PO 21. Recognize factors that influence the results of the pre-sidedress nitrogen test.
- Rotation
- Manure type and history
- Timing of sampling relative to weather patterns
- Depth of sampling
- Field variability
- Sample processing
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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). 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.
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