Introduction
Micronutrients
include boron, copper, iron, manganese, molybdenum and zinc. Plants use these
elements in much smaller amounts than macronutrients like nitrogen, phosphorous,
potassium, calcium and magnesium. Because such small quantities are required,
routine application is generally an unnecessary expense. However, micronutrients
are crucial to plant growth and deficiencies must be corrected.
Micronutrients
are usually found in much lower levels in the soil than macronutrients. Soil pH,
organic matter, clay and mineral content can strongly influence micronutrient
availability. This makes soil tests for estimating micronutrient availability
less reliable than those for the primary nutrients.
Which
to choose: soil or foliar fertilizers?
Both soil and foliar fertilizers
play a role in fruit crop production. The macronutrients (nitrogen, phosphorus,
potassium, calcium and magnesium) are required in relatively high amounts for
crop growth. As a result, soil application is almost always the most efficient
and economical method of getting these nutrients into the plant. Foliar uptake
occurs through the leaf's cuticle and the stomata. The amount of nutrients that
can enter the plant through these means is quite limited. Higher application rates
may lead to crop injury.
Since micronutrients are required in much lower
quantities, they can often be efficiently delivered through foliar applications,
especially when soil conditions limit micronutrient availability. If a micronutrient
deficiency is found, foliar application is the quickest way of addressing it.
This can be followed with a soil application to prevent a recurrence, depending
on the micronutrient and the soil pH.
Do not apply micronutrients to
fruit crops unless a deficiency is identified. Apply only the deficient nutrient
in sufficient quantities to correct the problem. The range between deficiency
and toxicity with micronutrients can be narrow.
Use caution when you
apply mixtures of several micronutrients, as crop injury may occur. Always follow
the product label. Do not combine micronutrients with insecticide, fungicide or
herbicide sprays unless there is information from the manufacturer of each product
that indicates the mixtures are compatible. Many chelated micronutrients will
consolidate in the spray tank if mixed with pesticides. Use caution when applying
micronutrients through fertigation systems. Certain micronutrient blends may plug
the emitters.
Foliar applied nutrient uptake can be improved through
the timing of the application and the use of surfactants. Younger leaves generally
have a less well-developed cuticle and are able to take up more of the nutrient.
Early morning applications may favour foliar uptake, and drought stress that results
in a thicker cuticle may hinder uptake. Avoid the application of foliar nutrients
during the heat of the day when leaves will dry quickly. Ensure good leaf coverage,
particularly on the underside.
Boron
Boron
plays an important role in the structure of cell walls, fruit set and seed development
as well as protein and carbohydrate metabolism.
Boron deficiency is most
likely to be found on alkaline soils or sandy knolls. Symptoms vary widely between
crops. Apples may exhibit internal breakdown and premature drop of highly coloured
fruit. Boron toxicity may occur when sensitive crops are planted in a rotation
where boron has been applied or over-applied.
There is no OMAFRA-accredited
boron soil test. Some soil test reports provide a soil boron value, however, soil
levels are often less than 1 ppm, making it very difficult to get an accurate
measurement. To correct deficiency, fertilizer manufacturers may mix boron sources
with other fertilizers to be applied. Boron can also be foliar applied for faster
results. If boron is required, refer to Table 3-15. Application
Rates for Micronutrients, below.
Some crops are very sensitive to
boron deficiencies, even at low levels. A soil pH between 5.0 and 7.0 provides
the best conditions for boron uptake. Boron deficiencies are more likely to occur
on soils with low organic matter and on exposed or eroded subsoils. Boron availability
decreases during periods of drought.
Copper
Copper
plays a role in chlorophyll production. It may also have a role in the suppression
of some diseases.
Copper deficiency is rare on mineral soils, except
perhaps very sandy soils.
Since soil tests for copper are unreliable,
there is no OMAFRA-accredited copper soil test. Plant tissue analysis is a more
useful tool. If copper is required, refer to Table 3-15c. Application
Rates for Micronutrients, below. Copper sulphate may injure leaves. Follow
label recommendations to minimize injury.
Iron
Iron
is needed for chlorophyll formation, plant respiration and the formation of some
proteins.
Iron deficiency, also called lime-induced chlorosis, is rare
in Ontario. Symptoms appear on the young leaves first. Leaves turn yellow between
the veins, but the veins will remain green except in extreme cases. Often symptoms
are seen in only one area of the plant. Factors associated with iron deficiency
include soils with high lime content (and therefore high pH), and gross imbalances
with other micronutrients like molybdenum, copper or manganese.
