Agriculture Reference
In-Depth Information
Table 3.13 gives examples of inorganic salts or chelates that supply micro-
nutrients.NaturalchelatescomprisingFe,Mn,orZncomplexedwithalig-
nosulfonate are marketed on the basis of better absorption through leaves or
from the soil than sulfates. Although this may be true for a soil application, it
is not so for a foliar application because the larger chelated molecule is less eas-
ily absorbed through the leaf cuticle. Seaweed extracts containing micro- and
macronutrients, vitamins, hormones, and amino acids are also marketed for use
as foliar sprays or soil drenches. They may provide benefits in addition to their
nutrient content because of growth-promoting compounds, but this extra ben-
eitishardtoquantify.
Because seaweed extracts and natural chelates cost more than the inorganic
compounds, the latter are preferred provided that “spray-grade” forms are used
(with negligible impurities that might cause leaf damage) and correct applica-
tion procedures are followed. Fifty percent of the nutrient may be lost through
spray drift, and up to 50% of the remainder may not be absorbed into the leaves.
Although this is important in the short term, remember that residual nutrients on
leaves will eventually reach the soil through rain wash and leaf fall, so in the longer
term a spray-on micronutrient can build up in the topsoil. Such has been the case
in many Bordeaux vineyards where Cu sprays have been applied to combat downy
mildew for many years.
Table 3.13
Examples of Micronutrient Fertilizers
Element content
(% dry weight)
Fertilizer
Comments
Borax
11 B
Soluble in hot water; apply in autumn to avoid
phytotoxicity and allow for translocation in
the tissues
Copper sulfate
26 Cu
Soluble;alsosuppliessomeS;notrequiredifCu
fungicidal sprays are used
Copper chelate
9-13 Cu
Content varies depending on whether it is a
natural or a synthetic chelate
Iron sulfate
20 Fe
Soluble, also supplies some S
Iron chelate
6-10 Fe
Content varies depending on whether it is a
natural or a synthetic chelate
Manganese
sulfate
25Mn
Soluble, also supplies some S
Zinc sulfate
22 Zn
Potentially phytotoxic if concentration is too
high; “neutral Zn” (ZnSO
4
.
Zn(OH)
2
.
CaSO
4
)
is safer
Zinc chelate
14 Zn
Maybeanaturalorsyntheticchelate
Sodium
molybdate
39Mo
Soluble
Compiled from Goldspink and Howes (2001) and Christensen (2005).
