Corn: Fertility Management

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Pub 811: Agronomy Guide > Corn > Fertility Management

Excerpt from Agronomy Guide for Field Crops
Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

• Nitrogen
  • Nitrate-Nitrogen Soil Test
  • Time of Sampling
  • Taking the Sample
  • Handling the Sample
  • Where Caution is Required
  • Table 1-24. Nitrogen Recommendations Based on Nitrate-Nitrogen
  • Laboratories
• Corn Nitrogen Rate Workshop
  • General Recommended Nitrogen Rates for Corn (Metric)
  • Table A. Base N Requirement
  • Table D. Previous Crop Adjustments
  • Table E. Price Ratio (PR) Adjustment for Nitrogen Relative to Corn Price
  • Table J. Timing Adjustment (Southwestern and Central Ontario only)
  • Table 1-25. Phosphate and Potash Recommendations for Corn Based on OMAFRA-Accredited Soil Tests
  • Nitrogen Application
• Phosphate and Potash
  • Seed-Placed Fertilizer
• Maximum Safe Rates of Fertilizer
• Secondary and Micronutrients
  • Magnesium
  • Sulphur
  • Zinc
  • Managanese
  • Table 1-26. Interpretation of Plant Analysis for Corn
  • Other Micronutrients
• Plant Analysis
• Foliar Fertilization

Nitrogen

Corn responds well to nitrogen, so adequate availability of nitrogen is critical to profitable corn production. However, excess nitrogen adds unnecessary expense and increases the risk of nitrate movement to groundwater.

Nitrogen deficiency first appears on the lower leaves, manifested as yellowing, beginning at the tip of the leaf and proceeding down the midrib. Eventually, the yellow areas will turn brown and die.

Plate 1. Nitrogen deficiency shows up on lower leaves first. Yellowing begins at the leaf tip and proceeds down the midrib.

In young plants, yield loss will occur long before nitrogen deficiency symptoms appear, so yellowing is not a reliable indicator of the need for nitrogen fertilizers.

Two methods can be used to determine optimum nitrogen rates:

• the nitrate-nitrogen soil test
• general recommendations based on expected yield, soil type, previous crop, location and fertilizer price adjusted for legumes, manure or other sources of organic nitrogen

It is common to see symptoms of nitrogen deficiency in the lower leaves as the plants near maturity, even when there is adequate nitrogen for optimum yield.

Nitrate-Nitrogen Soil Test

Soils can vary greatly in their ability to supply nitrogen. The amount of nitrate-nitrogen present in the soil at planting time, or just before side-dress, can be a useful indicator of a soil's capacity to supply nitrogen. Use of the soil test for nitrate-nitrogen should result in a more efficient and profitable use of nitrogen as well as a reduction in the risk of nitrate movement into groundwater.

Many of the factors included in the general recommendations will influence the soil nitrate levels, so the recommendations for the nitrate-nitrogen soil test should be viewed as separate from the general nitrogen recommendations. Research is ongoing to find methods to incorporate the soil test into the general recommendations as an adjustment.

Time of Sampling

The nitrogen recommendations based on the soil test for nitrate-nitrogen were developed using samples that were taken within 5 days of planting (before or after). However, this is often an inconvenient time for sampling. Seasonal differences in weather can dramatically change the soil tests at this time of year see Where Caution Is Required. Alternatively, sampling when the corn is 15-30 cm (6-12 in.) tall, before the application of side-dress nitrogen, has increased in popularity. This is referred to as the pre-side-dress nitrogen test (PSNT).

By delaying sampling past the busy planting season, the PSNT allows more time for sampling and receiving results from the laboratory. More importantly, considerable evidence indicates that nitrogen recommendations based on this later sampling time are superior to those based on a planting time sample. This is particularly true when there are organic sources of nitrogen, such as manure or legumes, in the cropping system. PSNT samples taken in June detect nitrate that has mineralized from these organic sources and will more accurately reflect total available nitrogen and fertilizer nitrogen requirements.

Taking the Sample

Nitrates are more mobile than either phosphorus or potassium, so a separate, deeper, soil sample must be taken for the nitrate-nitrogen test. The soil should be sampled to a depth of 30 cm (12 in.). It is important that all cores in a field be taken to the same depth and that the sampling depth be included with the information sent with the sample to the lab.

To ensure that the sample is representative of the field, use a sampling pattern similar to that recommended for the standard soil test, described in Soil Sampling. Since variations in soil nitrate content can have a large impact on nitrogen fertilizer recommendations, consider sampling more intensively for nitrate than for phosphorus or potassium.

Take separate samples of:

• areas with differences in past management
• areas with distinctly different soil types
• knolls and depressions

Handling the Sample

Place soil cores in a clean plastic pail, crushed by hand and well mixed. Take about 500 gm of soil (1 lb) from the pail and place it in a clean plastic bag or soil sample box.

