Agriculture Reference
In-Depth Information
and chelated species. In many cases these cation species are further grouped
into those available to plants and those that are unavailable. If a more detailed
grouping is needed, species can be further divided into readily, slowly, very
slowly, and nonavailable species.
Another simple cation commonly occurring in soil is ammonium (NH
+
).
Because of the unique role and chemistry of nitrogen and nitrogen species in
soil, it will be discussed separately in Section 10.3 [5,6].
10.1.2.
Complex Cations in Soil
Complex cations are those that can occur in several oxidation states in soil
and are often associated with oxygen and hydroxy groups while still carrying
a positive charge. The periodic table contains more than 65 metals that may
be present as complex cations. Of these, only a few are common in soil. Even
those that are not common in soil will, if present, occur predominately in one
oxidation state under normal soil conditions. No attempt will be made to cover
all these cations; rather, a limited number of the more important will be pre-
sented to illustrate the chemistry of complex cations in soil.
Examples of cations, which are present in significantly lower concentrations
than the simple cations, are iron, manganese, zinc, copper, nickel, and cobalt.
Except for cobalt, these have multiple oxidations states in soil as shown in
Table 10.1. Because of their multiple oxidation states they may be present as
Table 10.1. Common Important Ionic Species in Soil
a
Element/Compound
Cations
b
Anions
Nitrogen
NH
+
NO
-
,NO
-
c
Phosphorus
None
H
2
PO
-
, HPO
2-
,PO
3-
Potassium
K
+
None
Sulfur
None
S
2-
,SO
2-
Chlorine
None
Cl
-
d
Carbon
None
HCO
-
,CO
2-
Iron
Fe
2+
, Fe(OH)
+
, Fe(OH)
2+
,Fe
3+
None
Manganese
None
Copper Cu
2+
, Cu(OH)
+
None
Zinc Zn
2+
, Zn(OH)
+
None
Nickel Ni
2+
,Ni
3+
None
Boron None H
2
BO
-
Cobalt Co
2+
None
Molybdenum None MoO
2-
, HMO
-
a
Only elements and compounds commonly found in multiple oxidation states are listed.
b
In some cases species in this column may exist in soil under certain conditions; however, they
are not common.
c
Mn
2+
,Mn
3+
Mn
4+
e
Nitrogen oxides and nitrogen gas from denitrification will also be present.
d
Any halide found in soil solution will be present as its anion.
e
Often found in mixed oxidation states.
