Notes: Mon. 4 Nov


Issues to be addressed:

  1. what are they?
  2. factors in formation
  3. climate-related soil orders (we'll look at 6 of 11 major orders)

1. What are they?

The geological / geographical definition of soil is not the same as an engineering definition of soil. We are interested in soil as the interface between the atmosphere and the lithosphere, where the biosphere and hydrosphere are also active.

The formation of a soil results from transformation of a parent, or original, material. This parent material could be the underlying bedrock or an unconsolidated material from a different location.

A definition for "soil" from Birkeland says that a soil is a natural body composed of horizons with variable thickness. The horizons are composed of mineral and organic material and they differ from the parent material.

So the horizons are dynamic layers of physical and chemical activity. They are composed of:

  • minerals
  • organic material (from vegetation)
  • water
  • air (The water and air are in the pore spaces between the soil grains and there can be up to 50% pore space in soil.)

    Water is an important soil-forming tool. You need water to get any chemical activity. At the surface, there is organic matter and acids. Within the soil, there is moisture (already mentioned and as in lab 7). Some of this moisture is pulled down, or transported, to lower layers. This is called "gravitational water." Weathering is the physical and chemical breakdown of parent material. Water aids in this process and helps create the different soil horizons.


    These are the general horizons in an idealized soil cross-section.
    Horizon name Description
    O dark, rich, composed of organic material
    A composed of organic material and minerals (shows that material from O gets transported downward)
    E zone of leaching, where minerals are dissolved and transported downwards (eluviation)
    B zone of deposition (illuviation)
    C parent material in the earliest stages of weathering
    R "regolith," or unconsolidated bedrock

    Remember that this is idealized. Different soil profiles develop in different climates, and some layers may be missing. Also, the boundaries are not always straight lines. To find out what is missing in the E layer (due to leaching) and what's been added to the B layer, you must compare the composition with the parent rock in the R horizon.

    We will not be concentrating on all the little subdivisions within some of the layers, as the textbook does.

    2. Factors in soil formation

    1. Climate: mostly precipitation and temperature. Weathering occurs faster if there is water present and if temperatures are high. Also, wind can erode and deposit soils, but it's only effective in areas with little vegetation that would otherwise hold the soil down.
    2. Living organisms: biochemical activity (by worms, fungi, etc.) helps break down the organic components and create acids.

    3. Parent material: soil takes on the characteristics of its parent material
    4. Topography: the steepness of a slope (called "aspect" in your book) is related to water flow and drainage. You might expect that in areas of high precipitation, steep slopes would have high runoff and erosion. In contrast, areas of low relief might have well developed soils due to percolation of the water down into the ground.
    5. Time: soils take time to mature

    The first two factors are dynamic, or ongoing, and the last three are passive.

    Grain sizes

    The names "sand, silt, and clay" refer to different grain sizes, which are specifically defined in your textbook. Suffice it to say that sand grains are coarse and clays are very fine and small. Silts are somewhere in between. Note that these terms have nothing to do with the composition of the soil. Figure 24-2 in the book shows soil types mapped out as a function of composition. This diagram is called a "ternary" diagram. Instead of just plotting two things together, it has three. A similar diagram could be a color chart with red, blue, and yellow at each corner. Just as each color is a combination of some percentage of the three primary colors (e.g., about 50% yellow and 50% blue makes green), different combinations of the three types of grain sizes result in different soil types. There is one in particular called loam, which is about 40% sand, 40% silt, and 20% clay. This combination has the best water retention properties, so farmers like to use it for certain crops.

    Why is so much of the diagram that includes sand and silt called "clay"? Because even with that much sand and silt, the soil still behaves as we would expect a clay to behave (e.g., it is fairly sticky and rolls well into a ball). Thus, the soil types are named based on their expected behavior.

    3.Climate-related soil orders

    It is important to understand the soils of a region because the soils directly influence styles of agriculture that a particular culture employs.

    The hottest and wettest climates are in the tropics. In addition, soils there have been exposed for a long time and are very old. Therefore, we'd expect that they are highly weathered.

    An oxisol is the most-weathered soil order. It lacks horizons since it's practically one big B horizon. The color is red to red/yellow, and it's rich in clay. Since it is so weathered, many of the nutrients have been lost except for the very top layer of organic material. Therefore, as a whole, it is very infertile. It is so weathered that silica is no longer present. Iron and aluminum oxides are present, and you can mine bauxite, which is mostly an aluminum oxide.

    An ultisol is also very weathered but it still has silica.

    In these climates, a lot of minerals go into the water table because there is so much water to transport materials down to greater depths.

    Moving to higer latitudes, we get vertisols, which contain a clay ("montmorillonite") that expands and contracts with the amount of water present. Therefore, the soil appears to grow and shrink, hence the name.

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