Piece Together The N Puzzle

May 15, 2009 02:03 PM

Managing nitrogen (N) to maximize yields is a big task, but it is especially challenging during periods of volatile input prices. However, farmers have more tools than ever to successfully get the most from their N investment.

"Our goal is to understand how to successfully variable-rate nitrogen because it can help us manage the economics of production during input price volatility,” says Farm Journal Field Agronomist Ken Ferrie.

Using variable-rate technology (VRT) for N applications may not lead to an overall reduction of N applied, but rather N being distributed across a field in the zones where it could have the biggest impact on yield.

Pairing GPS technologies with advanced controllers and equipment, farmers can accurately identify and set up management zones to vary N rates on the go. However, the unknown factors in VRT N application are how much to apply and when to meet the plant's demand.

Ferrie says the answers come from understanding the foundation of the N cycle, assessing soil-supplied N and accounting for the impact environmental conditions can have on available N. 

"We know growing a bushel of corn requires 1.4 lb. to 1.6 lb. of nitrogen. Studies reference 1.2 lb. of nitrogen per bushel, but that amount is reflecting what needs to be applied. If you analyze the nitrogen in the grain and in the stover, it adds up to 1.4 lb to 1.6 lb. of nitrogen,” he says.

Soil's N supply. Researchers are trying to identify a testing method for farmers to measure how much N their soils can provide to the crop through the process of mineralization.

Across the country, researchers are working with soil test values, weather and historical models to better predict N needs. Their efforts, which lay the foundation for more precise VRT decisions, range from work with the Illinois Soil Nitrogen Test (ISNT) to slide rules and computer modeling.

ISNT, a sometimes controversial technique, measures amino sugar N present in the soil and uses that as an indicator for soil-supplied N. One of at least 11 universities studying ISNT is Cornell University.

"Our research started in 2002 with a set of 33 trials in New York,” says Quirine Ketterings, associate professor at Cornell. "We tried to evaluate the ISNT as a possible replacement for the pre-sidedress nitrogen test (PSNT) because the PSNT is easy to mismanage and it's not that well accepted.”

The first data set showed ISNT alone could not accurately identify if additional N was needed across a wide variety of soil types, but when soil organic matter was factored in, the test was an accurate reflection of the soil's N supply potential.

In a second independent data set of N rate studies with second or higher year corn after alfalfa or grass sods, the ISNT was 84% accurate with organic matter taken into account.
"The most powerful package to determine if nitrogen fertilizer rates can be reduced or eliminated is to combine ISNT sampling with a stalk nitrate test at the end of the season,” Ketterings says. "If stalk nitrate levels are high, corn had more nitrogen available than it needed that season, and it must have come from manure, fertilizer or soil. ISNT can help us identify fields where the soil itself has enough nitrogen-supplying capacity for the corn not to need additional nitrogen.

"If we can quantify the amount of nitrogen supplied by the soil, we'll be much closer to identifying where and how much fertilizer needs to be applied,” she explains. "Some people have tried to make ISNT a nitrogen rate predictor; you can't do that without field history. We suggest people take a stalk nitrate and ISNT test and then look at their manure and fertilizer management that year to determine if they can adapt lower nitrogen application rates the following year.”

The caution with incorporating organic matter into your consideration of N recommendations is that there are two ways used for finding organic matter values, and consistency in arriving at the values is key.

Another soil scientist, Jeff Strock, with the University of Minnesota, sees promise in ISNT for VRT.

"When we got started, we were looking for a test, something quantifiable for producers to use, that they could go out and measure and have some indication of the nitrogen-supplying capac-ity of their soil,” he says. "Conventional nitrate tests, which are very useful tools for managing nitrogen, are accurate at the time of sampling and can change dramatically over time. In contrast, ISNT values have been more stable and thus are potentially a better indicator of nitrogen-supplying capacity.”

In his research with ISNT, Strock's testing is based on crop rotation and soil types, collecting samples varying from loess soils to glacial till soils.

"What I am hopeful for is that we will be able to identify ISNT values that reflect potential crop productivity across fields. Then we could go across a field and at those areas with low to no crop response, we could trim the nitrogen back. And in other areas, where we would see a big nitrogen response, we would increase nitrogen,” Strock says.

With this approach, the same amount of N could be applied to the field overall, but the inputs would be in the right place to maximize production.

Strock notes that another benefit is being able to balance production with environmental concerns by applying N at its most efficient rates and in its most productive field sites, potentially minimizing N leaching.

