Introduction to Soil Chemistry

204 speciation
Table 10.2. Common Oxyanions in Soil, Their Chemical Characteristics and Mobility
Oxyanion Chemical Characteristics Mobility
Carbonate Forms insoluble carbonates with Precipitates
cations from solution
Bicarbonate Forms slightly soluble Mobile in
bicarbonates with cations solution
Nitrate Available to plants. Converted to N 2 Mobile
gas under anaerobic conditions
Nitrite Readily oxidized to nitrate Mobile
Monobasic phosphate Stable at low pH Considered
immobile
Dibasic phosphate Stable under neutral conditions Considered
immobile
Tribasic phosphate Stable under basic conditions Considered
mobile
Molybdate Reacts with various soil constituents Some mobility
Borate Acts as simple anion Mobile
through soil to roots with water taken up by plants and are readily converted
to nitrogen gas under the proper soil and environmental conditions, the most
important of which are temperature; between 20 and 40°C is optimal and saturated
soil.
Phosphorus oxyanions are entirely different from nitrogen oxyanions. The
oxyanion species present is controlled by the pH; also, phosphate oxyanions
are generally not mobile in soil; sandy soils and soils high in phosphorus are
exceptions to this rule. Any soil, however, can loose phosphate by erosion, and
this phosphate can cause environmental problems. Because of its unique
chemistry, phosphorus will be discussed separately below.
Boron and arsenic are natural components of soil and are both present as
oxyanions. Boron is present as boric acid or borate polymers, and arsenic is
present as arsenate. While boron is weakly held by soil, arsenic is similar to
phosphate in its interactions with soil constituents. Boron is an essential nutrient
for plants; however, it is also toxic at relatively low levels. Arsenic is toxic.
The laboratory chemistry of both of these elements is well understood but
their environmental chemistry and speciation is less well understood [19–23].
10.3. NITROGEN
Nitrogen is unique because it is found in soil as both reduced and oxidized
species, as part of organic molecules, and as oxidized gaseous species, as well as
in the elemental form (i.e., nitrogen gas). Many nitrogen species are soluble in
water and thus tend to be found in the soil solution as opposed to attached to
soil particles. There are, however, three important exceptions to this rule.
Ammonium having a positive charge can act as a cation and thus be attracted