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Corn+Soybean Digest

3 Best Nitrogen Tests

Think different 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.

It came as no surprise to Tim Mundorf last fall that many post-harvest corn stalk test samples contained high levels of nitrogen (N). The field representative for Midwest Laboratories in Omaha, Neb., says dry weather and poor yields are reminders of how much N can go unused by corn plants in a drought. The lesson to be learned is that you should test under all conditions as part of a long-term, successful N management plan.

“Whether your nitrogen application is all fall, all early spring or split applications, testing allows you to manage nitrogen better,” says Mundorf. “If you are going into soybeans after corn in 2013, then testing has less value than if you are going corn on corn. But nitrogen must be managed, given high prices of fertilizer and grain, nitrogen dynamics and environmental issues.”

N management must also go hand in hand with water management, stressesTom McGraw, soil sampling specialist from Buffalo Lake, Minn. “Nitrates move with soil moisture. But if you got only half a crop in 2012 does that mean you only used half the nitrogen? We don’t know. You can test what is left in the fall, but winter snow and spring rains may reduce or eliminate any residual or fall applications,” he says. “I recommend testing and putting on nitrogen as close as possible to its use.”


Timely testing is vital

Generally, industry experts suggest you evaluate N needs using fall, early spring and/or late spring tests, possibly in combination with a fall stalk nitrate test.

“If you only plan one test at one place, forget it. Incomplete information can be detrimental,” says McGraw. “You won’t get the right answer. You need 10 days to two weeks for multiple sampling with different soil types and water-holding capacities.”

Soil type may also influence test results and application decisions. For example, Michigan State University recommendations suggest fields with manure or legumes sampled in June will likely contain the most nitrate. Other fields that show the most nitrate traditionally have fine-textured soils (loam, clay loam and clay) that were heavily fertilized the previous year. Sandy soils, even though heavily fertilized the previous year, may not show much carryover.

Harold van Es, Cornell University soil and water scientist, notes that, because corn’s response to N is so highly variable, the economically optimal N rate can range from 0 to 225 lbs./acre, depending on the rates of nitrate leaching, ammonia volatilization and denitrification.

Farmers can input data into the online calculator Adapt-N( to determine N recommendations. The free web-based decision support tool is linked to a computer model and high-resolution climate data that can be accessed from the field, office or mobile devices. Adapt-N also allows farmers to receive daily updates on in-season N status of their fields through text or email messages.

Quick, accurate results are behind the soil analysis done from the field by Solum, a precision soil analysis company with a research and development lab in California and a full-service lab in Iowa.

“We can run a test and have results in three minutes,” says Brian Springer, field sales manager for Solum and certified crop adviser based in Illinois. “Our equipment can be mounted in trailers for mobile applications or at retail locations.”

He explains that three Stanford University Ph.D. physicists, known for “measuring small things very well,” founded the company and developed the technique. “We use an optical sensor and UV spectrum to measure nitrate in a soil slurry.

“Moist nitrate testing is just as accurate as a cadmium reduction test,” he adds. “The cost is comparable to other tests, and gets information to the field more quickly. Quick results allow for more timely nitrogen applications versus a best guess.”

McGraw has observed Solum’s efforts. “They are spot on and very exact,” he says. “Regardless of the process, farmers need to test at least twice per season. Three is better.”


Three-test regimen

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.”



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.


Nitrogen testing for soybeans

Farmers may not traditionally test for N deficiency in soybeans, but some experts predict such a test could have merit and is one or two years away. Minnesota agronomy consultant Tom McGraw says N fertilization for soybeans may not work in every instance, but notes Kansas State University university research shows N applications to soybeans through fertigation may enhance yield potential. The study demonstrated that N applied to N-deficient soybeans at pod development or early pod fill can increase yields. 

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