Corn: Nitrogen

Excerpt from Agronomy Guide for Field Crops (Chapter 3)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Introduction
2. Nitrate-Nitrogen Soil Test
3. Time of Sampling
4. Taking the Sample
5. Handling the Sample
6. Where Caution Is Required
   • Nitrogen Recommendations Based on Nitrate-Nitrogen - Table 3-18
7. Laboratories
8. Nitrogen/Corn Price Ratio
   • General Recommended Nitrogen Rates for Corn - Table 3-19
9. General Recommendations
   • Nitrogen Recommendations for Corn Grown in Rotation - Table 3-20
10. Sample Calculation
    • Phospate and Potash Recommendations for Corn Based on OMAFRA-Accredited Soil Tests - Table 3-21
11. Nitrogen Application
12. Updates on Corn: Nitrogen
13. Related links...

Introduction

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

Nitrogen deficiency shows up on the lower leaves of a plant first, 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. Colour Plates Guide to Nutrient Deficiency Symptoms. Reprinted with permission of the Saturday Evening Post © 1957 (renewed) BFL&MS, Inc.

Plate 5. Nitrogen deficiency shows up on lower leaves first. Yellowing begins at the leaf tip and proceeds down the midrib.

In young plants, however, 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 or general recommendations based on expected yield and location, 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.

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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 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 the groundwater.

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). Often, this is not a convenient time for sampling, and seasonal differences in weather can dramatically change the soil tests at this time of year (refer to the section Where Caution Is Required). Alternatively, sampling when the corn is 15-30 cm (6-12 in.) tall, before applying 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 particularily 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

As nitrates are more mobile than either phosphorus or potassium, 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 in the section Soil Sampling. Because variations in soil nitrate content can have such a large impact on nitrogen fertilizer recommendations, it may be worthwhile to sample more intensively for nitrate than for phosphorus or potassium. Sample areas with differences in past management separately, and knolls separately from depressions. Areas of a field with distinctly different soil types should also be sampled separately.

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Handling the Sample

Cores should be placed 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 its nitrate content quickly if it is not handled properly. Samples should be chilled or frozen as soon as possible. For shipping, samples should be packed with insulating material to keep them cool and sent 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 labs without any extra precautions. Do not dry the samples in a warm oven, as it can affect the nitrate content.

Where Caution Is Required

Sometimes the fertilizer recommendations based on the nitrate-nitrogen soil test should 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)

The nitrate-nitrogen soil test should be used with caution in these circumstances.

Table 3-18. Nitrogen Recommendations Based on Nitrate-Nitrogen, shows the recommended application rates of nitrogen for different levels of soil nitrate-nitrogen for samples taken to a depth of 30 cm (12 in.) when the nitrogen/corn price ratio is 5. (For an explanation of the price ratio, see the section Nitrogen/Corn Price Ratio.) If the price ratio is increased to 7 (i.e., the price of nitrogen fertilizer has increased or the price of corn has decreased), the recommended rates should be reduced 20 kg/ha from the rates in this table. If the price ratio drops to 3, the rates should be increased by 30 kg/ha.
Rates should be adjusted downward if the preceding crop was a legume sod (see Table 2-15. Adjustment of Nitrogen Requirement Where Crops Containing Legumes Are Plowed Down). If manure was applied, see Table 2-16, Average Amounts of Dry Matter, Nitrogen and Available Phosphate and Potash for Different Types of Organic Nutrient Sources.

Conversion Factors
To convert soil test results from kg/ha to ppm for a 30-cm (12-in.) sample, divide kg/ha by 4. For example, if the nitrate-nitrogen concentration of a sample taken from the top 30 cm (12 in.) of soil is 32 kg/ha, the nitrate nitrogen is 32 kg/ha ÷ 4 = 8 ppm.

¹Spring nitrate-nitrogen refers to samples taken within 5 days of planting (either before or after).
²Pre-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).

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Laboratories

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

Nitrogen/Corn Price Ratio

The most profitable rate of nitrogen application is also affected by the relative prices of corn and nitrogen, which can be expressed as a nitrogen/corn ratio. This ratio is calculated by dividing the cost of nitrogen, in dollars per kilogram of actual N, by the expected selling price of grain corn, in dollars per kilogram.

