Fertilizer Rates

Nitrogen Math

Matt Wilde
By  Matthew Wilde , Progressive Farmer Crops Editor
Many variables need to be considered when calculating how much nitrogen to apply. (Matthew Wilde)

Ian Plagge will apply 240 pounds of nitrogen (N) per acre this year on corn-on-corn acres and 190 pounds per acre on corn after soybeans.

Crunching the numbers on nitrogen applications is becoming increasingly important for the Latimer, Iowa, farmer and others as nitrogen and corn prices soar.

Determining ideal nitrogen rates can be tricky, University of Minnesota soil scientist Jeff Vetsch says. Using proper N math and taking advantage of N calculation tools can help.

"Nitrogen is often the most limiting yield factor for corn," Vetsch explains. "Finding the right [N rate] can be challenging ... but you can calculate the optimum rate for a given region, state and crop rotation."

Plagge finished spraying half of the N -- mostly UAN32 and some UAN28 -- planned for this year's corn crop on May 1 as part of a weed-and-feed mixture. The other half will be applied in-season before tasseling using Y-Drops.

The row-crop farmer believes he'll apply enough N, but not too much, to meet his overall yield target of 240 bushels per acre if the weather and other limiting factors don't get in the way. Nitrogen totals break down to 1 pound for every bushel of expected yield for corn on corn and 0.8 pounds for every expected bushel of yield for corn after soybeans.

"We're not real technical, but this works for our farm," says Plagge, who farms with family. "Corn after soybeans typically outyields corn after corn, but the yield goal is the same."


Vetsch says it's the No. 1 question he's been asked during a nearly 30-year career studying N management for corn.

With nitrogen and corn prices at or near all-time highs, and water quality a growing concern, he asserts the push by farmers to apply the right amount of N to maximize yields, minimize expenses and protect water resources from leaching and runoff has heightened. But, computing corn N rates to meet economic and environmental goals isn't easy. The weather, N form, soil type, geography, yield goals, application timing and other variables complicate the task.

Climbing corn yields -- an average of about 2 bushels per year since the 1950s, according to USDA data and calculations by Bob Nielsen of Purdue University -- further complicate matters. Vetsch says farmers often ask if N rates need to climb, as well.

"The answer is both yes and no," he adds. "You may need more fertilizer if your soil has low N-supplying power like if it's coarse-textured with low organic matter. But, current corn genetics and hybrids are much more efficient utilizing N to produce grain than 20 to 30 years ago.

"You may need a little more N due to higher yields, but not a lot more," Vetsch continues. "In my experience, yield increases and N needs aren't proportional."


Former DTN Contributing Agronomist Dan Davidson wrote the column "Nitrogen Math: Simple Calculations Give You the Right Rates" in 2016 to help growers grappling with N application decisions. The column, which can be found at https://www.dtnpf.com/…, continues to be a go-to source for many farmers seeking information on the topic based on views on www.dtnpf.com.

Davidson's column five years ago taught farmers how to calculate N application rates based on the percent of N in different nitrogen products. Urea (46-0-0), which contains 46% N, was one of many examples. If a farmer uses urea to fertilize corn and wants to apply 140 pounds of N per acre, divide 140 by 0.46 to find out how many pounds of urea is needed per acre. The answer is 304.

Vetsch says all of Davidson's N calculations apply today except for UAN28. He says farmers should use 3 pounds of N per gallon and not 2.9 pounds in their calculations.

As far as N recommendations from Davidson, here's an excerpt from his column:

"Gone is the day when we could use the old Iowa State approach of 1.2 pounds (of N) per bushel expected yield and take a credit for soybeans (1 pound per bushel soybean yield) if in a corn/soybean rotation. That seemed to work when N was inexpensive, hybrids weren't efficient and most N was put on pre-plant (fall) and we didn't worry about losing nitrates into our watersheds and the Gulf of Mexico.

"Today N is expensive, corn price is low (not the case today), and we can't let N escape into the environment," he continued. "Today's corn hybrids also are much more nitrogen efficient, and continuous corn is common. We see rates of 0.9 to 1 pound per bushel and some farmers are even pushing it to 0.7 to 0.8 pound per bushel yield goal. In general, I believe the 1 pound per bushel expected yield works quite well, but we need to take the right credits and apply that N smartly to protect it from losses."

If a farmer's corn yield goal is 180 bushels per acre following soybeans, Davidson recommended 140 pounds of N or a ratio of 0.78 pounds per bushel. That N total applies if one-third is applied in the spring at planting and the rest is sidedressed, and if organic matter is 2.5% or higher to mineralize additional N from the soil.

Vetsch says Davidson's 2016 N recommendations still ring true today.

"If we used the old 1.2-pounds-of-N-per-bushel formula, we would be applying 300 to 400 pounds [of N to corn], but modern hybrids don't need that much," Vetsch says.


Vetsch believes one of the best tools farmers can use to determine optimum N rates for corn is the Maximum Return to Nitrogen (MRTN) calculator. It was developed in the mid-2000s by university researchers and nutrient specialists in seven Midwest states. Find the MRTN calculator at cnrc.agron.iastate.edu.

Corn growers in Illinois, Indiana, Iowa, Michigan, Minnesota, Ohio and Wisconsin can input data such as the state in which they farm, what crop corn is following, corn price and N form and price to find the optimum N application rate and range. The calculator uses decades of N research trial data from each state or region within a state, which is continually updated.

"If you think of the law of limiting returns, as you get close to the optimum N rate, the amount of grain produced for the amount of extra N applied gets less and less," Vetsch says. "You get to the point that more N doesn't give you more yield, and your returns decline.

"What we don't like to see is growers putting out extra N as insurance not knowing what the weather conditions will be," he adds. "That's probably not giving you a return on investment during a normal growing season, and it increases the risk to the environment [unused nitrogen leaving the field and polluting the water]."

The seven states included in the MRTN calculator produced 58% of the 14.2 billion bushels of corn in the U.S. last year, according to USDA data. For farmers in other states, Vetsch recommends they start with N recommendation guidelines from land-grant universities when making N rate decisions.


-- Follow Matthew Wilde on Twitter @progressivwilde


Matt Wilde