Agriculture Reference
In-Depth Information
Change in concentration
in depletion zone
C
o,
bulk
soil
Nutrient
depletion
zone
Lower
concentration
at root
surface
Root hair
zone
Zone of cell
division
Figure 3.3
Diagram of a grapevine root
showing root hairs and the nutrient
depletion zone (White, 2003).
Mucilaginous
root cap
Depletion zones most commonly occur with P, K, Fe, Mn, Zn, and Cu
because the solution concentrations of these elements are usually low. The ions
cross the depletion zone mainly by diffusion, which is a slow process but none-
theless important in maintaining a supply to the root. On the other hand, the
solution concentrations of Ca
2+
,Mg
2+
, NO
3
−
, Cl
−
, and sulfate (SO
4
2−
) are nor-
mally large enough for root demand to be satisfied by the amounts swept along
in water flowing to the root (the “transpiration stream”), so that no depletion
zone develops. Overall, ions such as NO
3
−
, Cl
−
, and SO
4
2−
are considered to be
“mobile” in soil. Conversely, ions such as Ca
2+
,Mg
2+
, K
+
, Fe
3+
,Mn
2+
, Zn
2+
, Cu
2+
,
and orthophosphate (H
2
PO
4
−
), which are mainly adsorbed onto soil particles, are
“immobile.”
MycorrhizasandNutrientUptake
Mycorrhizasareasymbioticassociationbetweenafungusandaplantroot(the
host). The fungus enhances the host plant's uptake of immobile nutrients such as
P and, in return, obtains organic compounds for its growth. The most widespread
type of mycorrhiza is an endomycorrhiza, so called because the fungus grows
mainly inside the root where it develops branched structures called arbuscules
that release nutrients inside the root cells (figure 3.4). This association is called an
arbuscularmycorrhizalfungus(AMF).
