page 67

FUNCTIONS OF SOIL AND SOIL PROFILE

DEFINITIONS
soil - the outer weathered layer of the earth's crust. 
growing medium - the substrate in which plants grow.  Usually applied to manufactured or 
                              synthetic soils, i.e. "potting soils", or highly amended soils, ex. landscape 
                              beds. 

FUNCTIONS OF WATER OR  GROWING MEDIUM
1) Support and anchorage 
2) Supplies mineral nutrients 
3) Supplies water 
4) Allows gas exchange - especially 02 and CO2, but also ethylene 
 

SOIL ORGANIZATION
soil profile - morphology of horizons (layers) in a soil. 
A Horizon or topsoil
- highly weathered 
- abundant life, therefore, high in organic
   matter
- dark colored 
   plow pan - a compacted impermeable
                        layer in the A horizon due to 
                        repeated plowing or tilling 
                        (approx. 6" deep)
B Horizon or subsoil
- less weathered; higher in clay 
- less life, therefore, low in organic matter 
- lighter colored 
   clay pan - impermeable layer high in clay. 
   hard pan - impermeable layer high in iron.
C Horizon or parent material
- ittle weathered 
- little life, except deep rooted plants and
  little to no organic matter
D Horizon or bedrock
- rock base

page 68

SOIL COMPOSITION

TYPES OF SOIL BASED ON COMPOSITION
A) Organic soil - contain 20% or more organic matter 
    2 types
    1) peat soil - contains greater than 65% organic matter 
    2) muck soil - contains 20-65% organic matter 

B) Mineral soil (field soil) - contains less than 20% organic matter. 
    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 

 

Mineral Particle

Size

sand

0.05-2 mm

silt

0.002-0.05 mm

clay

<0.002 mm

    4)organic matter - typically about 1% in nature 

        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 


page 69

PROPERTIES OF SAND Vs. SILT Vs. CLAY

SAND
    1)Physical - structurally simple; relatively unweathered, physically broken down parent material
    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 
        d) very large internal and external surface area 
        e) very small internal and external pores 
    2) Chemical - very complex; negatively charged 
        a) very high cation exchange capacity (CEC); hence, nutrient holding capacity
        b) charge allows flocculation (aggregation) or de-flocculation (spread-out) 
            1) Ca+2 promotes flocculation of soil, and good soil structure
            2) Na+ promotes de-flocculation of soil, and poor 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. 


page 70

SOIL CHEMISTRY

cation exchange capacity (CEC) - milliequivalents per 100 grams dry soil; meq/100 g.

Component

CEC (meq/100 g)

sand

2-4

silt

4-10

clay

10-100

organic matter

150-300

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 

    How to Increase or Decrease Soil or Growing Medium pH

    Chemicals that Increase pH

    Chemicals that Decrease pH
    • calcitic lime - CaCO3 
    • dolomite - CaCO3 /MgCO3 
    • hydrated lime - Ca(OH)2 
    • burned lime - CaO  
    • basic fertilizers (nitrate) 
    • elemental sulfur -S  
    • aluminum sulfate - Al2(S04)3 
    • iron sulfate - FeSO4  
    • acidic fertilizers (urea, ammonia, ammonium) 

TYPE SOILS BASED ON CHEMICAL PROPERTIES
A) Acid Soils - soils with acid pH; in regions of high rainfall 

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 removes excess soluble salts. 
    b)add elemental sulfur (S) - acidifies the soil 
    c)add gypsum (CaSO4) - Ca promotes good soil structure, drainage and Na leaching


page 71

GROWING MEDIA FOR CONTAINERS

WHY PURE FIELD SOILS ARE NOT USED IN CONTAINER PRODUCTION
1) packs too much, which 
    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 orpotting 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. 
 

Organic Amendments

Inorganic Amendments

peat moss
bark
coir
sawdust or wood shavings
cedar chips
rice hulls
bagasse
cotton gin trash
municipal compost or processed sludge

sand
vermiculite
perlite
styrofoam
calcined clay or haydite
rice hull ash
rock wool

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


page 72

SOIL MOISTURE

FORMS OF SOIL MOISTURE (1 bar = 0.987 atm) 
1) chemically combined - occurs as a water shell around compounds and particles in soil; 
                                         plants cannot utilize. 
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; 
                        - at field capacity, water is held -1/3 bars. 
wilting- the loss of plant turgidity due to excessive water loss. 
incipient wilting - 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. 
2) Based on plant 
    a) experience - wilting, looks, etc. 
    b) measure plant water potential 


page 73

MULCHES

mulch- any material applied to the surface of the soil or growing medium 

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, leaves, sawdust, straw, hay, needles, paper 
2) inorganic - plastic, gravel 

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) mayadd 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 & growth of weed seedlings 
7) appearance - used for decorative purposes 


Go to:  Table of Contents | Introduction | Anatomy | Physiology | Hormones | Temperature |
| Light | Water | Soil | Nutrition | Propagation | Pruning | Pests |
Page: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100