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These three tests provide the best information for N management decisions.
- Preplant nitrate test. A soil nitrate test measures the amount of residual or carryover N in the active root zone.
- Pre-sidedress soil nitrate test allows you to apply more N for peak growth.Sample when corn plants are 6-12 in. tall, just before rapid growth and high N uptake.
- Fall stalk nitrate test can help farmers adjust N programs for the following spring.
You have to go back into the fields and test in the exact same spots where the nitrate tests were taken. Interpretation of the results is key.
Soil sampling calendar
Michigan State University Extension provides these recommendations for season-long nitrate testing:
Early spring. These soil samples primarily measure residual nitrate; the amount of N credit will be smaller. Soil nitrate content increases as soils warm. Testing for ammonium where manure has been applied provides a preliminary indication of available N release.
Spring. The greatest amount of soil nitrate usually is available three to four weeks after corn emergence (V6-V8 stage). Sidedress N when corn begins to take up N rapidly. Samples taken before sidedress can determine the appropriate N rate and measure both residual nitrate from the previous year and recently mineralized N from ammonium and organic matter.
A soil nitrate test provides the best information about available soil N when only small amounts of N have been broadcast preplant. When large amounts have been knifed in or broadcast and incorporated prior to planting, getting a good indication of available N is difficult.
Early summer. Samples taken in June from fields where N was broadcast before planting can guide N additions through irrigation or for planning next year’s application.
Fall. Samples taken in the fall help to evaluate how much N is left at the end of the season. Farmers with excess soil N in June or at harvest should consider reducing the next year’s fertilizer rate or using a PSNT to determine the appropriate rate.