Geography Reference
In-Depth Information
and chemical properties of the soil. The concept of the
cation
exchange capacity
is an important one. It is essentially a measure
of the soil's ability to hold and release various elements such as
plant nutrients. Experiments during the nineteenth century showed
that if you add ammonium chloride (as part of nitrogen fertiliser)
to the top of a soil a solution of calcium chloride came out of the
bottom. The cations of ammonium and calcium were rapidly
exchanged in this process. The process is also reversible. The negat-
ive charge on the clay and organic humus particles is balanced by
positively charged cations that are attracted to the clay and humus
particles. These cations are referred to as exchangeable cations
because cations in the soil solution can displace the cations on the
clay surface. This exchange between a cation in soil water solution
and another on the surface of a clay particle is cation exchange.
Cation exchanges are balanced reactions so that if an ion with two
positive charges such as calcium (Ca
2+
) is washed through by a
solution of sodium (which has a single positive charge; Na
+
) then it
will take two sodium ions to replace the one calcium ion. The
cation exchange capacity is the ability of an amount of soil to hold
cations and this depends on the overall negative charge of the clay
particles present. The cation exchange property controls fertility
and acidity and also means that soils act as an important buffer
between the atmosphere and groundwater thereby potentially
reducing pollution of water courses.
The acidity of a soil is important since it affects many soil pro-
cesses, the plants that grow and what happens to some pollutants.
Many polluting heavy metals become more soluble in water under
acid conditions and can then move downwards with water
through the soil to groundwater or river water. The concentration
of hydrogen ions in soil solution determines whether a soil is
acidic, neutral or alkaline. Concentrations of these ions are very
small and so the pH system was developed. Low numbers (starting
at 1) on the pH scale are acidic, 7 is neutral and large numbers up
to 14 are alkaline. In the pH scale a change of one unit represents
a 10-fold change in hydrogen concentration. So a pH of 5 means
the soil solution has 10 times the concentration of hydrogen ions
than at a pH of 6. Most soils have a pH of between 3.5 and 9 and
very low values are often associated with soils rich in organic
matter, such as peat.
