Outline:

  1. Because soils have negative charge they are able to hold positively charged cations.

    The soil solution consists of a water layer around the soil particle and water in the micropores.

    There are cations attached to the colloids and in the soil solution.

    Cation exchange takes place when one of the cations in the soil solution replaces one of the cations an the soil colloid.

    This exchange only takes place when the cations in the soil solution are not in equilibrium to the cations on the soil colloid. This is almost always the case

    Leaching, fertilizer addition, plant removal, etc.. all keep this system from remaining static.

  2. Cation Exchange Capacity

    • the ability or capacity of a soil colloid to hold cations.
    • this is directly dependent on the amount of charge on the soil colloid.


    The amount of cations a soil can hold is dependent on:

    The amount of cations a soil can hold is called its CATION EXCHANGE CAPACITY (CEC)
    It is expressed as milliequivalents per 100 g soil (meq/100g)

    A Cecil Ap horizon CEC = 5 meq/100g soil
    A Norfolk Ap horizon CEC = 2 meg/100g soil
    • Sandy soils with little organic matter have low CEC
    • Clayey soils with high organic matter have high CEC

    Type of colloid CEC
    1:1 clay (kaolinite) 3-15 meq/100g soil

    2:1 clays

    Montmorillonite

     

    100 meq/100g soil

    Illite

    10-40 meq/100 g soil

    Vermiculite

    100-150 meq/100 g soil
    Organic colloids (humus) 200-300 meq/100 g soil
    Fe and Al oxides very small, if any charge

Some interesting trivia about clay. Geophagy

and Quicksand

Remember that a meq of any cation is that amount of cation required to replace 1 meq of another cation.
Question then is how many meq of Ca++ would it take to replace 1 meq of H+ ?

The answer is 1 meq

How many grams of Ca does it take to replace 1meq of H+ ?


1 eq of Ca = 40g/2 = 20 g to 1 eq or 0.02 g to 1 meq
1 eq of H = 1g/1 = 1 g to 1 eq or 0.001 g to 1 meq

Therefore it would take 0.02 g of Ca to replace 0.001 g of H
Remember that 1 meq of anything equals 1 meq of anyother thing.

Additional Examples

    1. If a soil had 2 meq of Ca then it would have 2 * .020 = 0.040g Ca

      X / 2,000,000 = 0.040 g / 100 g

      X = 800 lbs/acre.
    2. How many lbs/acre of Ca would it take to replace 1 meq/100g of H+?

      It would take 1 meq/100g of Ca equivalent weight.

      40 / 2 = 20

      meq wt = 0.020g

      X / 2,000,000 = (0.020g * 2000) / 100g

      X = 400 lbs of Ca.
  1. Factors Affecting CEC
    1. Amount of Clay (% clay in the soil)
    2. Type of clay present
    3. Organic matter content
    4. Hydrous oxides of Fe and Al
    5. pH

  2. Soil Texture

    The amount of clay, which is the colloidal fraction, helps determine the CEC.

    As the clay increases the cation exchange capacity will also increase.

    CEC

  3. Organic Matter

    For each percent humus in the soil the CEC will increase about 2 meg/100g. m is the most practical and easy way for a farmer to increase the CEC of a soil. This is practical in your gardens, etc....

    The most practical way to increase the CEC of a given soil is to add organic matter and lime.

  4. Characteristic of Cation

  5. Order of the strength of adsorption of cations Al+3 > H+ > Ca+2 > Mg+ 2 > K+ >= NH4+> Na+

    This is called the lyotropic series

    The lyotropic series lists the common soil cations in order of their strength of bonding to the cation exchange surface. Ionic charge and size are the most important factors in determing an ion's position in the lyotropic series.

    Importance of CEC

    1. Indicates the nutrient holding capacity of a soil.

    2. Determines how often and how much lime must be added to a soil.

    3. The CEC determines how crop nutrients must be applied. Whether the material may be broadcast or placed in a band.

    5. Source of charge on colloids.

    Permanent charge of clays results from ismorphous substitution, which is the replacement of one atom by another of similar size in a crystal structure.. When a substituting cation has a smaller valence than the cation it is replacing, there is an increase in the net negative charge on the structure.

    pH dependent charge
    Organic matter
    Edges of Kaolinite (Si-OH)
    Oxides of iron and aluminum (Al-OH)
    amorphous clays at high pH