Agriculture Reference
In-Depth Information
example on acid, sandy soils with low humus content in South Africa (Beyers
and Terblanche,
c) or on readily-leached soils in high rainfall areas.
The concentration of B in soil solutions ranges very widely, from
µ
M,
a true trace element level, to
M. It is present in the inorganic form
as H
BO
and is probably taken up and found in the xylem in this form
(Robson and Pitman,
µ
). It is passively absorbed by roots into the free
space (Tanaka,
) and forms complexes with polysaccharides. There is
some active transport into root cells but it is thought that the net uptake of
boric acid is influenced by the transpiration rate and transport in the xylem
is probably directly proportional to the rate of transpiration (Raven,
).
Water stress due to drought and scarcity of irrigation water can be followed
by boron deficiency symptoms even on soils with sufficient B.
The B content of leaves and fruits increases throughout the season in apples
and there is a straight-line relationship between fruit weight and B per fruit,
indicating continuous B transport (Van Goor and Van Lune,
).
Accumulation in the bark follows a similar pattern to that in leaves and
fruits and is reduced in dry summers ( Johnson
et al.
,
). The supply of
B to the flowers comes from reserves in the wood of the branches bearing
them (Callan
et al.
,
) and the B content of flowers (of 'Italian' prune) is
increased much more by B sprays in the previous autumn than by spraying
immediately pre-bloom.
There is very little safety margin for B application. The normal range for
apple and pear leaf content of B is
-
ppm (Shear and Faust,
)but
toxicity can be shown at
). A soil application of
boraxor of Solubor (disodium actoborate tetrahydrate, containing
ppm in apple (Faust,
%B)is
effective on acid, sandy soils but may incur the risk of toxicity (Beyers and
Terblanche,
lfor
apple and pear can be effective at any time and should be applied at the first
symptoms of deficiency. Blossom-time sprays should be followed up four weeks
later. Autumn sprays, applied when symptoms such as external corking of fruits
appear late in the season, should be followed by post-harvest sprays before leaf
drop. This has a good residual effect in the following spring. An adequate
supply of compost or manure, which contains a small quantity of boron, good
drainage and irrigation to maintain optimum soil moisture, including after
harvest, can prevent deficiency arising.
c). Solubor sprays at a concentration of
g per
Iron nutrition
Although iron constitutes about
% by weight in the earth's crust and is only
foundintraceelementquantitiesinapplesandpears,irondeficiencysymptoms
can be found in both these crops. This is mainly because the activity of soluble
