Agriculture Reference
In-Depth Information
Biochemical substances in the rhizosphere increase Zn availability. Org-
anic amendments generally increase bioavailability although there are
exceptions, e.g. HCO
−
from decomposing organic matter immobilizes
Zn. Although total Zn is usually distributed universally with soil depth,
extractable Zn declines with depth.
High levels of P in the soil can induce Zn deficiency symptoms without
reducing total tissue Zn.
Zinc is predominantly taken up as a divalent cation Zn
+
at lower pH and
as a monovalent cation ZnOH
+
at higher pH. Its uptake may be inhibited
by Ca
+
and other divalent cations. It is bound in the apoplast as well as
being mobile there. Zn is transported in xylem both as a free cation and in
complexes with citric or malic acid. It is also found in the phloem, where
the concentration is much higher (Robson and Pitman,
). It is classed as
having intermediate mobility within the plant, but
Zn isotope applied to the
leaves does not appear to move from them although there is some evidence
for movement of Zn from roots and stems and remobilization of Zn from the
flag leaf of wheat (Swietlik,
).
There are three main techniques for applying Zn to fruit trees: soil applica-
tion, foliar or dormant-season spray application and trunk injection.
Soil treatments with zinc sulphate (ZnSO
) can be effective (Neilsen and
Hoyt,
). They may, however, need to be accompanied by
soil acidification or to be applied in concentrated bands, piles or peat plugs.
Toxicity may be induced. ZnEDTA (chelate) may be more effective but in
some soil types the compound is converted to FeEDTA.
Some fungicides such as Dithane M
; Swietlik,
and Zineb contain enough Zn for a
regular fungicidal spray programme to control incipient deficiency. Cessation
of such a programme may lead to Zn deficiency. Post-harvest and dormant-
season Zn sprays are recommended in many areas, e.g. a ZnSO
spray at the
silver-tip stage of bud development or one or more foliar sprays of zinc chelate.
Zinc chelate is not as effective as ZnSO
in correcting zinc deficiency but can
beusedtomaintainanadequateamountofZninthetreeoncethisiswithinthe
optimum range. Post-harvest and dormant-season sprays alone seem to have
little long-term effect owing to the limited translocation of the absorbed Zn.
Trunk injection, tried for other fruit trees, is not generally used for apples
or pears.
Effects of aluminium on nutrition
Although aluminium is a major component of the earth's crust there is no
evidence that it essential to the growth of any plant species. Very minute traces
