Introduction to Soil Chemistry

distribution between soil solids and soil solution 83
Many other methods of determining soil pH and exchangeable protons are
described in the soils literature. Among these are a number of methods
designed to determine a factor called the soil buffer pH, which is determined
by adding a highly buffered solution to the soil and measuring the change in
its pH. This change can then be related back to a property called the total
acidity in soil. It is also used as the basis for making liming recommendations
in agriculture. See Chapter 5, Section 5.3.1, for a detailed discussion of pH
electrodes for measuring soil pH.
It is often tempting to design a new protocol or change an existing method
for pH determination. Keep in mind that when or if this is done, it will change
all the analytical methods that use pH as an important component such as the
determination of CEC as discussed above. It will also change the interpretation
of analytical results and the recommendations for applications of some agricultural
amendments (see Chapter 10). Such a change is probably not advisable
unless there is some highly significant improvement in the new method.
4.10. THE SOIL SOLUTION AROUND PARTICLES
Dissolved ions and molecules are evenly distributed throughout a pure solution.
In soil, this does not take place because soil particles, particularly colloidal
particles, both clay and carbon, have sorbed, exchangeable, and
peripherally attracted components associated with them.
Cations attracted to colloid surfaces through their waters of hydration are
outer-sphere species, whereas those that react directly with the oxygens
present in the surface are inner-sphere species. Of the two, the latter species
will be more strongly bonded and harder to extract.
Cations form a diffuse layer of ions called the diffuse double layer or the
electrical double layer around soil particles as depicted in Figure 4.8. The existence
of the diffuse double layer means that the ions are not evenly distributed
throughout the solution; rather, they are more concentrated close to soil
particle surfaces and less concentrated farther away. This phenomenon must
be kept in mind, particularly when electrochemical analytical methods of
analysis are developed [5,6].
4.11. DISTRIBUTION BETWEEN SOIL SOLIDS AND SOIL SOLUTION
All components in the soil solution are to a greater or lesser extent distributed
unevenly between the solid and liquid phases. Anions are generally only
weakly attracted to soil solids, if at all. Cations are attracted to the soil colloids,
while the interaction between soil solids and organic compounds is
complex, depending on the structure and functional groups of the organic
compound and the nature of the soil solids. However, measuring the attraction
or having a measure of the distribution between the two phases is
extremely important in understanding the movement of material in soil. It will