Introduction to Soil Chemistry

anions 203
10.2.1. Simple Anions in Soil
There is only one simple anion commonly found in soil, and that is chloride
(Cl - ). Chloride is an essential nutrient for plants but is typically present in sufficiently
high concentrations that deficiencies are never observed. If other
halogens are present, they will also be simple anions. Most soils contain small
amounts of bromide as the second most common simple anion; however, in
some cases significant levels of fluoride and iodide may be present, although
this is rare. Inorganic combinations of all these anions are soluble in water and
thus this tends to be their predominate species. However, they may be combined
with other components and so may be present as other species; for
instance, fluorine is a component of phosphates and organic compounds, and
chlorine and bromine are components of chloro and bromo organic compounds
such as insecticides, dichloromethane, and other solvents. There are
also other nonionic species of these elements that may be present [18].
10.2.2. Complex Anions in Soil (Oxyanions)
Many important soil components are not present as simple cations or anions
but as oxyanions that include both important metals and nonmetals. The most
common and important metal oxyanion is molybdate (MoO 4 2- ). The most
common and important nonmetal oxyanions are those of those of carbon
[bicarbonate (HCO 3 - ) and carbonate (CO3 2- )], nitrogen, [nitrate (NO3 - ) and
nitrite (NO 2 - )], and those of phosphorus, [monobasic phosphate (H2PO 4 2- ),
dibasic phosphate (HPO 4 2- ), and tribasic phosphate (PO4 3- )].The soil chemistry
of oxyanions is complicated by the fact that some act as simple anions and
move readily through soil while others react with numerous soil constituents,
forming insoluble immobile constituents. Common oxyanions in soil and their
chemical characteristics and mobility are summarized in Table 10.2.
Molybdate, although present in small amounts in soil is an essential nutrient
for nitrogen fixation, specifically in the enzyme nitrogenase. The mobility of
molybdate in soil is limited, and so this anion does not move readily through soil.
Of the nonmetal oxyanions, those of carbon have a role in soil different
from those of nitrogen and phosphorus. Bicarbonate and carbonate can act as
counterions to cations to keep the soil electrically neutral.They are also important
because all pH changes in soil tend to involve either carbonate or bicarbonate,
and thus they are both involved in soil pH and buffering.
Both nitrogen and phosphorus oxyanions are important because they are
sources of nitrogen and phosphorus for plants and their potential for causing
water pollution. Nitrogen oxyanions, nitrite and nitrate are of great interest
because they are readily formed in soil from organic matter and inorganic
nitrogen containing compounds, particularly ammonia (NH 3). Soil must be
moist but not saturated, with a temperature above 20°C for rapid oxidation
of ammonia to nitrite and nitrate. Both oxyanions are mobile in soil and so
can be leached into groundwater and find their way into lakes, ponds, and
drinking water. Nitrite and nitrate are readily available to plants and can move