- -   AGRICULTURE CORE CURRICULUM  - -


(CLF7000)  Advanced Core Cluster:  PLANT AND SOIL SCIENCE

  (CLF7500)   Unit Title:  SOIL TILLAGE, LAND PREPARATION, AND CONSERVATION

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  (CLF7503)   Topic:  CROP RESIDUES AND          Time      Taught in Year(s) 
                      GREEN MANURES             2 hours         3 and 4      
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         Topic Objectives:  Upon completion of this lesson the student will 
         be able to:

         Learning Outcome #:

           (I-7)  -  Explain the advantages and disadvantages of 
                     incorporating crop residues or green manures 
                     into the soil.

         Special Materials and Equipment: Test plot sites.

         References: Donahue.  SOILS.  Pages 139 to 162.
                     Brady.  THE NATURE AND PROPERTIES OF SOILS.  
                       Pages 546-550.

         Evaluation: Quiz by instructor

TOPIC PRESENTATION:  CROP RESIDUES AND GREEN MANURES

A.  The organic matter content of the soil has a pronounced effect on its 
    biological, chemical and physical properties. 

    1.  Through decomposition of organic matter chemical elements become 
        available to our crop plants.

    2.  Soil organic matter is the nitrogen reservoir, and furnishes a 
        large portion of the phosphorous and sulfur. 

    3.  Since historic times turning under of a green crop to improve soil 
        conditions has been a common agricultural practice.

    4.  Stubble, roots and other crop residues are probably the most 
        important source of soil organic matter.

B.  Crop residues are simply defined as the portion of crop plants remaining 
    after harvest.

C.  Green manures are crops that are plowed under while still green and 
    growing to improve the soil. 


D.  There are many benefits of having organic matter in the soil. 

    1.  Organic matter (o.m.) is the source of 90 to 95 percent of the 
        nitrogen in unfertilized soils. 

    2.  It is the source of phosphorus and sulfur when o.m. is present at or 
        over two percent. 

    3.  Through microbial action o.m. supplies indirectly the soil aggregate 
        forming cements (polysaccharides).

    4.  O.m. contributes to the cation exchange capacity often furnishing 30 
        to 70 percent of the total amount.

    5.  O.m. is a carbon supply for many microorganisms that perform 
        functions such as denitrification.

    6.  It increases the water holding capacity, available water content,
        and increases both water and air flow through the soil.

    7.  O.m. acts as a chelate which increases the availability of metallic 
        ions such as iron, zinc, copper and manganese. 

    8.  When left on the soil surface o.m. reduces erosion, shades the soil 
        preventing moisture loss, and keeps the soil cooler in warm weather 
        and warmer in cool weather. 

E.  Soil organic matter from crop residues or green manure also has some 
    detrimental effects. 

    1.  Organic material is a source of food and energy for many plant 
        disease organisms. 

    2.  Excessive amounts of o.m. are difficult to incorporate into a soil 
        and hinder planting operations. 

    3.  Some plants produce phytotoxins (plant poisons; i.e., Juglonee from 
        black walnuts). 

    4.  Nonlegumes when used for green manure crops may depress the nitrogen 
        uptake in the following crop planted.

F.  There are several desirable characteristics of a green manure crop.

    1.  A green manure crop should be easy to establish and grow rapidly. 

    2.  It should produce an abundant growth of tops and roots in a short 
        time.  One should actually select the specie and variety that will 
        produce the maximum green tonnage. 

    3.  It should have the ability to grow on poor soil because this is 
        where the most benefits are needed. 

    4.  When other conditions are equal, a legume should be selected over a 
        nonlegume because of its nitrogen producing ability. 


    5.  Sometimes the growing of two crops together is recommended.  Larger 
        yields can be produced. (e.g., oats or rye and vetch)

    6.  It should "turn under" easily.

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          ACTIVITY:  
                      
          1.  Demonstrate several soil plots using various green 
              manure crops and crop residues.  Disc or spade them
              under and make observations. 

              a.  Note the length of time it takes for the organic
                  matter to break down. 

              b.  Does the amount of soil moisture at the time of 
                  incorporation effect the speed of break down?

              c.  How does the addition of nitrogen fertilizer to
                  the soil plot effect breakdown? 

              d.  How does temperature affect decomposition?

              e.  Is plowing or discing the best way to incorporate
                  a green manure crop?  Why?
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6/21/90 sg

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