Publication 360, Guide to Fruit Production: Micronutrients
Excerpt from Publication 360, Guide to
Fruit Production 2012-13,
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.
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 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 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 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 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.
Soil-applied manganese may be useful in acidic, sandy soils. In soils with a pH greater than 6.5, soil applied manganese will be unavailable to the plant. On alkaline soils banded applications are often more effective than broadcast. Foliar applied manganese is generally more effective where a manganese deficiency has been confirmed. 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 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.
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.
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