In-Season Testing For Corn N Management

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.

Source : psu.edu