Corn: Fertility Management
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811: Agronomy Guide > Corn
> Fertility Management
Table of Contents
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.
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:
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.
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.
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.
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:
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.
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:
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.
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.
1 Spring nitrate-nitrogen refers to samples taken within
5 days of planting (either before or after).
Adjust rates downward if the preceding crop was a legume sod see
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.
See Appendix C, Accredited Soil-Testing Laboratories in Ontario for a list of laboratories that are accredited to analyze soil samples for nitrate-nitrogen.
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
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
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.
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.
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.
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
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.
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.
Applying too much fertilizer to corn may result in crop injury, either from excessive salts or ammonia Plate 8.
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.
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.
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 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 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.
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 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.
2 Maximum normal concentrations are more than adequate but do not necessarily cause toxicities.
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 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
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
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.
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