Build Your Nitrogen Trust

March 21, 2014 10:42 PM
Build Your Nitrogen Trust

Variable-rate application acknowledges risk to boost yields

To build a successful variable-rate nitrogen program, it’s essential to have a working knowledge of the nitrogen cycle, nitrogen-supplying power of your soil types and nitrate measurement tools.

"Zone management is very complimentary to variable-rate nitrogen because it factors in the plant’s demand for nitrogen, the soil’s supplying power and the risk of loss. It’s easy to visually identify areas of yellow corn in a field and see the need for more applied nitrogen, but it takes more time and effort when using zone management to assess risk of nitrogen loss and implement variable-rate nitrogen," says Farm Journal Field Agronomist Ken Ferrie.

Understanding how nitrogen and soil work together can help your farm pocket greater profits, both by saving inputs and by applying enough nitrogen to realize maximum yields.

"Some years, you make profit by putting on less nitrogen than you normally would have," Ferrie says. "Other years, like 2013 with heavy spring rains, we had to kick up nitrogen rates. It became profitable by finding areas where nitrogen got away from us then shoring them up with VRT [variable-rate technology]. You made more money by making sure you didn’t run out of nitrogen."

Before adjusting nitrogen rates, farmers must first master application timing. Due to risk of loss, variable-rate nitrogen is most successful in a sidedress application.

Timing is critical. When farmers split nitrogen applications, Ferrie says, they pick up 10% efficiency by applying closer to the time when the plant will use those nutrients—making it easier to manage loss. The key to splitting the timing of nitrogen appli­cations is to sidedress green corn.

"I like to have half to two-thirds of nitrogen up front—enough for vegetative growth­—then sidedress the ear-fill nitrogen," Ferrie says. "When corn shows a strong visual response right after sidedress, it’s telling you there wasn’t enough nitrogen for the vegetative stages and growth was slowed. In an ideal program, you’d sidedress dark green corn that doesn’t need that supply of nitrogen for weeks out. As a program, we never want corn to have a bad day, and we always want the plant to have the nitrogen it needs."

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Variable nitrogen rates factor in the soil-supplying power as well as the risk of loss. Before sidedress, nitrate testing tells a zone’s nitrogen inventory.

After several years of playing catch-up with his nitrogen program, Aaron Gingerich, who grows corn and soybeans near Lovington, Ill., decided to switch from a rescue nitrogen program to a planned program.

"We were putting on all the nitrogen needed in the fall, and we’d lose those hot spots," Gingerich explains. "We would come back then put on extra in those areas."

Since 2009, Gingerich has modified his nitrogen program to include pre-emergence and sidedress applications. Today, 50% of his nitrogen is applied pre-emergence, and the remainder is with a variable-rate sidedress pass.

Planning a variable-rate nitrogen program should begin after harvest. First, use yield maps to pick out high-risk areas within management zones. "It’s trusting in the yield zones and management zones you created after years of collecting data for each field that make VRT nitrogen possible," Gingerich explains.

He estimates he saves 8% each year on his nitrogen costs using variable-rate application.

Prioritize zone management. A mix of nitrogen-friendly and nitrogen-thirsty soils, combined with varying topography, means farmers should test and document each zone to verify its supplying capacity.

"In fields with a lot of slope, you won’t have a lot of leaching on the slopes," Ferrie notes. "But when water runs down and ponds, there’s a risk of denitrification. If those lower areas are drained with tile, the nitrogen will still get away."

It’s the yield maps from dry years that provide perspective for variable-rate population; the wet years give the best insights for variable-rate nitrogen.

Yield maps from wet years are helpful in identifying field zones where leaching or denitrification might have occurred. That’s particularly true for Gingerich, whose fields are flat and might have standing water in low areas for a couple of months out of the year. The soils are dark and heavy with high water-holding capacity and high organic matter.

From year-to-year, tools such as the Illinois Soil Nitrogen Test (ISNT) measure potential soil nitrogen mineralization. The test assesses amino sugars, ammonium (NH4) and other readily mineralizable compounds in the soil.

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Illinois farmer Aaron Gingerich switched to a planned variable-rate nitrogen application in 2010 in order to address leaching problems.

About a week before you variable-rate sidedress corn in the V4 to V6 growth stage, Ferrie recommends pulling soil nitrate samples, which measure one of the mineral forms that plants take up. The other is NH4. These samples will provide a snapshot telling whether nitrogen is in a mineral form that can be taken up by the crop. Tissue tests can also provide valuable data.

"To determine nitrogen supply and demand, consider soil availability and plant uptake. You need to make up the balance in between," Ferrie says. "Do stand counts. Predict what your yield potential is. Then you can calculate your rates."

When moving to a variable-rate nitrogen program, identify the application rates you think will be necessary to meet your yield goals.

In general, variable-rate nitrogen recommendations from Monticello, Ill.-based Cropsmith might vary as much as 60 lb. to 100 lb., says owner Tim Smith. For Gingerich, the swing is lower at 35 lb. to 53 lb., depending on whether it is a low-need or a high-need zone.

"It’s a matter of redistribution based on what the soil provides first," says Smith, whose company performs soil-nitrogen testing, variable-rate consulting and contract research. "Sometimes in that redistribution you’re putting on a little more, sometimes a little less, than you normally would."

Being able to rely on your management zones and soil data will increase your confidence when it comes time to make variable-rate nitrogen decisions.

It sounds logical to say higher plant populations need more nitrogen, but that isn’t necessarily the case, Ferrie explains. In reality, deciding what rate to use goes back to the soil in each management zone. For example, Gingerich’s population ranges from 32,000 to 38,000 plants per acre, depen­ding on soil type.

"You increase population because of your heavier soil with more water-holding capacity, but that soil type could also mean it will supply more nitrogen," Ferrie says. "Typically, you pull population down in the hills and up in the low ground, but with nitrogen, it can be the opposite. The lighter soils might have lower organic matter and lower nitrogen-supplying capacity, and you’ll have to apply more nitrogen to them."

In Ferrie’s experience, nitrogen rates must be increased between 8 lb. and 10 lb. for every 1,000 plants of added population—although it can come from a combination of applied and soil-supplied nitrogen.

Trial and error. As farmers adopt VRT, they should actively test application rates to ensure they achieve the desired results.

"We encourage farmers to do comparison checks within management zones," Smith explains. "We suggest putting higher and lower rates within zones, as well as the rates we recommend. That increases their comfort over time with the VRT recommendation."

For farmers just getting started with variable-rate nitrogen, a background in the fundamentals will make the transition easier.

"The key is good data," Gingerich says. "If you have a minimum of three years’ worth of repeatable management zones within the field, it’s much easier to step into VRT."

Gingerich uses ISNT and in-season soil nitrate tests to determine appropriate rates. He uses a Fast Manufacturing coulter toolbar controlled by a John Deere 2630 control unit.

In the past four years, Smith has seen more farmers adopt variable-rate nitrogen than ever before. High commodity prices supported the initial investment, and he expects implementation to grow as farmers seek to become more efficient amid lower prices and potential increased government regulation.

"We can and should do a better job of managing nitrogen," Smith says. "If we don’t, someone else will."  

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To learn more about how to achieve success with variable-rate nitrogen applications, visit


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