Introduction to Soil Chemistry

206 speciation
reflecting the titration of each of these protons. At low (acidic) pH (~2), the
chief form is phosphoric acid. Above this the primary form is H 2PO 4 - , which is
called monobasic phosphate; at higher pH the chief form will be dibasic phosphate
(HPO 4 2- ), and finally at high pH the form will be tribasic (PO4 3- ). It might
assumed that the species of phosphorus found in soil would be controlled
solely by pH. This is not the case. The phosphate species found is dependent
in large measure on pH but is also dependent on organic matter. Lower soil
pH (i.e., 4–7) favors monobasic phosphate, while higher pH (i.e., 7–10) favors
dibasic phosphate.
Phosphorus occurs almost exclusively as either monbasic or dibasic phosphate
in the various soil compartments, including biological tissue. However,
these two species react with a host of both inorganic and organic components,
forming a multitude of other species. Often overlooked are the species that
form where phosphate reacts with surface constituents on soil particles, including
colloidal inorganic an organic particles. Species containing phosphorus,
other than phosphate or compounds containing phosphate, can sometimes be
found, but this is unusual.
Organic phosphorus associations can occur under both acidic and basic conditions.
Phosphorus can form esters with organic alcohol functional groups and
can be associated with amine groups in various ways.
Phosphate reacts and forms insoluble compounds with iron, aluminum, and
calcium. Under acid soil conditions both iron and aluminum become more
soluble, and thus as soil pH decreases, its “phosphate fixing power” increases.
This means that iron and aluminum react with phosphate to form insoluble
and plant unavailable iron and aluminum phosphate species. Under basic conditions,
high concentrations of calcium exist and insoluble calcium phosphates
form. Insoluble phosphates are formed with other metals that happen to be
present; however, the three mentioned are generally present in the highest
concentration, and so they represent the major reactants with phosphate. Iron,
aluminum, and calcium phosphates can also occur as coatings on soil particles.
When analyzing soil for phosphorus, all these forms or potential forms must
be kept in mind. It is to be expected that all soluble forms of phosphorus will
be available to plants while all insoluble forms will not. However, precipitation
processes will also play a role in phosphorus availability. Initial precipitation
results in small crystals with high surface areas and thus greater
reactivity and tendency to move into solution when the concentration of phosphorus
in solution decreases as with plant uptake. On the other hand, as time
passes, the crystals grow larger, thus decreasing surface area, reactivity, and
availability [28].
10.5. SAMPLING, SAMPLE STORAGE, AND SPECIATION
The problem of species changes during sampling and storage can be ameliorated
in three ways. First the soil component of interest can be measured in