Introduction to Soil Chemistry

phosphorus 205
to soil cation exchange sites and be an exchangeable cation. In the fine-grained
mica-type clays, ammonia can be trapped between clay sheets. In this condition
the ammonium is not available to plants and is seldom biologically available.
The third group is amino acids and proteins. While amino acids are soluble
in water and thus are expected to move readily, they are also zwitterions
(having both positive and negative charges) and thus may interact with charges
on soil particles, depending on the pH, and move slowly or not at all. Proteins
are polymers of amino acid and may or may not be soluble but can have
charges as do amino acids. There are also a large number of other types of
nitrogen containing compounds, for example, amino sugars and nitrogen
containing lipids, present in soil, and some will be soluble while others are
insoluble.
Otherwise, nitrogen moves between the various soil compartments easily,
depending on the soil environmental conditions. These movements are often
outlined as the nitrogen cycle (which can be found in the text by Coyne [24]).
The slowest and most energy-intensive part of the cycle is the “fixation” of
nitrogen gas (N 2) forming various nitrogen compounds with either oxygen,
forming oxyanions (hydrogen), forming ammonia, or with carbon to form
amino acids. Once these initial compounds are formed, subsequent changes in
nitrogen species are fairly rapidly and easily accomplished.
Ammonium (NH 4 + ) may be added as a fertilizer or released during organic
matter decomposition. It is the chief species present immediately after the
injection of anhydrous ammonium into soil (as a fertilizer). Ammonium will
be present in solution, as a cation, attracted to the cation exchange sites, and
it may also become trapped between clay layers. It is labile in soil in that, under
aerobic, moist, moderate temperature conditions, it is rapidly oxidized to
nitrite and nitrate. Ammonia may also be volatilized from the soil solution,
particularly under basic soil conditions.
Oxidized species, chiefly nitrite and nitrate, of nitrogen occur in all soils in
the soil solution. Although nitrite in the environment is of concern, its occurrence
is usually limited because the oxidation of nitrite to nitrate is more rapid
than the oxidation of ammonia to nitrite. Both nitrite and nitrate move readily
in soil, and nitrate is available to plants as a source of nitrogen and can move
to plant roots with water.
As a result of these reactions and volatilization, nitrogen species concentrations
are expected to change during sampling and sample storage. This
is particularly true if precautions are not taken to limit this eventuality
[2,23,25–27].
10.4. PHOSPHORUS
To understand the occurrence of phosphate oxyanions in soil, the titration of
phosphoric acid is instructive. Phosphoric acid is triprotic, and each of the
protons has a different pKa. Thus the titration curve has three inflections