Topic 7.  SOIL AND GROWING MEDIA

I. Introduction


    1. Functios of Soil

          -  Plant anchorage
          -  Provides water to plants
          -  Supplies mineral nutrients to plants

     2. Four Components of Soil
         - mineral
            - organic matter
            - water
            - air

    3. The Functional Phases of Soil

        a.  Solid phase - soil particles (clay, sand, silt, etc.)
          b.  Gas phase - provides oxygen
           c.   Liquid phase - supplies water
 


 
 

Field Capacity  -  the soil moisture condition obtained when all gravitational water is drained from the soil after field saturation

Container Capacity - same as field capacity for potted soil used in greenhouse
 

    4. Soil Texture     - Size of individual mineral particles
    - Varies by presence of sand, silt, clay, loam


            Heavy soil - high in clay and other fine particles
            Light soil - low in clay and high in sand and coarse particles

 5.  Soil Structure

- Arrangement of soil particles into aggregates formed by flocculation and granulation
  6.  Exchange Capacity

       a. Cation Exchange Capacity (CEC)
 

- The ability of soil particles to absorb and store cations (measured in meq/100g soil)
      Cation  =  positively charged ion (Ca2+, Mg2+, K+, NH4+)

                        Clay and organic particles have high CEC.
     

   b.  Anion Exchange Capacity (AEC)
    - The ability of soil particles to absorb (adsorb) and store anions (measured in meq/100g soil)

      Anion =  negatively charged ion (NO3-, SO42-, Cl-)

                        Most soils have little or no AEC


     
     

    Strength of cation replacement

                H+ > Ca2+ > Mg2+ > Mg2+ > K+ > Na+
     
     

II.   Soil Reaction (pH)

                                 pH  =  - log [H+]

     

     
1.  Definition  -  soil acidity or alkalinity expressed in pH

2. Why is optimum soil pH important?

    a.  Nutrient availability

                Deficiency, toxicity can be avoided

            - Fe deficiency at high soil pH
            - Al toxicity at low soil pH

    b.  Microorganism activity

                Needed for decomposition of organic matter (OM)

    c.  Nitrogen fixation and nitrification

                                                  Ammonifying nitrifying bacteria
                Actinomycetes            Nitrosomonas                 Nitrobacters
    OM  ------------------------ NH4+ ---------------------- NO2- ------------------- NO3-
     

3. The soil pH optimum
    a.  Most plants prefer  5.5 -7.0

    b.  Acid loving plants
     

      Blueberry - prefers a pH range from 4.0 to 5.5
      Azalea  -  prefers a pH range from 4.5 to 5.5

      Hydrangea
              - Flower color turns blue at pH 4.5 - 5.5
              - Flower color turns pink at pH 6.5 - 7.5
       

    c.  Alkalinity tolerant plants

            Asparagus - prefers a pH range of 6.5 to 7.9
            Alfalfa  - prefers a pH range of 6.5 to 7.7
            Sugarbeet - prefers a pH range of 6.5 to 7.7
     

4.  Adjustment of Soil pH
    a.  To raise soil pH, use:
            - ground limestone
            - dolomitic lime (mix of CaCO3 + MgCO3)
            - gypsum (CaSO4)

    b.  To lower soil pH, use:
            -  sulfur powder (S)
            -  aluminum sulfate (Al2 (SO4)3)

    -  iron sulfate (FeSO4)      Also for solutions, use:
            - sulfuric acid (H2SO4  ----->  2H+ + SO4-2)
            - phosphoric acid (H3PO4 -----> 3H+  + PO4-3)
            - nitric acid (HNO3 ----->  H+ +  NO3-)
IV. Synthetic Soils (Artificial Mixes) 1.  Soil Amendments             a.  Inorganic components

                Sand -  low water-holding, low CEC, heavy, size varies
                             inert medium

                 Vermiculite - expanded mica mineral
                              high H2O holding, good CEC,  high buffering

                 Perlite - heated, popped volcanic rock, inert
                              very light, no CEC, no buffering, no nutrient holding

                Calcined clay- Baked montmorillonite clay
                             aggregate particles, heavy, durable high CEC

                Pumice - crushed volcanic rock, inert
                             low waterholding, low CEC

            b.  Organic components

Peatmoss - 75% decomposed
                 low in pH, high in CEC, high waterholding
                Crop residues -Straw, peanut hull, dry foliage, etc.
                                  high in C, add N when decomposting

               Bark or sawdust - redwood, pine, fir, etc.
                                phenolic compounds phytotoxic, wait until decomposted

2.  Mixes Containing Soil

    a.  For heavy soils, use:

                1 part clay loam
                2 parts organic matter
                2 parts coarse aggregate

    b.  For medium-textured soil,s use:

               1 part silty loam
                1 part organic matter
                1 part coarse aggregate

    c.  For light soils, use:

               1 part sandy loam
                1 part organic matter

For all mixes, use base fertilizers:
Limestone -  6 to 8 oz/bushel
Superphosphate - 8 to 10 oz/bushel
    4.  Commercial Soil Mixes (used for nursery crops)

            a.  UC-Mixes
 
 

UC Mix  Percent sand  Percent peat 
Base fertilizers 
Mix A 
100 
0 
yes 
Mix B 
75 
25 
yes 
Mix C 
50 
50 
yes 
Mix D 
25 
75 
yes 
Mix E 
0 
100 
yes 
   b.  Cornell Mix (Soilless Mix)

        Peat-lite mix

               - Extensively used for greenhouse crops
                - Most commercial mixes are based on Cornell Mix formula
                        (e.g., Sunshine Mix, Pro-Mix)

               Mix A
1 part peat (sphagnum)
1 part vermiculite
plus base fertilizers
        - ground limestone
        - super phosphate
        - calcium nitrate
        - calcium nitrate
        - trace elements, and
        - wetting agent
               Mix B
1 Part sphagnum peat
1 Part perlite
plus base fertilizers
Mix C
2 parts peat (sphagnum)
1 part vermiculite
1 part perlite
Plus base fertilizers