Soils Chapter #3 - Soil Liquids & Gasses




Water participates in numerous chemical reactions that:

  • release or tie up nutrients
  • create acidity
  • wear down minerals so their constituent elements eventually contribute to the saltiness of the oceans



Certain soil-water phenomena seem to contradict our intuition



Soil-water interactions determine

  • rates of water loss by leaching
  • surface runoff
  • evapotranspiration
  • the balance between soil and water in soil pores
  • the rate of change in soil temperature
  • the capacity of soil to store and provide water for plant growth



Structure & Related Properties of Water



  • instead of lining up symmetrically on either side of the oxygen atom    ( H - O - H ), the hydrogen atoms are attached to the oxygen in a V-shaped arrangement at an angle of 105o.



Example of Covalent Bond:



     Water molecule showing a large oxygen
     atom and much smaller hydrogen atoms






Hydrogen Bonding



polarity causes water to be attracted electrostatically to ions and colloidal surfaces

Cations** (such as H+, Na+, K+ and Ca2+) become hydrated through their attraction to the oxygen (negative) end of water molecules

Negatively charged clay surfaces attract water through the hydrogen (positive) end of the molecule






Cohesion vs. Adhesion

Hydrogen bonding accounts for two basic forces responsible for water retention and movement in soils:

  • cohesion
  • adhesion (adsorption)














Surface Tension






Capillary Action

Adhesion (absorption) and surface tension cause capillary action


Hydrophilic: water loving

Hydrophobic: water hating






Height of Rise in Soils

Capillary forces are at work in all moist soils







Soil Air

Composition (O, N, CO2)

  • proportions can vary greatly caused by
    • plant root respiration
    • microbial respiration
    • saturation by water
    • contamination
    • soil compaction:
    • relative humidity: almost always 99-100%


Air Movement: two mechanisms move air:

  • diffusion
  • mass flow


Air - Water Relationships

Porosity is equal to the volume occupied by pores divided by the total soil volume.

  • total pore volume = volume of air + volume of water
  • total soil volume  = volume of air + water + solids
  • porosities generally vary from 30-60%


Soil Water

Pore Water Composition: Not pure, contains solutes including:

  • cations+ & anions- including:
    • Ca2+
    • Mg2+
    • K+
    • Cl-
    • NO3-: Nitrate
    • SO42-: Sulfate
    • H2PO4-: dihydrogen phosphate ion from Phosphoric Acid
    • HPO42-: a variation of Phosphoric Acid
    • HCO3-: bicarbonate, important in pH
    • H+
    • NA+
    • NH4+:Ammonium,
    • Mn2+
    • Al3+



Soil Water Amounts

The amount of soil solution (soil water) affects:

  • the composition of the soil solution
  • that rate at which water flows
  • the availability of water for plants





Oven dry- almost completely dry

Intermediate moisture states:

  • field capacity - max water soil can hold
  • permanent wilting point - water present but plants can't get it