- -   AGRICULTURE CORE CURRICULUM  - -


(CLF7000)  Advanced Core Cluster:  PLANT AND SOIL SCIENCE

  (CLF7400)   Unit Title:  SOILS

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  (CLF7408)   Topic:  SOIL COMPACTION            Time      Taught in Year(s) 
                                                3 hours         3 and 4
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         Topic Objectives:  Upon completion of this lesson the student will 
         be able to:

         Learning Outcome #:

           (G-7)  -  Identify four visible effects of soil compaction on 
                     plants and four effects it has on the soil.

           (G-8)  -  Define tilth.

           (G-9)  -  List two factors that contribute to soil compaction 
                     and the destruction of soil tilth.

           (G-13) -  List ways to improve soil structure.


         Special Materials and Equipment: Samples of compacted and 
                     non-compacted soils.

         References: Hudson, and Hartman.  PLANT SCIENCE.  Pages 176 to 179.
                     Donahue, Roy L.  SOILS.  Pages 57-64.

         Evaluation: Quiz by instructor

TOPIC PRESENTATION:  SOIL COMPACTION

A.  Soil compaction is a serious problem that is related to tillage, land
    preparation, and especially to the use of heavy harvesting machinery. 

    1.  Shearing action of tillage equipment is especially destructive to 
        soil structure. 

    2.  Compaction problems cannot be eliminated easily or quickly; it will 
        take time, possibly many years. 

B.  Plants are affected in several ways when they are grown on compacted 
    soils. 

    1.  Soil compaction limits root penetration and so limits the volume of 
        soil from which plant roots can take nutrients and water. 

    2.  Indirectly, this limit of soil volume can result in nutrient 
        deficiencies. 

    3.  Compaction reduces the number and size of soil pores, so reduces the 
        amount of respiration by plant roots and soil organisms.

    4.  Also decreased aeration will allow certain pathological organisms 
        that are harmful to plants to be present in the soil environment. 

    5.  All of these combined will certainly effect plant growth and reduce 
        quality.

C.  Soils are affected, also, in several ways when they become compacted. 

    1.  Compacted soil particles are pressed tightly together thus 
        reducing air space (reducing porosity) and increasing bulk density.
        Soil will develop a platy structure.

    2.  The infiltration rate of water deceases.

    3.  Since the soil is less porous, there is reduced root penetration and 
        reduced gaseous diffusion in the soil.

    4.  There is a greater accumulation of CO2 in dense soils leading to a
        possible increase of plant pathogens as mentioned above.

D.  Heavy equipment plus wet soil yields soil compaction.

    1.  When soil is wet individual soil particles are able to slide 
        together reducing the size and number of pore spaces.

    2.  The use of heavy equipment used for land preparation, tillage and 
        harvest add up to many trips across a field. 

        a.  Soil is not compacted all at once. 

        b.  The continued travel of heavy equipment over soils is 
            responsible for much soil compaction.

        c.  Sometimes many years pass before a decline in yield or quality 
            is noticeable. 

    3.  Soil compaction is easier to prevent than to correct.

E.  Compaction can be reduced by several management practices.

    1.  One needs to use tillage implements with the lightest possible 
        weight. 

    2.  Avoid unnecessary tillage. 

    3.  Keep equipment off the land when the soil is wet. 

    4.  Incorporate green manure crops, animal manures, crop residues or 
        other organic matter into the soil.


    5.  Simply leaving many loam soils alone will allow the physical 
        structure to return to its normal structural state.

    6.  On sandy soils physically tearing the surface particles apart by 
        chisels or subsoilers seems to be necessary to start them back to 
        recovery.

    7.  In orchards, the use of herbicides in strips down the tree row will 
        eliminate the need for cross disking.  Shredding of cover crops 
        instead of disking will also help.  (In both instances less 
        equipment passes over the soils - however the use of herbicides may 
        be more expensive and less enviornmentally sound.)

    8.  The degree of compaction should determine the method used.  No one 
        method will work everywhere. 

F.  Soil tilth is the physical condition of a soil related to its ease of 
    tillage, fitness as a seedbed, and suitability for plant growth.

    1.  Tilth includes the soil's moisture condition. 

    2.  Continuous tillage will destroy desirable soil structure. 

    3.  Permanent cover will in time improve soil structure. 

    4.  A grower usually makes more money from cultivated crops than from 
        pastures. 

        a.  There has to be a compromise between a continuous cover that 
            improves structure and continuous tillage that reduces 
            structure. 

        b.  By using approved practices we can raise the most profitable 
            crops with minimum damage to soil structure. 

          __________________________________________________________
          ACTIVITY:  
                      
          1.  Bring to class examples of soil compacted by heavy 
              equipment and traffic.  Observe the reduction in 
              pore spaces and lack of desirable structure. 

          2.  Have the class dig a trench across a farm lane or 
              other dirt road and observe the location of the 
              compacted layer and the lack of plants growing above
              them.
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7/25/90 sg

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