Introduction to Soil Chemistry

oxidative and reductive reactions in the soil solution 85
These constants are determined by mixing a solution of known concentration,
measured in mg/L, with a known amount of soil, measured in kilograms.
After a period of time the solution and solid are separated and the amount of
a given component in solution is measured. From this data a Kd can be calculated.
This procedure is often performed for various periods of time and at
various concentrations of a target compound in solution. In the determination
of K om, the amount of organic matter in the solid phase, namely, soil, is determined
(see Chapter 3), and this amount is used as the kilogram value of
organic matter.
From these two equations, the larger the amount of component sorbed to
the solid phase, the larger the K d or K om and the less likely it is to move in the
soil. Because K d and K om are determined using the compound in a pure
solvent, usually water, and a soil suspension, these constants do not necessarly
give information about how difficult the extraction of a component is likely to
be when a specific extractant is used, such as mixed solvents or extractants
using specific extracting or complexing agents [7,8].
4.12. OXIDATIVE AND REDUCTIVE REACTIONS IN
THE SOIL SOLUTION
Many chemical reactions occurring in soils are acid–base reactions; however,
oxidation–reduction reactions are more frequently the cause of confusion and
may complicate the interpretation of analytical results. The reason is that
reduced species, for instance, ferrous iron and the bivalent manganese, are
seldom assumed to occur in the liquid and solid phases in a well-aerated soil,
although they usually do.
During and after a rainfall the soil will quickly become anaerobic because
microorganisms and plant roots use up dissolved and trapped oxygen. Under
these conditions, reduction of various constituents takes place. When the soil
drains or dries, air replaces the water and there is movement of oxygen back
into the pores. However, not all pores drain immediately, and some pores
never drain at all. Both these situations lead to anaerobic zones. Even when
soils become aerobic, the reaction leading to oxidation of the reduced species
may not be fast enough to remove all reduced species before the next anaerobic
event.
Drying soil at an elevated temperature will result in the loss of water from
these normally filled pores, thereby allowing reactions that would not
otherwise take place. Also, chemical reactions take place faster at higher
temperatures, resulting in reactions taking place much more rapidly than
they normally would in soil. This not only changes the amount of compounds
in the soil sample but will also change the ratios of the components present
and may even lead to the formation of compounds not naturally present in
soil at all. For these reasons soil is not dried at elevated temperatures before
analysis [9].