4.8 Ion exchange and soil pH
Exchangeable ions are those that are held temporarily on materials by weak, elec-
trostatic forces. If particles with one type of adsorbed ion are added to an elec-
trolyte solution containing different ions, some of the particle-surface adsorbed
ions are released into solution and replaced by those from the solution (Fig. 4.20).
We have seen that the interlayer sites of clay minerals, particularly smectites,
hold ions weakly, giving these minerals a capacity for ion exchange. Clay mineral
ion exchange can also be a surface phenomenon. Edge damage to minerals can
break bonds to expose either uncoordinated oxygens (sites of net negative charge)
or uncoordinated silicon or other metal ions (sites of net positive charge). These
surface charges are balanced by electrostatic adsorption of cations and anions
respectively.
In soils with neutral or alkaline pH (see Box 3.5), ion-exchange sites are typ-
ically occupied by exchangeable base cations, for example Ca^2 +, Mg^2 +, Na+and K+
(Box. 4.11). These exchangeable base cations are in equilibrium with the soil pore
water and H+ions. However, changes in soil pH, for example to more acidic con-
ditions (higher solution H+concentration), encourage exchange of base cations
for H+ions, causing H+pore water concentrations to fall (pH increases) along
with an increase in pore water base cation concentration. The presence of the
base cations effectively buffers the soil water pH (see Section 5.3.1), minimizing
change. Clearly, a lack of exchangeable base cations, for example in oxisols
(Section 4.7), will often lead to low soilwater pH as there is little or no buffer
capacity. Understanding the buffer capacity of a soil to pH fluctuations is impor-
tant, as some fundamental chemical processes (e.g. dissociation, Box 4.5), chem-
ical species, for example phosphate (see Section 5.2) and elements, for example
aluminium (see Section 5.4), are very sensitive to pH change.
The Chemistry of Continental Solids 111NaNaHHMg KMgCaCa+++K
+K
+K
+K
+KK++K+K+K+K+K++++++12K+ Cl–
(solution)2Ca2+ + 2Mg2+ + 2H+ + 2Na+ + 12Cl–
(solution)Fig. 4.20Schematic diagram to show ion-exchange equilibria on the surface of a clay particle.
Potassium ions in solution are exchanged for other cations, causing the exchange equilibrium
to move from left to right.