Soils 205- General Soils

Lecture 19- Soil Reaction and trends

I.  Soil reaction- How acid or alkaline is the soil

    a.  common range: 

    b.  pH:

    c.  Al and soil acidity:

    In alkaline soils (pH > 7.3) Al is no longer on the exchange sites:

    d.  Types of soil acidity

    i.  Active acidity-

    ii.  Exchangeable acidity-

    iii.  Residual (reserve acidity)-

II.  Buffering:

 a.  buffering capacity: The coffee pot analogy

III.  Trends in pH

    a.  natural changes:

    1)  weathering releases base cations form minerals

                Humid regions                                                arid regions

    2)  formation of carbonic acid:

        CO₂ + H₂O --> H₂CO₃  -->  HCO₃^- + H⁺

    3)  decomposition of organic matter

    4) oxidation of nitrogen and sulfur

    5)  acids in precipitation:  The pH of "natural" rain in equilibrium with CO2 is 5.6.  Also there are various natural sources of S and N (volcanic eruptions, lightning, forest fires, etc) in the atmosphere which form strong acids (H₂SO₄ and HNO₃ ) which dissociate producing hydrogen ions.

                                H₂SO₄ ^--> H⁺ + SO₄^2-

   b.  Human induced changes

    1)  Chemical fertilizer usage:

    2)  Acid rain- increased use of fossil fuels has resulted in rain events with pH values between 4 and 4.5. 

    3)  Disposal of acid-forming wastes:

    4) Irrigation

    5) Drainage of some costal wetlands:

IV.  Higher plants and pH

    a.  pH tolerance

    1)  strongly acid soils (pH = 4-5)

    2)  Moderately acid soils (pH = 5-6)

    3)  slightly acid to slightly alkaline soils (pH = 6-7+)