Agriculture Reference
In-Depth Information
Change in concentration
in depletion zone
C
o
,
bulk
soil
Nutrient
depletion
Lower
zone
concentration
at root
surface
Root hair
zone
Zone of cell
division
Diagram of a grapevine
root showing root hairs
and the nutrient
depletion zone.
Figure 4.11
Mucilaginous
root cap
The Absorbing Root
The rate of nutrient ion uptake increases approximately linearly with the ion's
concentration in solution and levels off at high concentrations. The ion concen-
tration at the root surface falls when the uptake rate exceeds the rate of replen-
ishment at the root surface. A “depletion zone” develops around the root. For
plants with root hairs, the depletion zone is a cylinder with the internal surface
of the cylinder at the root surface and the outside surface at the tips of the root
hairs. For grapevine roots, there is a zone of root hairs ca. 20 mm long, some 2
mm back from the root tip (fig. 4.11). Each root hair is up to 0.2 mm long, and
there can be 300-400 hairs per mm
2
of root surface (Richards 1983).
Depletion zones are most common with ions that are strongly adsorbed or
form insoluble compounds, so their concentration in solution is low. The contri-
bution of mass flow in ion transport to the root is therefore small. Phosphate is
a good example. Similarly, ions such as K
, Fe
3
, Zn
2
, and Cu
2
may show
depletion zones around absorbing roots. For these ions, the rate of diffusion across
the depletion zone is very important in determining the rate of supply to the root;
the effective diffusion rate being inversely related to the sorption capacity of the
soil.
4.7.2
For ions such as Ca
2
, Mg
2
, NO
3
, and SO
4
2
, however, the concentra-
tion in the soil solution is normally high enough for mass flow alone to meet the
root demand, and no depletion zone develops. For grapevines, the rate of N up-
