growing medium - the substrate in which plants grow.  Usually applied to manufactured or 
                        symthetic soils, i.e. "potting soils", or highly amended soils, ex. landscape beds. 
 

FUNCTIONS OF SOIL OR GROWING MEDIUM
1) Support and anchorage - allows the plant to anchor in place

2) Supplies mineral nutrients - virtually all nutrients are absorbed from soil

3) Supplies water - virtually all water is absorbed from soil 

4) Allows gas exchange - roots are alive, so the soil must be porous enough to allow gas exchange,
                                         especially 0₂ and CO₂, but also ethylene

A) Organic soil - contain 20% or more organic matter

     2 types
     1) peat soil - contains greater than 65% organic matter; northern peat bog, cranberry farming

     2) muck soil - contains 20-65% organic matter; very dark and more decomposed
 
B) Mineral soil (field soil) - contains less than 20% organic matter; most common farmland

     4 Major Components (in a well watered, but well drained loam soil)

     1) air- approximately 25% of volume; in larger pores

     2) water - approximately 25% of volume; in smaller pores

     3) mineral particles - 44-49% of volume
 

         litter - partially decayed organic matter on the soil surface. 

         humus - highly decomposed, fine, amorphous organic matter in the soil.

         Functions of Organic Matter:
         1) stabilizes soil structure
         2) increases water retention and availability
         3) increases drainage and aeration
         4) increases cation exchange capacity
         5) supplies nutrients upon decay (only if low C:N ratio)
         6) stabilizes pH
         7) food source for microorganisms 

    2) Chemical- relatively inert; results in:
        a) little effect on soil chemistry and pH 
        b) poor nutrient holding capacity (i.e. CEC)

    3) Pore Space
        a) less total pore space
        b) more large (macro) pores, fewer small (capillary) pores; thus sand causes:
            1) increased aeration
            2) increased drainage
            3) decreased water holding capacity

SILT - intermediate chemical and physical properties between sand and clay 

CLAY
    1)Physical - structurally complex
        a) colloidal - sub-microscopic and held in suspension in solution
        b) when wet - viscous and gelatinous, sticky; when dry - hard, packed and cohesive
        c) composed of micelles = flat, sheet-like plates laminated into stacks; like expanded mica vermiculite
        d) very large internal and external surface area 
        e) very small internal and external pores

    2) Chemical - very complex; negatively charged
        a) cations attach to and neutralize the negative charges.  Roots exchange and absorb these cations.
        b) very high cation exchange capacity (CEC); hence, nutrient holding capacity
        c) charge allows flocculation (aggregation) or de-flocculation (spread-out)
            1) Calcium Ca promotes flocculation of soil particles, and good soil structure
            2) Sodium Na promotes de-flocculation of soil particles, and good soil structure

    3) Pore Space
       a) greater total pore space
        b) more small (capillary) pores; fewer large (macro) pores; thus clay causes:
            1) decreased aeration 
            2) decreased drainage
            3) increased water holding capacity
            4) but not all water is available 

TYPICAL AGRICULTURAL SOIL
An agriculturally productive soil is a balanced mixture of sand, silt, and clay. For example, a typical loam soil is composed of 40% sand, 40% silt, and 20% clay. This yields a balance between aeration vs. drainage vs. water holding capacity vs. fertility. 

percent base saturation - % of total CEC occupied by basic nutrients, such as Ca⁺², Mg⁺², K⁺,
                                         and/or Na⁺ (as opposed to acidic H⁺ and Al⁺³) 
 
pH- negative log of the hydrogen ion concentration;    pH  =  -log [H⁺]  =  log 1/[H⁺]

pH is a measure of the acidity of a solution.  Below pH 7 is acid, pH 7 is neutral, and above pH 7 is basic.  pH is seldom high or low enough to directly effect plants. Its major effect is on the solubility of nutrients in the soil solution, i.e. nutrient availability. 
 