Microbial action in the sample can change the nitrate content quickly if it is not handled properly. Chill or freeze samples as soon as possible. For shipping, pack samples with insulating material to keep them cool and send them by courier to ensure quick delivery to the lab.

Samples can also be air-dried. Spread the sample in a thin layer on a clean plastic sheet, breaking up any large lumps in the process. It should be dry in 1-2 days, and can be shipped to the lab without any extra precautions. Do not dry the samples in a warm oven, as this can affect the nitrate content.

Where Caution Is Required

Sometimes the fertilizer recommendations based on the nitrate-nitrogen soil test need to be modified.

The nitrogen in manure or legumes applied or plowed down just before sampling will not have converted into nitrates and will not be detected by the soil test. Information will be provided with the test results on how to make appropriate adjustments.

The nitrate-nitrogen soil test has not been adequately evaluated for:

• legumes or manure plowed down in the late summer or fall
• legumes in a no-till system
• soil samples taken prior to planting before the soil has warmed up significantly (i.e., in mid- to late April)
  

In these circumstances, use the nitrate-nitrogen soil test with caution.

Table 1-24, Nitrogen Recommendations Based on Nitrate-Nitrogen shows the recommended application rates of nitrogen for different levels of soil nitrate-nitrogen for 30-cm (12-in.) deep samples when the nitrogen/corn price ratio is 5. If the price ratio is increased to 7 (i.e., the price of nitrogen fertilizer has increased or the price of corn has decreased), reduce the recommended rates by 20 kg/ha (18 lb/acre) from the rates in this table. See Price Ratio Adjustment, in Appendix B.

¹ Spring nitrate-nitrogen refers to samples taken within 5 days of planting (either before or after).
² Pre-side-dress nitrate-nitrogen refers to samples taken when the corn is 15-30 cm (6-12 in.) tall (usually within the first 2 weeks of June).

Adjust rates downward if the preceding crop was a legume sod see Table 9-7, Adjustment of Nitrogen Requirement, Where Crops Containing Legumes Are Plowed Down. If manure was applied, see Table 9-8, Typical Amounts of Available Nitrogen, Phosphate and Potash From Different Types of Organic Nutrient Sources.

Laboratories

See Appendix C, Accredited Soil-Testing Laboratories in Ontario for a list of laboratories that are accredited to analyze soil samples for nitrate-nitrogen.

Corn Nitrogen Rate Worksheet

General Recommended Nitrogen Rates for Corn (Metric)

The figures in this worksheet are based on a review of N response trials from 1961-2004. The fertilizer rates calculated here are designed to produce the highest economic yield when accompanied by good or above-average management. Research shows that higher rates will occasionally produce higher yields, but usually not enough to pay for the additional fertilizer.

A version of the worksheet using Imperial measure, as well as notes that explain each section can be found in Appendix B, Corn Nitrogen Rate Worksheet (Imperial) With Detailed Explanation.

¹ Manure N Credits can be found in Chapter 9, Soil Fertility and Nutrient Use.

100 kg/ha = 90 lb/acre
¹ HR, MR, LR, RR, and NR denote, respectively, high, medium, low, rare and no probabilities of profitable crop response to applied nutrient. Profitable response to applied nutrients occurs when the increase in crop value, from increased yield or quality, is greater than the cost of the applied nutrient.
² Where manure is applied, reduce fertilizer applications according to the amount and quality of manure see Table 9-8, Typical Amounts of Available Nitrogen, Phosphate and Potash From Different Types of Organic Nutrient Sources.
³ When the response rating for a nutrient is "NR," application of this nutrient in fertilizer or manure may reduce crop yield or quality. For example, phosphate applications may induce zinc deficiency on soils low in zinc and may increase the risk of water pollution. Potash application on soils low in magnesium may induce magnesium deficiency.

Nitrogen Application

The major portion of the nitrogen should be applied in the spring, preplant, pre-emergence or side-dressed before the corn is 30 cm (12 in.) high. Fall application is not recommended because of the potential for high losses.

A portion of the nitrogen may be applied in a band at planting. Ensure that safe rates of fertilizer near the seed are not exceeded. Where it is desirable to apply high rates of nitrogen at planting, it should be placed in a separate band greater than 10 cm (4 in.) from the seed row.

Solid forms of nitrogen or urea-ammonium nitrate solutions (UAN) may be applied to the soil surface without incorporation. However, incorporate urea or UAN applied on crop residues into the soil immediately to prevent ammonia volatilization losses. Under dry conditions, the effectiveness of all forms of nitrogen may be improved by incorporation. If UAN solutions contact leaves, burning and yield reductions may occur.

