Agriculture Reference
In-Depth Information
Values of
k
depend not only on the type of organic material, but also on changes
in soil conditions such as occur during repeated cycles of air-drying and rewet-
ting, or freezing and thawing.
Insoluble Hydroxides of Micronutrient Metals
and How Complex Formation Increases
Their Availability to Plants
Appendix 5
We use the reactions of hydrated Fe
3
as an example. The first step in the hy-
drolysis of this ion is
Fe(H
2
O)
6
3
H
2
O
FeOH(H
2
O)
5
2
H
3
O
(A5.1)
The hydroxy-Fe ion continues to hydrolyze, releasing H
ions, until finally fer-
ric hydroxide Fe(OH)
3
precipitates. The reaction is driven to the right by the pres-
ence of OH
ions, which combine with the H
3
O
ions. The overall process is
summarized as
Fe
3
(soln)
3OH
(soln)
Fe(OH)
3
(solid) (A5.2)
Equation A5.2 is a dissolution-precipitation reaction for which the
solubility
constant K
sp
is defined by
(Fe
3
)(OH
)
3
K
sp
(A5.3)
(Fe(OH)
3
)
Taking the activity of pure, solid Fe(OH)
3
as 1, equation A5.3 becomes
K
sp
(Fe
3
)(OH
)
3
(A5.4)
Two points follow from equation A5.4:
1. The smaller the value of
K
sp
, the lower is the solubility of the compound,
and the lower the concentration of Fe in solution.
2. The higher the concentration of OH
ions, the lower the concentration of
Fe in solution.
The availability of metal cations can be increased by the formation of solu-
ble complexes, using specific chelating agents (section 2.3.4.2). For example, Fe
3
forms very stable complexes with several organic ligands. If the soluble organic
complex is more stable at a given pH than Fe(OH)
3
, Fe will be maintained in so-
lution and not precipitated. The stability constant of the complex determines
which chelating agent is chosen to form a soluble complex with Fe, to increase its
availability at high pH, especially on calcareous and chalky soils. Ethylenediamine
tetra-acetic acid (EDTA) is a synthetic chelating agent that forms stable complexes
with most micronutrient cations. Figure A5.1 shows that the complex FeEDTA
is normally more stable than Fe(OH)
3
to around pH 9, above which Fe(OH)
3
is
more stable. But in calcareous soils, the crossover occurs around pH 8 because of
the increasing stability of the CaEDTA
2
complex at high pH. Thus, chelating
agents that have a higher affinity for Fe than for Ca, such as EDDHA (ethylene
diamine di(
o
-hydroxyphenylacetic acid)), should be used on calcareous soils. Nat-
ural chelating agents for Fe and other metals occur in the soil organic matter and
leaf litter (box 1.5).
