- -   AGRICULTURE CORE CURRICULUM  - -


(CLF7000)  Advanced Core Cluster:  PLANT AND SOIL SCIENCE

  (CLF7400)   Unit Title:  SOILS

____________________________________________________________________________
  (CLF7404)   Topic:  BASIC SOIL COMPONENTS      Time      Taught in Year(s) 
                                                1 hour          3 AND 4      
____________________________________________________________________________

         Topic Objectives:  Upon completion of this lesson the student will 
         be able to:

         Learning Outcome #:

           (G-4)  -  List the four basic components of soil.

         Special Materials and Equipment: None

         References: Donahue, Roy L.  SOILS.  Pages 139 to 207.

                     Brady, Nyle C.  THE NATURE AND PROPERTIES OF SOILS.  
                         Pages 12 to 17.

         Evaluation: Quiz by instructor

TOPIC PRESENTATION:  BASIC SOIL COMPONENTS

A.  Mineral soils are made up of four major components:  mineral materials, 
    organic matter, water and air. 

    1.  In a typical soil these components exist in a close mixed condition. 

    2.  This closeness encourages interactions among the components and 
        yields a complex of variation in the environment for the growth of 
        plants. 

    3.  As the soils moisture content varies there will be wide fluctuations
        in the proportions of air and water in the soil pores.

B.  The volume composition of an average silt loam surface soil when in 
    good condition for plant growth will be: 

    45% mineral matter   }                     25% water    }
                         }  solid space                     }  pore space
     5% organic matter   }                     25% air      }

C.  The mineral (inorganic) portion is variable in size and composition. 

    1.  The rock fragments are remnants of the parent rocks from which this 
        soil was formed.

    2.  Other components such as the colloidal clay particles are so small 
        that an electron microscope must be used to see them. 

    3.  In our study of soils we normally break down the sizes of inorganic 
        particles as: 

        a.  gravel  =  very coarse

        b.  sands   =  coarse

        c.  silt    =  fine

        d.  clay    =  very fine

    4.  This particular size definitely affects properties.

D.  Soil organic matter represents an accumulation of plant and animal 
    residues.

    1.  This material is constantly being decomposed, so it must continually 
        be renewed by adding plant or animal residues or both. 

    2.  The organic matter content of soils is small compared to the other
        components, about 2 to 6% by weight of a typical loam soil.

    3.  The influence of the organic matter, is however, of great 
        significance.

    4.  Organic matter is largely responsible for the loose, easily managed 
        characteristics of productive soil.

    5.  It is the essential source of soil nitrogen and the major source of 
        sulfur and phosphorous.

    6.  It increases the water holding capacity of soils as well as 
        increasing water available to plants.

    7.  Organic matter is the main source of energy for soil organisms.

E.  Water is held in the pore spaces between soil particles. 

    1.  Soil water, with its dissolved salts, makes up the "soil solution" 
        which is the media which supplies nutrients to growing plants. 

    2.  Not all of the water in the soil is available to the growing plants.
        As the amount of water in the soil decreases, the remaining water is 
        held more tightly by small pores and as a film around the particles. 

    3.  Only about half of the water is available to plants because of the 
        mechanics of these water holding forces in the soil. 

    4.  Nutrients are exchanged between the soil solids and the soil 
        solution and then between the soil solution and the plants. 

F.  Soil air is located in the soil pores separated by soil particles.
    These pores are the passageways for the movement of oxygen needed for 
    plant roots and other organisms living in the soil. 

    1.  Soil aeration can become limiting when the soil pores are filled 
        with water or when the soil is compacted allowing smaller or fewer 
        pores.

    2.  Generally, soil air contains less oxygen and more carbon dioxide 
        than the above ground atmosphere. 

    3.  Flooding or water logging will reduce the oxygen content in the 
        soil. 

    4.  Many plants suffer when the oxygen percent in the soil drops below 
        10 to 12 percent (normal atmosphere contains 20.0 percent oxygen) or 
        the carbon dioxide percent goes up to 5.0 percent.  (Normal 
        atmosphere contains 0.03 percent carbon dioxide.)

7/24/90 sg

#%&C