Iowa corn growers are evaluating their nitrogen management programs and looking ahead. Predicting how much crop-available nitrogen will be in fields next spring is challenging. Many factors influence the availability of N, including the type of nitrogen fertilizer applied, application method and timing, soil type, weather conditions, use of a nitrogen stabilizer, and the previous year’s crop.

Sampling and testing your soil for N can help. Using a combination of three nitrogen tests is an even more powerful decision-making tool, providing estimates of available nitrogen at various times: late fall, preplant and pre-sidedress.

Each test is designed to provide farmers with the best information for making (and adjusting) nitrogen management and application decisions. Whether growers are applying nitrogen in fall, early spring or split, these tests help farmers better manage nitrogen based on science and laboratory results instead of guesswork.

Use the right test at right time
More agronomists and industry experts are suggesting farmers evaluate nitrogen availability and needs by using a combination of late-fall, early spring and/or late-spring (pre-sidedress) soil testing. The fall stalk nitrate test is also a consideration. Experts say it’s important for farmers to use a combination of tests at various locations in a field to get an accurate reflection of nitrogen availability and future needs.

“Thinking you can get by with just one test in one place is a flawed approach,” says Kent Klingbeil, director of precision agriculture for Landus Cooperative, based in Ames, Iowa. “Employing only one test can lead to incomplete information, inaccurate data, and most important, flawed nitrogen application recommendations. For the best results, farmers should use a combination of tests with multiple sampling locations in different soil types with varying water-holding capacities.”

Here’s a look at the three key tests
• Preplant soil nitrate test. This test measures the amount of residual or carryover nitrate-nitrogen in the soil profile, typically sampled to a 2- to 3-foot depth in the fall or early spring. A wet spring increases the likelihood of leaching and denitrification, which is why the preplant profile sampling is not common in the humid areas of the Midwest as the nitrate-N amount measured in the fall or early spring can change quickly. The preplant test should be considered if a large amount of nitrogen carryover is expected where corn yields were low due to dry conditions in 2017 and corn will be planted again next year.

• Pre-sidedress soil nitrate test. If nitrogen applications are lowered based on soil profile nitrate-N sample results, farmers should consider the pre-sidedress soil nitrate test. This test allows farmers to adjust nitrogen applications before peak growth at critical nutrient uptake stages. Sample the top foot of soil when corn is 6 to 12 inches tall, typically in late May to early June. Sample the areas that are similar and 10 to 20 acres in size. If nitrogen or manure was band-applied, the soil samples must represent the area within and between bands. Therefore, many cores collected at set distances between corn rows are needed (sets of eight cores). Take 24 cores per sample, using three sets of eight cores. Soil cores can be collected randomly if nitrogen or manure is broadcast.

• Fall stalk nitrate test. This test provides information on the relative nitrogen program results that year. Samples for the stalk nitrate test are collected shortly after corn reaches black layer stage. Aerial imagery can help determine where to collect samples in a field and can help identify spots that suffered nitrogen stress. Taking fall stalk nitrate tests over a period of years can help farmers fine-tune nitrogen management over time. The fall stalk test results are not simply to be used to adjust your nitrogen application rate for the next year, but rather for longer-term evaluation.

Takeaways from 2017 growing season
Dry conditions in parts of Iowa during 2017 raise several questions related to soil fertility. “If corn plant vegetation and/or grain yield was drastically affected by drought conditions, then N uptake would have been reduced and the unused N in the soil could be accounted for in determining the N fertilization rate for the 2018 corn crop,” says John Sawyer, Iowa State University Extension specialist in soil fertility and nutrient management.

Sawyer suggests two methods to estimate carryover nitrogen. The direct method is to sample the soil profile (a minimum depth of 2 feet) after harvest and measure the nitrate-nitrogen concentration. Samples should be taken in 1-foot increments. If dry soil conditions persist, most applied N should remain in the top 2 feet. To add up nitrate-N in the sampled profile, multiply the concentration in each foot by four to get the nitrate-N amount per foot and then add the amounts together.

A second method to estimate carryover nitrate-N is to use the 2017 corn grain yield. Take the total nitrogen applied for the 2017 corn crop and subtract the 2017 grain yield in bushels per acre. Then assume 50% of that amount will remain available to the 2018 corn crop if precipitation is normal or below normal for the fall/winter/early spring. Nitrate-N amounts can change quickly, so sampling in the spring may be more beneficial than fall sampling in geographic areas like Iowa where rainfall can accumulate quickly.

Timing of fall N application
If you are applying N in the fall, whether it’s anhydrous ammonia or manure, remember to wait until the 4-inch soil temperature is below 50 degrees F and the forecast indicates it will stay below 50, says Sawyer. You can monitor soil temperature online. This is critical because the conversion to nitrate is much more rapid above 50 degrees F than below 50. Nitrate is the form of N that is subject to leaching and denitrification.

A word about water quality

Nitrogen application can be one of the major contributors of nitrate-N loss to surface water and groundwater. As nitrogen application rates increase, the likelihood of nitrate-N concentration in tile flow grows. Carefully managing nitrogen inputs and using precise nitrogen rates are important to protecting water quality.

Using a nitrogen stabilizer such as N-Serve or Instinct has long proven effective in reducing nitrogen loss. “N-Serve protects applied nitrogen in the ammonium form and slows the conversion process to nitrate nitrogen which is moveable in the soil,” says Kent Klingbeil, precision ag specialist for Landus Cooperative. “N-Serve and Instinct slow down the conversion for when corn plants actually need the nitrogen, which is later in corn’s life cycle. These products help ensure that applied nitrogen will still be there in the soil where a corn plant can use it and won’t end up moving lower in the soil profile out of corn plants reach.”

Several years of independent field trials, many conducted by the Landus agronomy team, demonstrate the benefits of using a nitrogen stabilizer with nitrogen applications. Trials show improved nitrogen use and efficiency as well as environmental benefits. “To me, N-Serve is one of the most proven products on the market today,” says Klingbeil. “There’s no arguing the yield advantages. You’re always going to break even, but more often will increase revenue per acre by using it. Ultimately, it makes you more profitable and is a great way to protect our water supplies. It makes sense, and we recommend our customers use N-serve and Instinct with all their nitrogen applications, spring or fall.”

The regional Corn Nitrogen Rate Calculator website, which has helped farmers determine profitable nitrogen rates for more than a decade, can be found at cnrc.agron.iastate.edu. This tool provides a way to calculate economic return from nitrogen application with different nitrogen and corn prices, and to find profitable nitrogen rates directly from recent nitrogen rate research data. To calculate your return when using Instinct or N-Serve, use the profit calculator at nitrogenmaximizers.com.

Sources: ISU Extension; Landus; Dow AgroSciences