Corn: Secondary and Micronutrients

Excerpt from Agronomy Guide for Field Crops (Chapter 3)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Magnesium
2. Sulphur
3. Zinc
4. Manganese
5. Other Micronutrients
6. Updates on Corn: Secondary and Micronutrients
7. Related links...
8. For More Information.....

Magnesium

Magnesium is plentiful in most soils in Ontario, but a deficiency can occur on acid, sandy soils. The symptoms appear first as yellow striping of the lower leaves (see Plate 1 and Plate 6). As the deficiency worsens, the upper leaves may become striped while the lower leaves turn reddish-purple.

Plate 1. Colour Plates Guide to Nutrient Deficiency Symptoms. Reprinted with permission of the Saturday Evening Post © 1957 (renewed) BFL&MS, Inc.

Plate 6. Magnesium deficiency appears first as yellow striping of the lower leaves. These may turn reddish-purple later as deficiency progresses.

All soil samples analyzed under the OMAFRA-accredited soil testing program are tested for magnesium. This test is a reliable guide for determining magnesium requirements.

Dolomitic lime is an excellent source of magnesium. Where limestone is required to correct soil acidity, dolomitic lime should be used whenever the magnesium test is less than 100 ppm. For further information, refer to OMAFRA Factsheet, Soil Acidity and Liming, Order No. 91-086, or visit the Web site at www.omafra.gov.on.ca/english/crops.

Few soils that do not need lime will require magnesium. Magnesium application is recommended only if the magnesium test is under 20 ppm. On these soils, magnesium can be supplied either by magnesium sulphate or, if potassium is also required, by sulphate of potash magnesia. Apply 30 kg of water-soluble magnesium/ ha (27 lb/ac).

Over-application of potassium can induce magnesium deficiency. For this reason, it is important to monitor soil potassium levels closely and restrict potash application rates to those recommended by the OMAFRA-accredited soil test.

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Sulphur

Sulphur deficiency has not been observed in corn in Ontario. The corn-growing areas of the province receive adequate sulphur as acid precipitation.

Zinc

Zinc deficiency occurs on corn in Ontario. Visible symptoms on the leaves (see Plate 7) are the best indications of deficiency, but soil tests are also useful. Zinc deficiency usually appears as a broad white band near the base of the younger leaves on a corn plant. In severe deficiencies, the entire leaf in the whorl will be white (known as "white-bud"). Response to zinc should not be expected unless deficiency symptoms are quite marked.

Plate 7. Zinc deficiency appears as a broad white band near the base of the leaf on younger plants.

When zinc is required, it may be applied to the soil mixed in the fertilizer at rates supplying 4-14 kg/ha (3.5-12.5 lb/ac). The higher rate should be sufficient for up to 3 years. Not more than 4 kg/ha should be banded at planting. Zinc may be applied as a foliar spray at rates supplying 60 g/100 L (0.6 lb/100 gal). A wetting agent should be added. Spray to leaf wetness. For further information, see the section on Micronutrient Fertilizers.

Manganese

Manganese deficiency in corn is rare, although there have been a few occurrences reported on muck soils with high pH in southwestern Ontario. Corn is much more tolerant of low soil manganese levels than soybeans or cereals. Manganese deficiency in corn appears as an olive-green discolouration of the leaves, occasionally with faint striping. Foliar application of manganese is the most effective way to correct a deficiency.

Correct the deficiency as soon as detected by spraying the foliage with 2 kg of actual manganese/ha from manganese sulphate (8 kg of manganese sulphate/ha) in 200 L of water. A "spreader-sticker" in the spray is recommended. If the deficiency is severe, a second spray may be beneficial. When applying micronutrients, take care to first clean out the spray tank of a sprayer that has been used to apply herbicides.

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Other Micronutrients

Other micronutrients are not likely to be deficient in corn in Ontario. Some micronutrients, such as boron, can be toxic if applied to corn, particularly if applied in a band or in the starter/pop-up fertilizer.

Updates on Corn: Secondary and Micronutrients

OMAFRA Factsheet, Soil Acidity and Liming, Order No. 91-086 is no longer available.

• Magnesium Soil Test Ratings and Interpretations for All Crops
• Manganese Requirement Table and Zinc Requirement Table

Related links...

• Corn

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