By Douglas Beegle, Agronomist

Most people know to make adjustments for yield, manure and legumes, but even those adjustments are based on averages. There are some things we can do to go beyond average nitrogen (N) management.

The Pre-sidedress 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 absolute correct answer to your N management decision, but research has shown that both, if used correctly, can significantly improve N management. 

A summary of research data over a number of years where we know the actual economic optimum N rate, showed that these tests, while far from perfect, roughly doubled the accuracy of the sidedress N recommendations compared to not using these tests. 

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 5-6. 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 at least 12 inches tall — sampling too early can give misleading results. Late is better than early, as long as you still have time to sidedress if necessary.
  • Do not sample immediately following heavy rains, wait 2–3 days.
  • Take a 12-inch deep soil sample. Dry the sample the same day 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 pounds of N, especially starter N, was applied per acre 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 fifth 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 l15 pounds of fertilizer N is the limit for using the two-step procedure. This procedure 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.

Another option is to use a N/weather model to estimate what is happening to the N in a field. There are several models commercially available. Examples include: Adapt-N from Agronomic Technology Corp., Encirca from Pioneer and Climate Fieldview Pro from Monsanto. These models use local weather data and information input by the farmer about the field characteristics and management to model N behavior and then use this to make a sidedress recommendation. In addition to the recommendation, these models provide useful insights into what is happening with N in the field that can be used to improve N decisions.

None of these will always give the absolute correct answer, but all of these approaches provide information that the farmer can use to significantly improve sidedress N recommendations beyond just using the basic recommendation. They also all have limitations, one of which is that they cannot predict what is going to happen after sidedressing time. 

Also, it may not be practical to sample or model all fields, so sampling several fields using the PSNT or Chlorophyll Meter or running the model on fields 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.