Soils 205- General Soils

Lecture 18- Cation and Anion Exchange

I.  Cation exchange:

    a.  Types of charge:

        1) constant-

        2) pH dependent (variable)-

        Al-O^-  +  H^{+  --}>  Al-OH  +  H⁺ 

        -C-O^-  +  H^{+  --}>  -C-OH

        Total charge = sum of these two types of charge

    b.  Cation exchange capacity (CEC):

    i. CEC depends on:

        1)

        2)

        3)

    c.  adsorbed cations

    i.  affinity  for surface depends on the charge and hydrated radius of the cation.  In general, the higher the charge and the smaller the hydrated radius of the cation, the more strongly it will adsorb to the colloid.  The order of strength of adsorption for the most common cations is:

    ii.  base saturation: Base saturation is the proportion of exchange sites occupied by basic cations (Ca⁺², Mg⁺², K⁺, Na⁺).  Usually a higher base saturation is better for plants not only because these bases are often plant nutrients, but also because a low value means that the system is dominated by H⁺ and Al⁺³ which acidify the soil:

     c.  Measurement of CEC:

    1)  a concentrated solution of a particular exchanger cation (usually NH₄⁺) is used to leach a soil sample

    2) The CEC can be determined by measuring the individual replaced cations in solution, or

    3) excess adsorbed NH₄⁺ is removed from the sample, and the adsorbed NH₄⁺ is replaced by a different   exchanger cation (K⁺ is commonly used).  The displaced NH₄⁺ is then measured.

    d.  Example of CEC and %base saturation calculation

    A concentrated solution containing NH₄⁺ was used to leach a soil.  After leaching, the following cations were measured in solution:

exchangeable cations

Ca = 3 cmol+/kg

Mg = 1 cmol+/kg

K = 1 cmol+/kg

Na = 2 cmol+/kg

H = 2 cmol+/kg

Al = 1 cmol+/kg

CEC = sum of cations =

% Base saturation = sum of bases/CEC * 100 % =