"Our experience with ISNT is that it isn't a perfect predictor, but those few exceptions are opportunities to find out how it can work better,” he says.

This fall Strock will release new data with insights into how ISNT, carbon, N and sulfur interact for crop nutrient availability and productivity.

Although University of Nebraska–Lincoln (UNL) researchers don't support using ISNT values, they acknowledge the importance of soil-supplied N. They recently introduced a new tool for the state's corn growers. The Nitrogen Slide Rule marries yield expectations, corn, N prices and soil-supplied N, using organic matter levels and nitrate readings.

"It is a slide rule that has a single purpose, to calculate how much nitrogen you need for corn and how you can adjust with current economic conditions,” says Charles Shapiro, UNL
Extension soils specialist. "Farmers need a recent soil test that has soil nitrates to at least 2' and soil organic matter in the surface 8". The slide rule also adjusts for the price of nitrogen and value of corn.”

Shapiro notes the slide rule is an effective way to show farmers the impact of changing soil N values or yield goal on N rates as well as the impact of economic factors. These changes can be made easily without a calculator or computer.

"Farmers can move the slide rule and factor in all the effects, so it's more than punching numbers. It's not as accurate as a spreadsheet, but it gets within 5 lb.”

The slide rule is based on UNL's N recommendation, which incorporates soil-supplied N values.

"Since we have less spring rain than eastern areas of the Corn Belt, a nitrate reading is an adequate measure and we have records that back up this approach,” Shapiro says. "For variable rate nitrogen applications, one would use yield maps as the basis for yield goal.”

Supply and demand. The Nebraska researchers calculate the supply and demand sides of corn nutrition—the amount supplied by the soil as well as the uptake by the plant.

"Our research is trying to understand the production of nitrogen from soil and its timing in relation to corn's nitrogen requirement,” says Dan Walters, UNL soil scientist. "We at
Nebraska are firm believers that nitrogen rate is tied to yield potential. We believe that yield obtained is a function of available nitrogen, so recommendations need to be made on basis of yield potential. In the eastern Corn Belt, they've removed the yield potential from the equation.”

This summer, Walters plans to release an online universal fertilizer N model called Maize-N.

"We are working to have a model to predict the nitrogen needs for corn worldwide. We use the model to estimate the soil nitrogen supply from residue decomposition and soil nitrogen mineralization delivered to the crop on average,” Walters explains. "The biggest part of precise nitrogen management is being able to pinpoint the efficiency of how and when the plant uses its nitrogen supply.”

Walters notes that despite sophisticated models based on reliable historical data, the economically optimum N rate is still strongly related to the weather experienced during a season. "The trouble is, when we make a nitrogen recommendation, we don't know what kind of year we'll have,” he says.

He also notes that strategically timing N applications can protect growers from unforeseen weather challenges and improve efficiency.

"The unfortunate thing with nutrient management is that corn yield is made in the last half of grain fill, which is past the time when corrective measures can be made. We generally do a great job with stand establishment and upfront nutrient management, yet late-season temperature can effect respiration loses, maturity date and eventually yield,” Walters says.

Real-time rates. By tapping all N measurement tools, farmers can make better in-season N decisions.

"Variable-rate application of nitrogen helps to correct the variations in soil nitrogen supply, and new plant nitrogen–sensing technologies allow us to apply supplemental nitrogen in a real-time fashion,” he says.

Walters encourages farmers to split N applications and, if using irrigation, to apply N in multiple splits with the irrigation water. He says investing time in N management will complement the other areas of production.

To make field-by-field assessments, Walters relies on intensive data collection. He says farmers should use all the technology available: yield monitors, remote sensing and soil sampling.

"Farmers must have good record keeping—not just overall yields but detailed yield maps and chart changes in management practices,” he says.

"Farmers can use yield monitor data to understand yield variability from season to season. They should be able to identify hot spots and dull spots within a field's yield map, and then look at variability in yield from one year to the next.”

In Walters' work, drainage often corrects low-yield areas. And on eroded knolls that show low yields year after year, increasing the N rate in those zones can be the remedy.

"In adopting variable-rate, farmers should seek help. If you don't fully understand the intricacies of nitrogen management, get help from Extension agents and consultants,” Walters says. "Farmers need to know everything they can about their fields in order to fine-tune nitrogen management.”

You can e-mail Margy Fischer at mfischer@farmjournal.com.

Variable-rate technology (VRT) is an important management tool for improving agronomic efficiency. This is the first story in a series of straightforward articles that provide step-by-step considerations to take into account for VRT adoption. The series includes production management decisions extending from preplanting through harvest.

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