Agriculture Reference
In-Depth Information
soil surfaces. As solutes are removed from the soil solution by root uptake, the
sorbed solutes tend to buffer the soil solution against the resulting changes in
concentration. The influx across the root surface,
F
in moles per unit area per
unit time, is related to the concentration in solution at the root surface with a
'root absorbing power',
α
, such that
F
=
αC
La
(
6
.
8
)
To calculate the inflow,
C
La
must be found from the concentration in the soil
bulk taking into account rates of transport through the soil. Kirk and Solivas
(1997) have done this for N uptake by rice growing in flooded soil and used the
resulting model to assess the relative importance of root uptake properties and
transport through the soil. Their results are summarized in the following.
Kirk and Solivas measured the time course of N uptake by soil-grown rice
plants and the simultaneous changes in soil solution NH
4
+
and root length den-
sity, and then compared the results with the calculated minimum root length
densities required to explain the uptake. The calculation was based on the fol-
lowing picture of events.
(1) All the N is absorbed as NH
4
+
.
(2) The rate of uptake per unit root length for a given concentration of NH
4
+
at the root surface is maximal, as indicated by a Michaelis-Menten rela-
tion derived from measurements with plants grown hydroponically under
moderate N-deficiency (Section 6.3).
(3) The concentration of NH
4
+
in the soil solution at root surfaces is related to
the mean concentration in the bulk soil solution by an equation for steady-
state diffusion through the soil. The diffusion coefficient of NH
4
+
in the soil
was measured.
The corresponding equations are as follows. For roots uniformly or randomly
distributed in volume
V
of soil at density
L
V
(length per unit volume), the rate
of uptake is
d
U/
d
t
=
2
πaFL
V
V
=
2
πaαC
La
L
V
V
(
6
.
9
)
where
a
is the mean root radius. For steady-state diffusion across a cylinder
of depletion around a root of radius
x
, the concentration maintained at the root
surface is (from Tinker and Nye, 2000, Equation 10.24)
C
L
C
La
=
1
(
6
.
10
)
x
2
(αa/Db)
(x
2
1
2
(αa/Db)
+
ln
x
a
−
−
a
2
)
where
C
L
is the mean concentration in solution in the soil around the root,
D
the diffusion coefficient of NH
4
+
in the soil and
b
the buffer power for NH
4
+
.
The assumptions inherent in this equation are discussed by Kirk and Solivas. The