1. To determine the cost per kg of actual nitrogen, divide the cost per tonne of fertilizer material by 10 times the percentage of nitrogen in the fertilizer. Application costs should be included.

   Cost of N/kg = Cost of Nitrogen Fertilizer per Tonne ÷ (10 x (% Nitrogen in fertilizer))

2. To determine the selling price per kg of corn, divide the price per tonne by 1,000. Include any anticipated payments from stabilization or market revenue programs.

3. To determine the nitrogen/corn price ratio, divide the cost of N per kg by the selling price of grain corn per kg.

   Nitrogen/Corn ratio = Cost of N/kg ÷ Price of Corn/kg

The recommendations in Table 3-18. Nitrogen Recommendations Based on Nitrate-Nitrogen, and Table 3-19. General Recommended Nitrogen Rates for Corn, are based on a nitrogen/corn price ratio of 5 (e.g., a nitrogen cost of $0.65/kg of actual nitrogen and a corn price of $0.13/kg or $130/tonne).

100 kg/ha = 90 lb/ac
1 1/ha = 16 bu/ac

*See Updates section below.

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General Recommendations

In the absence of a soil test for nitrate-nitrogen, Table 3-19. General Recommended Nitrogen Rates for Corn, can be used to determine an average recommendation for nitrogen for various regions of Ontario based on the expected yield and what is expected from a field in a normal year, based on previous experience. Nitrogen rates can be reduced following some crops (see Table 3-20. Nitrogen Recommendations for Corn Grown in Rotation). Fertilizer rates should also be adjusted for legumes plowed down (see Table 2-15. Adjustment of Nitrogen Requirement Where Crops Containing Legumes Are Plowed Down) and manure applied (see Table 2-16, Average Amounts of Dry Matter, Nitrogen and Available Phosphate and Potash for Different Types of Organic Nutrient Sources).

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Sample Calculation

If the expected yield of grain corn in a field in western Ontario is 7 t/ha (112 bu/ac), the nitrogen recommendation would be 110 kg/ha. (See Table 3-19. General Recommended Nitrogen Rates for Corn ). If the soil tests are 11 for phosphorus and 110 for potassium, the phosphate requirement would be 50 kg/ha and the potash requirement 30 kg/ha. (See Table 3-21. Phosphate and Potash Recommendations for Corn Based on OMAFRA-Accredited Soil Tests.) The phosphate could be supplied by 160 kg/ha of 8-32-16 (50 x 100÷32 = 160). This would also supply 12 kg of nitrogen/ha and 25 kg of potash/ha. The remaining 98 kg of nitrogen/ha could be supplied by 120 kg of anhydrous ammonia/ha (98 x 100÷82 = 120).

100 kg/ha = 90 lb/ac

¹ Where manure is applied, reduce fertilizer applicaitons according to the amount and quality of manure (See Table 2-16. Average Amounts of Dry Matter, Nitrogen and Available Phosphate and Potash for Different Types of Organic Nutrient Sources.)
² When the soil test rating for a utrient is "Excessive," applicaiton 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.

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Nitrogen Application

The major portion of the nitrogen should be applied in the spring, either 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. Take care 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 from the high phosphate starter at a distance from the seed row greater than 10 cm (4 in.).

In southwestern Ontario, side-dressed applications have shown a slight yield advantage over preplant applications and anhydrous ammonia has produced 3%-5% higher yields than urea or ammonium nitrate. Throughout the rest of the province, there has been no yield advantage for side-dressed applications, and no yield differences have been found comparing the various nitrogen materials.

Solid forms of nitrogen or urea-ammonium nitrate solutions (UAN) may be applied to the soil surface without incorporation. However, urea or UAN applied on crop residues should be incorporated into the soil immediately. Urea or UAN that is left on the surface can lose a significant amount of nitrogen to the air through ammonia volatilization. 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.

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Updates on Corn: Nitrogen

Addition to Table 3-19 footnote - "Nutrient recommendations for use in nutrient management plans should be calculated using the NMAN software or the Nutrient Management Workbook."

OMAFRA Publication 811: Agronomy Guide for Field Crops Table of Variance

• Nitrogen Soil Test for Corn
• Nitrogen Requirements - Annual Field Crops
• General Recommended Nitrogen Rates for Corn: Corn N Worksheet

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Related links...

• Corn
• Soil Fertility & Crop Nutrition

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