Agriculture Reference
In-Depth Information
where [P
L
] is the concentration of P in the soil solution, [C
L
] is the concentration
of DOC in the soil solution,
a
,
b
,
c
are coefficients and
k
is the rate constant,
which is soil specific.
The values of
k
were 0.18, 1.98 and 1
.
16
×
10
−
6
(mol dm
−
3
s
−
1
basis) for
Soils A, B and C in Figure 3.15, respectively. These values are more than four
times
k
for the solution system. The values of the inhibition coefficients-
a
=
−
3854—were smaller than in the solution system. As a
result the concentrations of P and DOC required to halve the rate of precipitation
were 10 times those in the solution system. Also the interaction between [P
L
]and
[C
L
] was negligible in the solution system but important in the soils. Figure 3.16
shows plots of Equation (3.54) for different values of [P
L
]and[C
L
]and
w
=
0
.
75 g dm
−
3
. For the values used, which are realistic for submerged-soil solutions,
the combined inhibitory effect of P and DOC was such that an order of magnitude
greater degree of supersaturation [
(
Ca
2
+
)(
CO
3
2
−
)/K
SP
] is necessary to produce
the same rate of precipitation as in the absence of inhibitors.
This sensitivity of precipitation in soil to organic ligands and other inhibitors
explains why the soil solutions of submerged soils may be as much as a hundred-
fold over-saturated with respect to solid phases in the first few weeks following
submergence (Chapter 4).
1686
,b
=
6
.
13
,c
=
3.7.5 EQUATIONS FOR SOLID—SOLUTION INTERACTIONS
In the initial few weeks following submergence, the properties of the soil surface
change markedly as a result of reductive dissolution and precipitation reac-
tions. But in time, a steady- or quasi-steady-state is reached, and then the same
rules govern the distributions of exchangeable ions between the soil solid and
9
8
0, 0
7
6
5
0, 200
4
0.5, 200
3
0.5, 0
2
1
0
20
40
60
80
100
Saturation index
Figure 3.16
Inhibition of calcite precipitation by P and water-soluble organic matter
calculated with Equation (3.54) derived using the data in Figure 3.15. Numbers on curves
are concentrations of P and DOC in the soil solution (mM)