Anhydrous ammonia, applied with conventional equipment, should be placed a minimum of 15 cm (6 in.) deep in the soil. For preplant applications, applicator outlets should be no more than 50 cm (20 in.) apart. For wider spacings, a 4-day waiting period before planting is recommended to avoid damage to seedlings.

When appropriate equipment is used, ammonia may be applied with a cultivator or disc, a minimum of 10 cm (4 in.) deep with the ammonia outlets spaced no more than 50 cm (20 in.) apart.

Phosphate and Potash

Adequate phosphorus and potassium are necessary for optimum corn growth and yield, although the response to these nutrients is not as evident as with nitrogen. Phosphorus deficiency does not show any obvious symptoms, although phosphorus-deficient plants will be stunted and may have a darker green or purplish colour. Purple leaves may also be an indication of cool weather stress or root injury.

Plate 2. Purple corn. Purple leaves on corn is most often caused by cool weather stress or root injury. Occasionally, it is an indication of phosphorus deficiency.

Potassium deficiency symptoms appear on the lower leaves of the plant first, showing as yellowing and browning beginning at the tip and proceeding back along the outside margin of the leaf. Both of these nutrients will exhibit "hidden hunger," where yields are reduced by a deficiency of one or both of these nutrients even though no deficiency symptoms are visible.

Plate 3. Potassium deficiency shows up on lower leaves first, as yellow and browning at the leaf tip, and proceeds back along the margin of the leaf.

Phosphate and potash recommendations for corn are presented in Table 1-25, Phosphate and Potash Recommendations for Corn Based on OMAFRA-Accredited Soil Tests.

For information on the use of this table or if an OMAFRA-accredited soil test is not available, see Fertilizer Recommendations.

Where soil tests indicate that large amounts of phosphorus and potassium are required, the major portion may be broadcast and incorporated in the fall or spring. Where soil tests show a moderate or small requirement for these nutrients, apply a fertilizer containing nitrogen (preferably in the ammonium form) and phosphorus, or nitrogen, phosphorus and potassium as a starter at planting. All of the phosphorus and some of the potassium may be applied in a band 5 cm (2 in.) to the side and 5 cm (2 in.) below the seed Table 9-21, Maximum Safe Rates of Nutrients.

Seed-Placed Fertilizer

Field trials over several years have shown that an application of 10-15 kg/ha (9-13 lb/acre) P₂O₅ directly with the seed will give greater yield increases than 20 kg/ha (18 lb/acre) P₂O₅ in a side band. At phosphorus soil tests of 13-45, this "with-seed" application is more likely to give a profitable response than a side-band application. At soil tests below 13, application of 10-15 kg P₂O₅/ha (9-13 lb P₂O₅/acre) with the seed may also be profitable but cannot replace the requirement for additional phosphorus in the side band or broadcast.

Fertilizers applied with the seed that contain nitrogen in the ammonium form must be low in salt and must not contain either urea or diammonium phosphate Table 9-21. They must also be distributed uniformly to avoid toxicity to the germinating seed. Application of more than 15 kg/ha (13 lb/acre) P2O5 with the seed in 75-cm (30-in.) wide rows is not recommended.

Maximum Safe Rates of Fertilizer

Applying too much fertilizer to corn may result in crop injury, either from excessive salts or ammonia Plate 8.

Plate 8. Fertilizer injury burns the primary root, delaying growth until secondary roots develop. Plant emergence will be uneven.

The more concentrated the fertilizer and the closer it is to the seed, the greater the risk of crop injury and the lower the safe rate. Maximum safe rates are given in Table 9-21. Note that slight reductions in crop growth and yield are possible with these application rates under adverse weather conditions.

Secondary and Micronutrients

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. As the deficiency worsens, the upper leaves may become striped while the lower leaves turn reddish-purple.

Plate 4. Magnesium deficiency appears first as yellow striping of the lower leaves. These may turn reddish-purple later as deficiency progresses.

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.

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/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 recommended by the OMAFRA-accredited soil test.

Sulphur

Sulphur deficiency in corn has not been observed in Southern Ontario. Most corn-growing areas of the province receive adequate sulphur as acid precipitation. Corn grown in Northwestern Ontario may respond to the inclusion of 15 kg/ha (13 lb/acre) of sulphate sulphur with the fertilizer.

Zinc

Zinc deficiency occurs on corn in Ontario. Visible symptoms on the leaves (Plate 5 on page 279) 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 5. 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/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/ha from manganese sulphate (8 kg/ha (7.1 lb/acre) of manganese sulphate) in 200 L of water. A "spreader-sticker" in the spray is recommended. If the deficiency is severe, a second spray 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 changes for different nutrients. 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 1-26, Interpretation of Plant Analysis for Corn, for normal concentrations of nutrients.

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 seven leaves 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.

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. The exception is correction of some of the micronutrient deficiencies, but even in these cases it is often more economical to apply the nutrient to the soil.