Introduction to Soil Chemistry

80 soil basics iv
or both of their oxidations states simultaneously. Manganese presents a completely
different situation in that it can exist in several oxidation states simultaneously
such that it appears to have some intermediate oxidation state.
4.7.2. Multielement Cations
Ammonium and hydrogen (protons) are both present in the soil solution as
multielement cations. Ammonia gas reacts with water to produce the ammonium
cation, NH4 + [Figure 4.6, reaction (1)]. Ammonium acts as a cation in all
senses and will be attracted to cation exchange sites on soil particles. Ammonium
both in the soil solution and on exchange sites is available to plants.
The hydrogen ion or proton represents a very different situation. When
hydrogen (H + ) is released into the soil solution by ionization it loses its electron,
the naked proton is naturally attracted to the partially negative oxygen
of water and its lone pair of electrons [Figure 4.6, reaction (2)]. The result of
this interaction is the species H 3O + , which is called a hydronium ion. This is
the true species in the soil solution even though both scientific papers and texts
will use the simpler term H + when writing equations. The hydronium ion does
not act as common cation because of its greater chemical reactivity, especially
in cation exchange reactions.
It is important to remember that exchangeable cations (see Figure 4.6),
including NH 4 + and H3O + , when attached to exchange sites, cannot be measured
directly; they must be brought into solution before analysis can be
effected. Thus, extracting solutions must contain a cation capable of replacing
all the cations of interest on the exchange sites of the soil. Once in solution,
analysis can be carried out.
4.7.3. The Simple and Oxyanions in Soil
Simple anions in soil solution are the halogens, chlorine (Cl - ) and bromine
(Br - ). If present, the other halogens will also occur as simple anions. Because
the compounds of these anions are generally very soluble, they leach readily
out of the soil and so are generally present at low concentrations. Exceptions
occur in low rainfall regions where significant, sometimes deleterious (to
plants and animals), levels of simple anions can be found.
The two important oxyanions in soil are nitrate and phosphate. Nitrate
(NO 3 - ) is the predominant oxyanion of nitrogen; however, nitrite (NO2 - ) can
also occur in the soil solution. Phosphate can occur as one of three species,
H 2PO 4 - , HPO4 2- , and PO4 3- , depending on the soil pH. Both nitrate and phosphate
are important in plant nutrition and, because of contamination concerns,
environmental work. Other important oxyanions are bicarbonate (HCO 3 - ) and
carbonate (CO 3 2- ) and the various oxyanions of boron (H2BO3 - ) and molybdate
(MoO4 2- ).
It is reasonable to expect that since anions and most colloidal particles in
temperate region soils have a negative charge, they will repel each other. The