SPECIATION - Introduction to Soil Chemistry - page 204

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 sat-
urated 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 nutri-
ent 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