Agriculture Reference
In-Depth Information
3.7.4
INHIBITION OF PRECIPITATION
Rates of precipitation in soil are sensitive to inhibition by organic and inorganic
ligands that may be sorbed on soil surfaces and thereby interfere with nucleation
and crystal growth. This is particularly important in submerged soils because
large concentrations of organic and inorganic ligands develop in the soil solution
following submergence.
Inskeep and Bloom (1986) measured inhibition of calcite precipitation by
organic ligands in simulated soil solutions prepared from CaCl
2
,
KHCO
3
and
seeds of CaCO
3
, and maintained at constant pH and CO
2
pressure. The data
fitted the rate equation:
R
=
ks
[
(
Ca
2
+
)(
CO
3
2
−
)
−
K
SP
]
(
3
.
51
)
where
R
is the rate of CaCO
3
precipitation,
k
is the rate constant,
s
is the surface
area of CaCO
3
seeds, and
K
SP
is the solubility product of pure CaCO
3
.The
value of
k
was 117 dm
6
mol
−
1
m
−
2
s
−
1
and it decreased to zero in the presence
of 0.15mM water-soluble soil organic matter or 0.028mM fulvic acid.
However, rates of precipitation in soil systems may be quite different from
those in solutions because: precipitation is catalysed by adsorption of the reacting
solutes onto soil surfaces; the nature of the solid phases formed may be different;
and sorption may also alter the effects of inhibitors. There are very few data
in the literature on these effects actually measured in soils. Figure 3.15 shows
data of Huang (1990) for calcite precipitation in three soils incubated with urea.
Precipitation was induced as the pH increased during urea hydrolysis:
CO
(
NH
2
)
2
+
3H
2
O
−−−→
2NH
4
+
+
HCO
3
−
+
OH
−
(
3.52a
)
Soil
2
—Ca
+
2NH
4
+
+
HCO
3
−
+
OH
−
−−−→
2Soil—NH
4
+
CaCO
3
(
s
)
+
H
2
O
(
3.52b
)
Simultaneously, in all the soils the concentrations of P and dissolved organic
carbon (DOC) in the soil solution increased as the pH increased, and this will
have inhibited CaCO
3
precipitation. By analysing the combined data by multiple
regression, Huang (1990) developed a rate equation for the three soils allowing
for inhibition by P and DOC. The rate equation was developed from an earlier
equation developed from studies in solution systems:
R
=
kw
0
.
379
[
(
Ca
2
+
)(
CO
3
2
−
)/K
SP
]
(
3
.
53
)
where
R
is the rate of precipitation in mol dm
−
3
soil solution s
−
1
,
k
is the rate
constant and
w
is the weight of newly formed CaCO
3
in kg dm
−
3
soil solution,
which is a surrogate for the surface area term in Equation (3.51), surface area
being unmeasurable in soil. The rates calculated with this equation are comparable
to those with Equation (3.51). The modified equation for soil systems is:
R
=
kw
0
.
379
e
a
[P
L
]
e
b
[C
L
]
e
c
[P
L
][C
L
]
[
(
Ca
2
+
)(
CO
3
2
−
)/K
SP
]
(
3
.
54
)
