Introduction to Soil Chemistry

conclusions 207
situ, thus removing problems associated with sampling and extraction. This
approach, although optimal, is feasibly in only a few instances. The second
approach is to make the analysis as quickly after sampling as possible, preferably
in the field, so as to eliminate storage problems. In this case minutes or
seconds between sampling and analysis are desirable.
When these approaches are not possible, steps must be taken to account
for changes occurring during sampling and storage. These can be in either
direction; that is, the concentration of a species may increase or decrease or
be converted to an entirely different species. It is not possible to give a prescription
for sampling or storage that will guarantee the stability of a soil
species over a period of time. Some species will be sensitive to oxidation while
others will be changed by the lack of oxygen. In some cases species will be
stable in air dry soil while others will be more stable in moist soil. In some
cases refrigeration at 0°C is best while other species will require storage at
-40°C or even lower temperatures. In yet other cases storage at room temperature
will be sufficient. The solution to sampling and storage problems is
to study the stability of the species of interest under various sampling and
storage conditions and thus determine which is best.
Some generalized sampling and storage recommendations, however, can be
made. During sampling and storage samples must be kept under the same
oxygen concentration, temperature, and pressure conditions occurring in the
field, if species integrity is to be maintained. Samples taken from anaerobic,
low- or high-temperature, or high-pressure conditions should be maintained
under these conditions during sampling, storage, and until actual analysis
is carried out. A sample taken from the bottom of a lake will be under
pressure and low-oxygen or anaerobic conditions. It may also be at a significantly
lower temperature than surface water, air, or soil temperatures. To
maintain sample integrity, it must be kept under these conditions [2].
10.6. CONCLUSIONS
This discussion is not intended to be a comprehensive discussion of all the elements,
cations, anions, and organic species present in soil. It includes a range
of such species that illustrate the common situations related to the analysis of
cations, anions, and oxyanions in soil. Different species have different biological
reactivity and availability, and thus it is important to know which species
is present. Cations have water molecules surrounding them, and some cations
react with water, forming oxy and hydroxy cations. Other cations can be
present in multiple oxidation states and may be present in mixtures of these
oxidation states. Some species added to soil will rapidly be converted to other
species, such as the conversion of Cr 6+ to Cr 3+ , such that analysis for the original
species may be fruitless, especially if the soil sample has been stored for
any length of time. All species may be associated in various ways with the inorganic,
organic, and colloidal components of soil to form species of interest.