Agriculture Reference
In-Depth Information
Fe(II) oxidation was comparable to that released from the roots to balance excess
cation uptake. But in both soils the H
+
generated in these two processes exceeded
the acidification calculated from the pH profile and the soil pH buffer powers.
This was possibly because of CO
2
uptake by the roots and, in the Maahas soil,
where acidity diffusion was fast because of the high pH and high CO
2
pressure,
because the acidification spread beyond the zone of soil analysed.
6.4.2 THE pH PROFILE ACROSS THE RHIZOSPHERE
The pH profile across the rhizosphere resulting from the above processes depends
on the rate at which acidity is generated versus the rate at which the resulting pH
change is propagated away through the soil. In the Iloilo soil (Figure 6.15), which
is a highly weathered sandy loam, the pH at the root surface fell by more than
2 units, whereas in the Maahas soil (Figure 6.16), a less-weathered humic clay,
it fell by only 0.2 units. Apart from differences in the rate of acidity generation,
the Maahas soil had a greater initial pH and a greater organic C content resulting
in a greater CO
2
pressure when flooded. These together result in faster acidity
diffusion through the soil.
The continuity equation for diffusion of acidity in the soil surrounding a root
is (Equation 2.32 expressed in radial coordinates)
rD
HS
dpH
d
r
∂
pH
∂t
1
r
∂
∂r
=
(
6
.
12
)
where
D
HS
is the soil acidity diffusion coefficient, given by:
2
.
303
θ
L
f
L
b
HS
(D
LH
[H
3
O
+
]
+
D
LC
[HCO
3
−
]
)
D
HS
=
(
6
.
13
)
The explanation of Equation (6.13) is that a small portion of soil near the root
may gain acidity either by access of H
3
O
+
(
S—
+
H
3
O
+
=
S—H
+
+
H
2
O
)
or
by dissociation of CO
2
and removal of HCO
3
−
though the soil solution
(
S—
+
H
2
CO
3
=
S—H
+
+
HCO
3
−
)
(Section 2.2). Since [H
3
O
+
]and[HCO
3
−
] are both
sensitive to pH, the rate at which pH changes are propagated is also sensitive
to pH and passes through a minimum in the pH range 4.5-6.0 where [H
3
O
+
]
and [HCO
3
−
] are both small. In this pH range, therefore, a flux of acid or base
through the soil causes steep pH gradients.
Figure 6.17 shows pH profiles calculated with Equations (6.12) and (6.13) with
parameters appropriate for a rice root in anaerobic soil. The pH change at the
root surface is small at pHs above neutral because
D
HS
is large, and it increases
towards the pH at which
D
HS
is minimal. Because of this, the combined effect of
the two sources of acidity in the rice rhizosphere is greater than the sum of their
effects in isolation. So in spite of
θ
L
f
L
and the CO
2
pressure both being large
in submerged soils, if the rate of generation of acidity is sufficiently large there
may be substantial pH changes at the root surface. These effects are summarized
