1. Field View of Soils

    Soils are quite variable from one location to another. They have different physical appearances and properties which are a result of chemical and physical processes occuring in the environment.

    Soil horizons are soil features that are esy to observe.

    Soil Horizon - A layer of soil approximately parallel to the land surface and differing from adjacent genetically related layers in physical, chemical, and biological properties or characteristics such as color, structure, texture, consistency, kinds and number of organisms present, degree of acidity or alkalinity etc.

    Horizons are seen by exposing a soil profile.

    Soil Profile - A vertical section of soil through all of its horizons, extending into the parent material.

  2. MASTER HORIZONS

    O

    organic material at the surface ( top of ) soil -- leaves, twigs, branches, dead grass etc.  These horizons are usually seen in forested soils, generally not in cultivated soils.

    A

    horizon where decomposed O.M. accumulates -- usually has a dark color area where most biological activity occurs most - soil microbes are here, as are earth worms etc. - usually at the surface. If the field is plowed this horizon extends to the depth of cultivation. This is why it is called the plow layer. Area of clay and chemical loss. It is called the zone of eluviation.

    E

    horizon of maximum removal or leaching. This is also a zone of eluviation. Maximum clay and chemical losses have occurred here. -- generally light colored and sandy

    B

    horizon where the material from the A and E horizon will collect. This horizon is known as the zone of illuvaition. -- clay and chemicals are generally at maximum levels

    C

    layer of nearly unaltered mineral material. no biological activity here. neither a zone of eluviation or illuviation. No changes have really taken place. Very little if any rooting has occurred in this horizon. Generally considered to be the lower limit of the soil. -- This is called the soil's parent material.

    R

    the underlying bedrock, such as limestone, sandstone or granite.


    These soil horizons may be subdivided.

    A vertical section of the earth's crust showing all of the soil's horizons is called a soil profile (shown in the figure below).

    divisions of soil

    1. There are 4 processes involved in horizon differentiation:

      1. Additions to the soil

      2. Losses from the soil

      3. Translocations within the soil

      4. Transformations within the soil

        1. Additions

          • water as precipitation, condensation, or runoff
          • O2 and CO2 from the atmosphere
          • N, Cl, and S from the atmosphere and precipitation
          • organic matter from biotic activities
          • material from sediments
          • energy from the sun

        2. Losses

          • water by evapotranspiration
          • N by denitrification
          • C as CO2 from oxidation of O.M.
          • soil by erosion
          • energy by radiation
          • water and material in solution or suspension

        3. Translocations

          • clay, organic matter, iron oxides, and chemicals by water
          • nutrients circulated by plants
          • soluble salts in water
          • soils by animals

        4. Transformations

          • decomposition of organic matter
          • reduced particle size by weathering
          • mineral transformations (primary to secondary)
          • clay and organic matter reactions


        All of these processes are going on at the same time in the soil. They are all working together to help form the different horizons.


    2. Soil is distinguished from parent material by:

      • roots and soil organisms; biological activity
      • higher organic matter contents
      • more intense weathering
      • characteristic layers or horizons

        Soil profile is a vertical section of a soil showing all of its horizons and parent material but it is two dimensional.

        Although the soil profile is a useful tool in describing a soil, it really only represents a point on the landscape and is two dimensional. Soil profiles don't give any information on irregularities in horizon thickness. In order to take care of these irregularities, soil scientists study a volume of soil known as the soil pedon. The soil pedon is a three dimensional soil body.

        Pedon is the smallest volume that may represent a soil. It is generally 1 to 10 meters across and hexagonal in shape.

        If we put similar pedons together we get a polypedon. The pedons in a polypedon have similar features and characteristics.

        Polypedons are put together into groups known as soil series This is a term that we will work with throughout the semester. There are over 14,000 soil series throughout the world. Each of these series has different chemical and physical properties that affect management.

        Soil Series are simply groups of pedons whose properties fall into a certain range. When the landscape changes so that the pedons no longer fit within the given range, they are placed into another grouping or another soil series.

  3. SOIL GENESIS

    Remember that soils are a natural body at the earth's surface that are capable of supporting plant growth. Because they are natural bodies, they are acted on by natural processes.

    Five factors control the formation of soils:

    1. Parent material (texture, structure, chemical and mineralogical composition)

    2. Climate (temperature and precipitation)

    3. Topography

    4. Biotic (Vegetation and animals)

    5. Time


    The relationship of each of these factors to each other is seen in the following sentence:

    "Soil is a product of climate, organisms, and topography acting on parent material over a period of time."

    Soil = f(parent material, climate, topography, vegetation, time)

    Soils have different features because of differences or changes in these five factors. These different effects show up as different soil horizons and soil profiles.

    The study of soil formation is called soil genesis. This means interpreting the origin of soil. It is important to understand the beginning of anything so that you have a foundation for the future. A thorough understanding of soil formation processes is a valuable tool to use in interpreting soils for specific uses. Understanding formation factors of the past allows us to understand what management practices of the future will most likely do in the soil.