Calicification reduces the soil's cation-exchange capacity (or CEC).  This term sounds a little frightening, but really is just describes the lack of illuviation and eluviation taking place due to the hard pan (similar to gleization below).   


    4. LOCATION:

    - climate type:

    - vegetation:

     

  4. Gleization – this process is similar to calcification as a layer of soil limits eluviation and illuviation. In gleization, clay layers limit the porosity of the soil, causing plant litter and animal waste to be accumulated on the surface. Soil depth is limited and humus collects along the surface. We call these soils histosols (see page 551) or peat bogs (our source of commercial peat moss). These soils are found in cold climates (which limit percolation) and areas with poor drainage.


    5. LOCATION:

    - climate type:

    - vegetation:

     

  5. Podzolization: this is the process of soil acidification caused by the type of plant litter found in such regions. Some vegetation, such as tomatoes, pine trees, walnuts, and tobacco, have acidic leaves. As this plant litter decays and percolates through the soil with precipitation, an acidic environment is created in which little vegetation can exist. These conditions naturally occur in pine forests, creating spodosols or ultisols, (p. 548).

Humans can stimulate podzolization by planting acidic plants in fertile soils, such as those noted above. Lime can be added to the surface to reduce the acidity.

LOCATION:

- climate type:

- vegetation:

 

 

 

Other elements creating soil fertility and infertility

We have discussed the dominate processes which determine soil fertility and soil infertility. Once you understand the processes above, further discussion of soil color, soil texture and soil porosity are easy to comprehend. Soil texture and soil porosity are addressing the type of parent materials which make-up each soil region. The key here is clay. Clay limits the movement of water, and therefore limits eluviation and illuviation. This can cause salinization, calcification and gleization. Soil color is an indicator of soil fertility. Red soils reflect too much water (oxidation and laterization), white soils reflect too much heat (causing calcification or salinization). Mollisols are nice, soft, dark soils which reflect a high amount of humus throughout the soil layers, leading to fertility.

If you were a pioneer in the Oklahoma Land Grab, what indicators would you look for to obtain the prime soils?

As noted throughout the discussion above, humans can cause infertile soils through mismanagement. We can stimulate:

Yet, we could also learn how to use our soils on a sustainable basis by understanding the processes of eluviation, illuviation and humification. Methods to stimulate natural fertility would include:

And many others….see physical hyperlinks