Magnesium
Magnesium is plentiful in most Ontario soils, but deficiencies can occur on acidic, sandy soils. The symptoms appear first as yellow striping of the lower leaves (Figure 1). As the deficiency worsens, the upper leaves may become striped while the lower leaves turn reddish-purple.
Dolomitic lime is an excellent source of magnesium where limestone is required to correct soil acidity and should be used whenever the magnesium test is less than 100 ppm. For further information, see Soil acidity and liming.
Soils that do not need lime will seldom 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/ha (27 lb/acre) of water-soluble magnesium.
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 suggested by the OMAFA-accredited soil test.
Sulphur
Sulphur deficiency in corn has not been widely observed in southern Ontario. However, in the past two decades, sulphur deposition from the atmosphere has steadily declined to the point that most corn-growing areas of the province no longer receive adequate sulphur as acid precipitation. Sulphur shortages are becoming more common in corn on light-textured soils, such that sulphur is more frequently added to broadcast and banded fertilizer applications. Generally, an application rate in the range of 10–20 kg/ha (9–18 lb/acre) of sulphate sulphur with the fertilizer is adequate.
Zinc
Zinc deficiency occurs on corn in Ontario. Visible symptoms on the leaves (Figure 2[cmb2] ) 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.
When zinc is required, it may be soil applied by mixing with fertilizer at rates supplying 4–14 kg/ha (3.5–12.5 lb/acre). The higher rate should be sufficient for up to 3 years. Not more than 4 kg/ha (3.5 lb/acre) 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.
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/ha (1.8 lb/acre) of actual manganese from manganese sulphate (8 kg/ha (7 lb/acre)) in 200 L of water. A “spreader-sticker” in the spray is suggested. If the deficiency is severe, a second application may be beneficial. Prior to applying micronutrients, take care to properly clean out the spray tank of a sprayer that has been used to apply herbicides.
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.
Plant Analysis
The most appropriate growth stage for sampling corn for plant analysis depends on which nutrient is being tested for. For most nutrients, sampling the mid-third of the ear leaf at silking is most appropriate. For phosphorus and zinc, sampling the whole plant when 5–6 leaves are visible is more appropriate. See Table 1, Interpretation of plant analysis for corn for normal concentrations of nutrients.
Table 1. Interpretation of Plant Analysis for Corn
| Nutrient | Critical Concentration1 | Maximum Normal Concentration2 |
| Seedling Corn (five to six leaves) | ||
| Phosphorous | 0.35% | 0.70% |
| Zinc | 20.0 ppm | 70.0 ppm |
| Silking (mid-third of leaf opposite ear) | ||
| Nitrogen (N) | 2.5% | 3.5% |
| Phosphorous (P) | 0.28% | 0.50% |
| Potassium (K) | 1.2% | 2.5% |
| Calcium (Ca) | – | 1.5% |
| Magnesium (Mg) | 0.10% | 0.60% |
| Sulphur (s) | 0.14% | – |
| Boron (B) | 2.0 ppm | 25.0 ppm |
| Copper (Cu) | 2.0 ppm | 20.0 ppm |
| Manganese (Mn) | 15.0 ppm | 150.0 ppm |
| Zinc (Zn) | 20.0 ppm | 70.0 ppm |
| 1 Yield loss due to nutrients deficiency is expected with nutrient concentrations at or below the “critical” concentration. 2 Maximum normal concentrations are more than adequate but do not necessarily cause toxicities. | ||
For sampling at times other than those indicated above, take plant samples from both deficient and healthy areas of the field for comparative purposes. For plants with six leaves or less, sample the total above-ground plant. From V7 to silking, sample the youngest fully developed leaf. Take a soil sample from the same areas and at the same time as the plant samples. For more information, see the section Plant analysis.[BR3]
Foliar Fertilization
The foliar application of nutrients to corn has not proven effective in most instances. The rates of nutrients required cannot be applied as a foliar spray without causing damage to the leaf, unless numerous small applications are made. Correction of some of the micronutrient deficiencies are the exception, but even in these cases, it is often more economical to apply the nutrient to the soil.