Nitrogen Tests For Corn
Feasible Even After Red Clover
By Ken Janovicek, Tony Vyn and Eric Beauchamp,
Departments of Plant Agriculture and Land Resource Science, University of Guelph

Nitrogen recommendations for corn have traditionally been based on historical yield response data; higher nitrogen (N) fertilizer rates have been recommended as yield expectations increased.

However, Ontario research conducted over the past 30 years suggests that the actual N requirement to achieve maximum economic yield (MEY) is only weakly related to yield expectation. In fact, it is not uncommon for fields with high yields to have relatively low N requirements or, conversely, for low yielding fields to respond economically to unexpectedly high rates.

A better indicator of the maximum economic nitrogen rate for corn is the quantity of available N already present in the soil during late spring. This article discusses the potential and limitations of using soil N tests to more accurately determine corn nitrogen fertilizer requirements.

How does the soil N test work?

The soil N test is based on the discovery that the amount of nitrate present in the upper one foot of soil on normal planting dates (May 1-15) or just prior to side-dress application (June 1-15) can be reliably used to estimate the maximum economic nitrogen rate. The concentration of nitrate in the upper one foot of soil is used to calculate the N test value, which is representative of the amount of available nitrogen already present and is related to what is likely to become available to corn throughout the rest of the growing season. The N test value can be considered an index of the potentially available N to corn for the current growing season.

The N test value increases throughout the spring period mainly because of N release associated with decomposition of previous crop residues and soil organic matter. Therefore, the accuracy of the soil N test is dependent on sampling time. It should be conducted between May 1-15 (planting) or June 1-15 (before side-dress) to ensure the most accurate results.

Nitrogen rate recommendations for various N test values for planting and pre-sidedress sample dates are shown in Table 1. Corn producers with N tests in excess of 115 at planting or 150 at side-dress need not apply any nitrogen fertilizer, except for a small quantity with a phosphorous-based starter fertilizer, since enough nitrogen will likely be available naturally to attain MEY.

How should fields be sampled?

The soil N test is based on a soil sample taken to one foot. Samples should be collected throughout the field with at least one core per acre. The starter fertilizer band should be avoided if sampling after planting. A field which is not uniform, due either to previous management, soil texture or topography, should be partitioned into more uniform sections and each section sampled separately. For more information on sampling, sample preparation and interpretation of results, consult either Ontario Field Crop Recommendations (Publication 296), your local OMAFRA office or your local fertilizer dealer.

When can the soil N test be used?

Soil N tests can be used for corn planted following all of the common field crops grown in Ontario, with some special considerations when corn is planted following forage legumes such as alfalfa or underseeded red clover. Soil N tests may not be accurate where manure was applied the previous fall or in the spring of the sampling year. The soil N test also may not be accurate where living cover crops were plowed under or burned-down in the spring of the sample year. However, if an error occurs based on the soil N test following recently applied manure or spring killed cover crops, it will be to overestimate N fertilizer requirements. Also, the soil N test should not be used to confirm N sufficiency after N fertilizer has been broadcast applied because it has not been properly evaluated for this purpose.

How should the soil N test be used following red clover?

We have conducted on-farm evaluations of the soil N test on a wide range of soil types (sandy loam to clay loam) in both plow and no-till systems. Our research has shown that the nitrogen fertilizer requirement for corn following underseeded red clover can be accurately determined with the pre-sidedress N test (PSNT) provided that clover was plowed or chemically killed in the fall. The planting date test was not as accurate. Soil nitrate concentrations can increase rapidly in the spring period following red clover, with the rate of increase related to the amount of clover growth and weather conditions. Therefore, the planting date test should not be used since later sample dates (i.e., PSNT) will better reflect N availability from red clover. These special considerations for the soil N test following underseeded red clover also apply to corn following alfalfa.

An example of the potential reliability of the PSNT was obtained over a four-year period on sandy loam farms near Ayr (Table 2 ). Plots were fall-plowed after wheat alone or wheat plus red clover, and corn was fertilized with multiple N rates. The actual (experimentally determined) nitrogen requirement was often closer to the PSNT recommendations than to the yield-goal based recommendations of 125 lb N/ac following wheat alone and 85 lb N/ac following underseeded red clover. Overestimation of requirements by the PSNT during 1996 was due to an abnormally cool and wet May which delayed N release from clover and organic matter until after the PSNT sample date.

We are confident that the PSNT will only rarely underestimate the maximum economic N rate required for corn following fall-killed or fall-plowed clover. Furthermore, using PSNT recommended rates will help minimize the risk of nitrate leaching losses, which can be particularly high after red clover and/or in fields with recent history of manure application when no adjustments are made in N fertilizer rates. Even if corn farmers arbitrarily added 20 lb N/ac to their PSNT-based recommendations to help avoid the risk of underfertilizing, they are still better stewards of their wallets and the groundwater than if they applied high N fertilizer rates without regard for available soil N.

Conclusion

The soil N test is the only extensively evaluated method available for predicting corn nitrogen requirements based on current field conditions. If properly conducted, the soil N test offers Ontario’s corn producers an opportunity to increase their profitability. The PSNT is even reliable after red clover. So this spring, why not try the PSNT between June 1 to 15 (i.e., before you select your side-dress N rate)?

Acknowledgements
This project was partly based on M.Sc. thesis work by John Faber and Zak Zizza, Crop Science. Funding was provided by the OMAFRA Plant Program, OCPA and the AAFC Green Plan.

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