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Competency Area 3: Soil Testing and Plant Tissue Analysis

PO 21. Recognize factors that influence the results of the pre-sidedress nitrogen test.

  1. Rotation
  2. Manure type and history
  3. Timing of sampling relative to weather patterns
  4. Depth of sampling
  5. Field variability
  6. Sample processing

Most universities in the Northeast do not use a soil nitrogen test, because nitrate is very mobile and soil nitrate tests do not correlate well with actual N supply from the soil.  Rather, nitrogen requirements are based on yield potential, N supply from the soil, and other organic N sources (composts, animal and green manures) and N uptake efficiency. The exception to this is the Pre Sidedress Nitrogen Test (PSNT).

The PSNT is an in-season nitrate test for corn that predicts organic N to be mineralized. It is designed to:

  1. Estimate the soil's nitrate supplying potential.
  2. Decide if that is enough N to meet crop needs.

Vermont determines application rates based on PSNT results. Other states (e.g. NY) use the PSNT as an indicator only. By avoiding the unnecessary application of N, producers save money and reduce the risk of nitrate leaching.

Testing is done at sidedressing time, just before the period of major N demand. It is most useful for fields with a history of manure and/or sod incorporation. However, it cannot be used if broadcast or pre-plant N has been applied (over 40 lb N/acre), as "leftover" nitrate will be detected and will overestimate soil supply.

Several factors influence the results of the PSNT.

  • Rotation: PSNT is appropriate for corn fields in the second or later year after a sod. First-year corn after grass and/or alfalfa does not need additional N beyond a small starter N application (20-30 lb N/acre). The PSNT results for first-year corn fields have been shown to be unreliable as indicators of soil N supply.
  • Manure type and history: the PSNT is appropriate when the manure history of a field is uncertain.
  • Timing of sampling relative to weather patterns: wait 2-3 days after a significant rainfall, to avoid sampling after nitrate leaching.
  • Depth of sampling: samples should be taken from between rows (avoid sampling the starter band), to a depth of 12 inches.
  • Field variability: a sample area should be limited to 15 acres or less, and separate samples should be taken for areas with different corn stands (population densities, stages of development, color, etc), crop histories, fertility managements, changes in slope, etc.
  • Sample processing: samples should be dried immediately to stop N mineralization - spread them thinly and allow to dry in the sun or under a fan.

For fields with <21 ppm:

  • If you took a PSNT on a field that you expected to need sidedress N (for instance, a field that received less manure than needed to meet N needs), add the extra N.
  • If you took a PSNT on a field that you did not expect to need sidedress N (for instance, a field where manure application should have provided sufficient N), make sure the field actually received the planned manure application and that the field history is correctly recorded. Check N needs with the NYS Corn N Calculator, using the lowest manure application estimates. If the calculator still shows that no additional N is needed, organic N mineralization rates early in the season were like lower than average, but should improve once mineralization conditions improve (warm and moist soils). The field soil will then provide sufficient N. If the calculator shows that additional N is needed and the PSNT is <21 ppm, consider adding the extra N.