Introduction to Soil Chemistry

organic ions in solution 81
consequence is that anions will pass through soil and not be adsorbed or even
retarded. For the simple anions and some of the oxyanions, this is exactly what
happens. All the halides, nitrite, nitrate, bicarbonate, and carbonate act in this
fashion. However, there are some oxyanions that do not act as expected, and
chief among them is phosphate.
Monobasic (H2PO 4 - ), dibasic (HPO4 2- ), and tribasic (PO4 3- ) phosphates react
with iron and aluminum at low pH to form insoluble phosphates. In a similar
fashion, calcium reacts with phosphate at high pH to form insoluble calcium
phosphates. In addition, phosphates will react with clay minerals and organic
matter to form insoluble compounds. For these reasons phosphate seldom
moves in soil. Exceptions occur when phosphate is associated with some kinds
of organic matter, which is moving through the soil and where there is a large
excess of phosphate in soil such as in areas where phosphate is mined, such as
around the Florida phosphate mines. Other oxyanions will also behave like
phosphate, although the exact nature of the reactions leading to their attraction
to soil particles and organic matter is not well understood.
Some soils, particularly those in the tropics, have significant anion exchange
capacity. For these soils, there is an attraction between soil colloids and the
simple halogen and nitrate anions. Bringing these anions into solution for
analysis will require an extraction or replacing anion just as does the analysis
of exchangeable cations.
4.8. ORGANIC IONS IN SOLUTION
There are three types of organic functional groups—acid, phenolic, and nitrogen—which
are commonly ionized in soil (Figure 4.7). Whether the group is
H
C
H
H
O
C
OH
Carboxylic acid
Phenol
OH
C N
R
H R
+ H +
H
C
H
O
C
O –
Carboxylate ion
Phenoxy ion
H
H
C N
R
H R
+
O –
+ H +
+ H +
3 o Amine
4 o Alkyl ammonium ion
Figure 4.7. Reactions leading to the formation of charged organic species in soil. Note that the
unsatisfied bond on the left is attached to some larger organic component in soil.