An iron
soil test does not correlate well with plant uptake or fertilizer response in
Ontario. Consequently there is no OMAFRA-accredited iron soil test. Plant analysis
is a much more reliable indicator of iron availability. Iron deficiency is easily
corrected with the foliar application of iron chelates, whereas soil application
is not generally effective. Refer to Table 3-15c. Application
Rates for Micronutrients, below and consult the manufacturer's label for information
on rates and timing.
Manganese
Manganese is
involved in photosynthesis and chlorophyll production. It helps activate enzymes
involved in the distribution of growth regulators within the plant.
Manganese
deficiency causes yellowing between veins of young leaves. Leaves gradually turn
pale-green with darker green next to the veins. Manganese toxicity can occur on
soils with a low pH. It causes brown spots or yellow mottled areas near leaf tips
and along the leaf margins and usually develops on older leaves. Brown spots may
also develop on veins, petioles and stems.
The OMAFRA-accredited manganese
soil test uses a manganese availability index. This index evaluates manganese
availability based on soil manganese level and soil pH.
Foliar applied
manganese is generally preferred because the higher soil pH that may lead to deficiency
tends to make added manganese unavailable. Soil-applied manganese may be useful
in acidic, sandy soils. On alkaline soils banded applications are often more effective
than broadcast. If a deficiency is confirmed, apply foliar sprays when the plants
are about one-third grown or sooner. Two or more sprays may be necessary at 10
day intervals.
If manganese is required, refer to Table
3-15c. Application Rates for Micronutrients, below.
Manganese availability
is greatest at a soil pH of 5.0 to 6.5. It is important not to add more limestone
than is needed to correct soil acidity. High organic matter levels decrease manganese
availability. Foliar applications may be required for crops grown on muck soils.
Zinc
Zinc is important in early plant growth
and in seed formation. It also plays a role in chlorophyll and carbohydrate production.
Zinc is relatively immobile within the plant. Deficiency symptoms appear
first on younger leaves. Young leaves become mottled and show interveinal chlorosis,
striping or banding. In advanced stages in tree fruits, small, narrow terminal
leaves are arranged in whorls. This results in the typical "rosette"
and "little leaf" description for zinc deficiency symptoms. Use leaf
and soil analysis to test for zinc deficiency.
The OMAFRA-accredited
zinc soil test is reported as a zinc index value, which estimates availability
based on soil zinc level and soil pH. Zinc deficiency can be prevented by the
application of zinc fertilizer to the soil at a rate of 4 kg of zinc/ha. Broadcasting
up to 14 kg of zinc/ha will correct a deficiency for three years. No more than
4 kg zinc/ha should be banded. Early in the growing season, foliar sprays can
be used to correct a deficiency after the symptoms have appeared. If zinc is required,
refer to Table 3-15c. Application Rates for Micronutrients,
below.
Zinc deficiencies are most often seen on sandy soils with high
pH levels. Heavily eroded knolls may also have deficiency problems. Large applications
of phosphorus may aggravate zinc deficiencies. Livestock manure is often an excellent
source of zinc.
Where a zinc deficiency exists, check manufacturer's
recommended rates and timing of applications of zinc products. If zinc is required,
refer to Table 3-15. Application Rates for Magnesium, Calcium and Micronutrients,
page 40. Foliar applications early in the growing season can be used to correct
a deficiency after the symptoms have appeared.
Table
3-15c. Application Rates for Micronutrients (PDF
77 kb)
A number of micronutrients are available as
chelates, with various formulations and nutrient contents. Check the product labels
for crop specific recommendations. The effective use rate for chelated products
is the same as for other formulations.
| Nutrient | Source | %
Nutrient | Other nutrients | Soil
Application | Foliar Application |
| Boron (B) | various
granular materials |
12-15% |
| yes | no |
| sodium
borate | 12-21%
| | yes | yes |
| Solubor | 20% | | no | yes |
| Copper (Cu) | copper
sulphate | 13-25%
| 6.5-12.5%
sulphur | yes | no |
| copper
chelates | 5-13%
| | no | yes |
| Iron (Fe) | ferrous
sulphate | 20
% | 11%
sulphur | no | yes |
| iron chelates | 3-13%
| | no | yes |
| Manganese (Mn) | manganese
sulphate | 28-32%
| 16-18%
sulphur | no | yes |
| manganese
chelates | 5-12% | | no | yes |
| Molybdenum (Mo) | sodium
molybdate | 39%
| | no | yes |
| Zinc (Zn) | zinc
sulphate | 36%
| 17% sulphur | yes | yes |
| zinc oxysulphate |
8-36% | | yes | no |
| zinc chelate | 9-14%
|
| no | yes |
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