Agriculture Reference
In-Depth Information
and can increase fruit set and fruit size and reduce fruit drop (Greenham and
White,
). The residual effect of such sprays is minimal and soil
application of MgSO
or of dolomitic (magnesian) limestone is essential for a
longer term solution, especially on soils high in potassium.
; Ford,
Manganese nutrition
Although manganese deficiencies can occur, Mn toxicity is a much more
widespread problem.
Manganese is usually found only in very low concentrations in the soil
solution (
M) where, like Fe, Cu, Zn and Co, it is present mainly in a
complexed form (Robson and Pitman,
<
.
µ
). The concentration of available
Mn (Mn
+
) in the soil solution is dependent on the soil pH and its oxidation-
reduction potential. Mn availability decreases very sharply as the pH rises to
a certain level, below the neutral point, and deficiency can be induced by the
application of large amounts of agricultural limestone. It may also occur on
naturally alkaline soils. The necessary pH to avoid deficiency varies with soil
type (Beyers and Terblanche,
a). Exudates from plant roots may solubilize
amorphous Mn oxides, bacterial activity in the rhizosphere may affect the
solubility of these and use of NH
+
instead of NO
−
fertilizers can also increase
Mn availability.
Transport of Mn within the plant is in both the xylem and the phloem, in
the cationic form in the former and largely in the cationic form in the latter
(Robson and Pitman,
).
Manganese deficiency often occurs simultaneously with zinc deficiency, the
symptoms shown reflecting the element in greatest deficiency. It is often con-
trolledadequatelybyroutinespraysofDithaneM-
fungicide,whichcontains
Mn. Additional control can be achieved by MnSO
sprays, either in the dor-
mant season or as foliar applications. Soil treatment with Mn compounds is not
effective on alkaline or heavily limed soil unless applied in large quantities, e.g.
kg MnSO
plus compost in holes around each tree. The soil can, however,
be acidified by use of acid fertilizers or sulphur to reach desirable pH levels.
Manganese toxicity is expressed as leaf chlorosis, early leaf abscission, re-
duced growth and flower bud formation and internal bark necrosis (IBN). If
IBN develops early in the life of the tree, it will fail to become productive
(Ferree and Thompson,
). Toxicity is associated with very high Mn con-
centrations, up to
ppm, in the affected part of the bark and seems to be
associated with high Mn availability under low Ca supply conditions (Domoto
and Thompson,
). Calcium influences both Mn absorption and transloca-
tion (Fucik and Titus,
). These effects may involve ion exchange processes
independent of pH. However, under conditions where Mn toxicity is likely to
