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Soils and Soil Moisture

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					Fig. 21.1, Cain et al. (p. 454)
                                  Soils and Soil Moisture
Soil—
“The collection of natural bodies occupying parts of
the earth’s surface that support plants and that have
properties due to the integrated effect of climate and
living organisms acting upon parent material, as
conditioned by relief, over periods of time.”
 —N.C. Brady, in The Nature and Properties of Soils, 9th ed.
                                          The general makeup of soils




Fig. 4.8, Smith & Smith 7th ed. (p. 64)
Soil porosity—
 • Definition
 • What’s in the soil pores?
 • How much pore space is there?
 • Determinants of air and water in pore volume
 • Why is air essential to plants?
    And how does it get into the soil?
Soil texture—
  What determines soil texture?
  Particle sizes:
        • Gravel: >2.0 mm
        • Sand: 0.05-2.0 mm
        • Silt: 0.002-0.05 mm
        • Clay: <0.002 mm (microscopic)
Importance of soil particle size—
    Critical in determining soil physical properties.
    Clay has much higher surface:volume ratios (due to small
   particle size) than larger particles.
Influence of particle size on surface:volume ratio
  Fig. 6.7, Campbell & Reece, 7th ed.
Importance of soil particle size—
    Critical in determining soil physical properties.
    Clay has much higher surface:volume ratios (due to small
   particle size) than larger particles.
    Electrostatically negative charges on clay micelles permit
   stronger adsorption of water and nutrients, reducing leaching,
   especially of cationic nutrients (e.g., NH4, Ca, Mg, K, Na, Cu,
   Mn, Zn, Fe all have + charges)
    Too much clay results in reduced pore sizes
    Sand—with its larger particle sizes—has characteristics
   opposite to those of clay
    Silt is between clay and sand in size and traits
    Best agricultural soils are classed as loams, varying broadly
   around a 40:40:20 ratio of sand, silt, and clay, respectively
Parent material—
  “The physically and chemically weathered mineral matter
  from which the mineral portion of a soil is derived.”

  Possible origins:
    • Residual parent material: originates from weathering of
      underlying bedrock
    • Transported parent material
Macronutrients, micronutrients, and
         cation exchange
Macronutrients—
     Generally >0.1% of plant tissue dry weight

    C, H, and O—the basic constituents of organic
     molecules—make up about 96% of plant dry weight.
     What types of biological molecules are composed
     largely of these elements?

     Other macronutrients include N, P, K, Ca, S, Mg
                                            Their functions
                                                              Macronutrients—




Table 6.1, Smith & Smith 7th ed. (p. 120)
Micronutrients—
   Usually <0.01% by weight
   Nonetheless, they’re essential
   Often serve as cofactors, facilitating enzyme activity
   E.g., Cu, Mn, Zn, Fe
Using hydroponic culture to identify essential plant nutrients



        Fig. 37.2, Campbell & Reece (6th ed)
Identifying essential plant nutrients—
       Magnesium deficiency in a tomato plant




                                      Fig. 37.3, Campbell & Reece (6th ed)
Cation exchange capacity (CEC) of soil—
    Capacity of the soil to adsorb cations
    Strength of cation retention depends on number of
   “exchange sites” and the intensity of the charges on them.
    Largely determined by clay and humus content
    Clay micelles have numerous negative charges. Because
   of the tiny size of the clay particles and the resulting
   small soil pore sizes, clayey soils tend to a high CEC.
    CEC is affected significantly by soil pH. Why?
Plant roots use cation exchange to assist in nutrient uptake from
the soil—
   (a) Soil moisture surrounding the roots
   (b) Absorption of soil mineral nutrients by cation exchange




                                                   Fig. 37.6, Campbell & Reece (6th ed)
Absorption and leaching of soil nutrients in the soil solution




                                         Fig. 4.11, Smith & Smith 7th ed. (p. 66)
Cation exchange on soil particles—effects of soil pH




                                                 Fig. 5.5, Smith & Smith 5th ed. (p. 86)
Soil moisture and water uptake by plants—
Gas exchange and the uptake of water and nutrients by
a plant: an overview




          Fig. 37.1, Campbell & Reece (6th ed)
Determinants of soil moisture availability—
  Fig. 4.18, Cain et al. (p. 97)
Soil moisture and water uptake by plants—
    Water moves in response to a gradient of water potential,
   from higher water potential to lower water potential.

   Net water potential in the plant is determined by

       (a) osmotic pressure—due to cross-membrane differences in
       solute concentration,

       (b) water pressure—due to atmospheric pressure, and

       (c) the transpiration stream—which pulls water up through
       xylem tissues, based on transpiration from leaves and the
       adhesion and cohesion properties of water.
Water movement through a plant—
     the “transpiration stream”




                                  Fig. 6.3, Smith & Smith (5th ed), p. 105
A stomate functions in gas exchange—
Stomatal openings respond to environmental cues to
control gas exchange rates—
            Open                                  Closed




                        Fig. 36.14, Campbell & Reece, 7th ed. (p. 749)
Ascent of water in a plant—the transpiration stream,
driven by a water potential gradient



        Fig. 36.11, Campbell & Reece (6th ed)
Ascent of water in a plant—the transpiration stream,
driven by a water potential gradient



        Fig. 4.20, Cain et al. (p. 98)
The diffusion of solutes across membranes




                              Fig. 8.10, Campbell & Reece (6th ed)
Osmosis—a lab demonstration




                              Fig. 8.11, Campbell & Reece (6th ed)
The water balance of living cells




                                    Fig. 8.12, Campbell & Reece (6th ed)
Turgor pressure in plant cells




                                 Fig. 4.19, Cain et al. (p. 97)
A wilted tomato plant regains its turgor when watered




Fig. 36.5, Campbell & Reece (6th ed)
Fig. 5.4, Smith & Smith (5th ed), p. 84
                                          Soil moisture
Biotic factors in soil—Organic matter
  Consists of plant and animal materials in the soil in various
  stages of decomposition.
  Varies widely in texture and chemical composition from litter
  (coarse, freshly deposited dead material, e.g., leaves, dead
  insects and other animals, animal wastes) to humus (the more or
  less stable portion that remains after most of the decomposition
  has occurred).
  Humus is crucial for water and nutrient retention, behaving
  similar to clay.
Biotic factors in soil—Resident soil organisms
  Detritivores—Important in breakdown of organic matter to
  release nutrients in a form that is usable by living plants. Their
  types, abundances, and activity levels determine turnover rates
  for nutrients. What factors would affect the activity levels of
   the detritivores?
  Mycorrhizal fungi—symbiosis between fungi and plant roots
  Nitrogen-fixing bacteria—either symbiotic or free-living
  Other soil organisms: soil mixers, root parasites, pathogens, etc.
Mycorrhizae, symbiotic associations of fungi and roots

         Fig. 36.10, Campbell & Reece, 7th ed. (p. 745)
N-fixing bacteria: Symbiotic associates with the roots of legumes




                     Fig. 37.10, Campbell & Reece, 7th ed. (p. 765)



                                                                      Nodules




                                                                        Roots
The decomposers—a community of heterotrophs




                                         Fig. 5.8, Smith & Smith (5th ed), p. 91
Soil horizons




                Fig. 37.5, Campbell & Reece, 7th ed. (p. 760)
Soil horizons




                Fig. 21.4, Cain et al. (p. 458)
Fig. 5.9, Smith & Smith (6th ed), p. 94
                                          A generalized soil profile
The Return of ClORPT—
  How the soil profile develops
        Climate
        Organisms
        Relief
        Parent material
        Time

				
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