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In-Season Testing For Corn N Management

By Douglas Beegle
Distinguished Professor of Agronomy
Does your corn crop need additional Nitrogen to produce a top yield? Here are two ways to find out.
This is the annual reminder that the Presidedress Soil Nitrate Test (PSNT) and the Chlorophyll Meter Tests are available to help improve N management decisions at sidedress time. Neither of these tests will give you the perfect answer to your N management decision, but research has shown that both, if used correctly, can significantly improve N management. One key is they must be used correctly to be valid. Here is a brief reminder of how to use these two tests. 
Pre-sidedress Soil Nitrate Test (PSNT)
With the PSNT, for optimum N nutrition for corn, the soil nitrate-N level in the PSNT should be above 21 ppm when the corn is 12 inches tall or around leaf stage 6. If it is above this level, there is adequate N in the soil to meet the needs of the crop. However, if it is below this critical level then sidedressing additional N is recommended, even if adequate N was applied earlier. 
Agronomy Fact # 17, “Pre-sidedress Soil Nitrate Test for Corn” provides all of the details on the PSNT including the sampling procedures and how to make recommendations based on the results. 
A couple of important points to keep in mind when using the PSNT:
  • Wait until the corn is 12” tall, sampling too early can give misleading results
  • Do not sample immediately following heavy rains, wait 2 – 3 days.
  • Take a 12” deep soil sample.
  • Dry the sample the same day as it is collected.
  • Use a quick test kit or send the dry sample to a soil testing lab for NO3-N analysis.
Note: Because of the narrow window for sidedressing, most soil testing labs including the Penn State Ag Analytical Services Lab will provide 1 day turn-around once they receive the samples for the PSNT
Chlorophyll Meter Test
All of the details for using this test are available in a factsheet on using the chlorophyll meter, Agronomy Facts #53 (Revised) The Early-season Chlorophyll Meter Test for Corn. This factsheet updates the procedure for testing fields with a high N reference area and includes the two-step procedure for fields with recent manure or a forage legume without a high N reference area. 
A couple of important points to keep in mind when using the Chlorophyll Meter:
  • Take samples only after the corn has reached at least the 6 leaf stage
  • The chlorophyll meter cannot be used if more than 15 lb N fertilizer/A, especially starter N, was applied at planting. Not following this condition could result in a false indication of adequate N for the crop and thus if no N is sidedressed an N deficiency is likely.
  • Take readings on the 5th leaf about ¾ of the way out the leaf between the edge of the leaf and the midrib
  • Take readings on at least 30 plants in a field
  • Use the worksheet in Agronomy Facts #53 (Above) to make a sidedress N recommendation
  • For best results a high N reference area should be established shortly after corn planting in fields to be tested.
  • If you did not establish a high N reference area, you can still use the chlorophyll meter test on fields with a significant amount of manure or following a forage legume, but again less than 15 lb of fertilizer N, using the two-step procedure. This involves taking a reading at the 6 leaf stage and then if necessary taking another reading a week later. See the factsheet #53 above for details.
It may not be practical to sample all fields, so sampling several fields using the PSNT or Chlorophyll Meter that are representative of different N management strategies on your farm this spring can be very helpful in making a decision about whether additional sidedress N is needed on similar fields. N is too expensive to just put some extra on just in case.

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