Nutrients That Are More Available at
Low pH (below pH 5.5)	Intermediate pH (pH 6-7)	High pH (above pH 6.5 )
Fe, Zn, Cu, Mn, B	P	N, K, Mg, Ca, S, Mo

B) Basic or Alkaline Soils - soils with basic pH; in arid regions
    3 Types:
    1) saline soil - pH 7-8.5, and has greater than 2,000 ppm total soluble salts.
    2) sodic soil- pH 8.5-10, low to moderate total salts, but 15% or more of CEC is occupied by Na.
    3) saline-sodic soil- pH 8-8.5, greater than 2,000 ppm total soluble salts and 15% or more of 
                                    CEC occupied by Na.

    How to improve saline, sodic or saline-sodic soils
    a) leach - application of large volumes of water to remove excess soluble salts.
    b) add elemental sulfur (S) - acidifies the soil
    c) add gypsum (CaSO4) - Ca promotes good soil structure, drainage and Na leaching

    a) decreases total pore space, esp. large pores

    b) decreases aeration

    c) decreases drainage

2) must sterilize for disease and weed seeds

3) heavy - increases shipping costs and heavy to manually handle

4) hard to obtain good supply in many areas
 
GROWING MEDIA
Most container production uses specially prepared mixes called growing media, artificial media, soilless media or potting soils, which are made from various organic and inorganic growing medium amendments.  The highest quality peat moss is sphagnum peat moss.  All the organic amendments, except peat moss, must be composted before use. 

Typical Growing Medium Should Contain:
50-75% organic amendments - usually sphagnum peat moss, composted bark or coir 
25-50% inorganic amendments - usually vermiculite, perlite, sand or styrofoam
plus: lime, starter fertilizer and sometimes a wetting agent and gypsum

2) hygroscopic water - water adsorbed onto soil particles, held at less than -31 bars of tension;
                                     plants cannot utilize.

3) capillary water - water held by capillary attraction in the capillary pores in soils; held at  -1/3 
                               to -31 bars; plants can extract water in the larger capillary pores down to
                               approximately -15 bars.

4) gravitational water - water in large pores immediately after watering or a rain, which drains
                                     from the soil (within 24 hr.) by the force of gravity; held at greater 
                                     than -1/3 bars (0 to -1/3 bars); plants can utilize when present. 

SOIL WATER TERMINOLOGY
field capacity - the amount of water a soil can hold against the force of gravity; 
                      - water is held at -1/3 bars at field capacity.

wilting- the loss of plant turgidity due to excessive water loss.

incipient wilting (go to 1 min) - when a plant wilts, but recovers when placed in a saturated atmosphere
                             (100% R. H.), ex. overnight.

incipient wilting point - the soil moisture content when a plant wilts, but recovers when placed
                                      in a saturated atmosphere (100% R. H.), ex. overnight.

permanent wilting - when a plant wilts, but cannot recover when placed in a saturated 
                                atmosphere (100% R.H.).

permanent wilting point - the soil moisture content when a plant wilts, but cannot recover
                                         when placed in a saturated atmosphere (100% R.H.). 

HOW TO DETERMINE WHEN TO WATER PLANTS
1) Based on soil
    a) experience - feel, looks, etc.
    b) measure soil water potential, usually use a tensiometer - an instrument that is inserted
        in the soil and measures the soil moisture tension.  Can automate an irrigation system.

2) Based on plant
    a) experience - wilting, looks, etc.
    b) measure plant water potential 

Mulches are almost always beneficial to use, and their use is highly recommended.  In nature, the soil under plants is covered by a natural mulch of composting litter (leaves, twigs, etc.)

TYPES OF MULCHES

1) organic - bark, sawdust, leaves, grass clippings, pine needles, straw, hay, municipal compost, newspaper 

2) inorganic - plastic (black or white, clear only for solarization), gravel and rock  

BENEFITS AND USES OF MULCHES

1) stabilizes soil temperature - cooler in summer; warmer in winter under a mulch

2) conserves water - decreases evaporation of water from soil surface

3) better water infiltration - more rain or irrigation water soaks-in due to slower runoff

4) controls erosion - due to slower runoff

5) may add nutrients - upon decomposition, if it is a) organic and b) has a low C:N ratio

6) decreases weed growth - decreases germination of weed seeds and growth of weed seedlings

7) appearance - used for decorative purposes