Agriculture Reference
In-Depth Information
lime-induced Fe chlorosis (Beyers and Terblanche,
d; Swietlik and Faust,
). Iron chelate can also be used as a foliar spray at petal fall and three or
four times subsequently at
-weekly intervals. If the Fe deficiency is associated
with poor drainage, attention should be given to this.
Zinc nutrition
Apple and pear demand for zinc is very low and unless deficiency symptoms
are shown application of additional zinc usually has no demonstrable effects.
In those circumstances where Zn is deficient there is a strong positive rela-
tionship between Zn content of leaves within the range of
.
to
.
ppm dry
weight and apple, cv. 'McIntosh', yield (Stiles,
). Treatments to ameliorate
Zn deficiency are essential in a number of important areas of apple production,
e.g. southern British Columbia, Canada. Susceptibility varies between culti-
vars, the low-chilling-requirement apple 'Anna', which is very widely grown in
the tropics and subtropics, can become totally unproductive under conditions
whereitistheonlycultivartoshowzincdeficiencysymptoms.Themostimpor-
tantfunctionsofZnconcernenzymeactivation,carbohydratemetabolism,cell
membrane integrity and auxin synthesis (Swietlik,
). The precise mecha-
nism of the dramatic effects of Zn deficiency on shoot extension and foliage
development is not established.
Zincinsoilsispresent(
a
)inthesoilsolution,(
b
)inanadsorbedexchangeable
form associated with alumino-silicates, hydrous oxides of Al, Fe and Mn and
solid organic matter, (
c
) associated with organic matter, e.g. by incorporation
into organic molecules and by chelation, (
d
) in association with hydrous oxides
and carbonates and (
e
) in soil minerals, e.g. silicates (Swietlik,
). The
causes of deficiency, and the responses to soil treatments directed towards
its correction, are correspondingly complex. The following seem to be most
important.
The amount of zinc available in soils depends very much on the Zn content
of the parent material. It is low in soils derived from granite and basalt
(Tagwira,
) and in soils containing little clay or organic matter. Con-
sequently Zn can be very low in sandy soils, including those derived from
quartz which is low in this element, even when these are acidic.
In most circumstances water-soluble Zn, and thus total available Zn, de-
creases dramatically with increasing pH: there can be a hundred-fold re-
duction in Zn activity for each unit of pH increase (Swietlik,
). Soil
acidification can increase leaf Zn under these circumstances. Availabil-
ity is generally low in high-pH calcareous soils and deficiency can follow
repeated heavy liming.
