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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.
Mundorf agrees with McGraw that three tests provide the best information for N management decisions.
1. Preplant nitrate test. A soil nitrate test measures the amount of residual or carryover N in the active root zone. “The carryover nitrate test gives the amount of nitrogen in the nitrate form present in the soil,” says Mundorf. “If it is wet in the spring, you have an environment for denitrification. But a test should be made if a large amount of nitrogen was applied the previous year and yields were much lower than expected. There still may be large amounts of nitrate available in the soil.”
2. Pre-sidedress soil nitrate (PSNT) test. If applications were reduced based on carryover readings, Mundorf says top-dressing urea in-season is an option. A PSNT allows farmers to apply more N for peak growth. He recommends taking samples when corn plants are 6-12 in. tall, just before rapid growth and high N uptake.
“I am a proponent of testing as close to sidedress time as possible, ideally at the 6-7 leaf stage,” says McGraw. “That provides ample opportunity to make adjustments.”
Some sources say the PSNT is most applicable as an indicator of N availability in soils where manure has been applied or a legume has been plowed down.
3. Fall stalk nitrate test. After harvest, McGraw encourages farmers to take a fall stalk nitrate test. Like a yield monitor, the 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,” he says. “It is a valuable evaluation tool to gauge economic and environmental benefits, and it should be used with multiple nitrate tests in the spring, with multiple cores to prepare for the next year.”
According to Tracy Blackmer, director of research for the Iowa Soybean Association On-Farm Network, the stalk nitrate test is best done after black layer and before any fall soil sampling. “We use aerial imagery to help determine where to collect samples. It can help identify spots where nitrogen stress occurs. We can look at soil types in combination with other factors on the maps and compare test results,” he says. “Interpretation of the results is key.”
Fall stalk nitrate tests, done over a period of years, can help farmers correct and fine tune N use over time, Blackmer says. This “adaptive management” approach, learning from experience and making adjustments, means having perfect hindsight in understanding whether the plant had enough N to maximize yield.
Most experts agree that farmers should use a commercial company to do the sampling and talk with a local ag retailer for N recommendations.
“Farmers will find that testing pays for itself,” McGraw adds. “Use nitrogen aggressively where needed. The return on investment in testing is two to three times what you spend if you manage it well.”