By: The Nature Conservancy
March 2025

A tractor on a field applies nitrogen
Optimal nitrogen rates vary widely depending on the weather, management practices and soil composition.

Nitrogen is one of the most expensive inputs, often claiming the No. 2 spot after seed costs in the list of farmers’ input expenses. It’s a major driver of a corn crop’s yield potential and one of the hardest nutrients to pin down. That’s why it’s important to be as efficient as possible with nitrogen application.

Introducing N-FACT

The 4 Rs — the right source, the right rate, the right place and the right time — vary from field to field and from season to season. It’s difficult to predict a given season’s optimum nitrogen rate because so many management, weather and soil health factors come into play, explains Melissa Miller, project director for the Iowa Nitrogen Initiative (INI).

For the past several years, INI has conducted on-farm nitrogen rate trials to get a better handle on nitrogen use and the many factors that influence nitrogen needs. “Farmers want to apply the amount of nitrogen the plant needs — no more, no less,” Miller explains. Optimized application means less nitrogen loss to the environment and more efficient use of an expensive input.

N-FACT, short for Nitrogen Fertilization Application Consultation Tool, is powered by data from those INI trials. It uses millions of simulated data points leveraged with hundreds of trials to help farmers determine the best rates for their fields and goals.

Custom nitrogen recommendations

trial fields
Trial fields must be big enough and have the equipment necessary to apply five separate rates of nitrogen.

While it would be convenient to have a simple number or formula to determine optimum nitrogen rates, the reality is this is a moving target. Optimum nitrogen rates vary from year to year and field to field.

A “high” rate in one field may be low for another, Miller points out. “Instead of looking solely at the optimum nitrogen rate, we are encouraging farmers to benchmark against their nitrogen use efficiency, or the pounds of nitrogen fertilizer used per bushel corn.”

Not only is understanding that variability and determining the most efficient nitrogen application rates critical to the individual farmer’s profitability, but it also impacts Iowa’s economy and its environment. That’s why the project is supported by nearly 20 partner organizations.  

Backed by extensive data

“The sheer amount of data we have from on-farm trials sets N-FACT apart from other tools,” Miller says. “Previous decision tools might be based on information collected from 10 or 12 trials. But N-FACT is backed by data from hundreds of trials across Iowa.”  

The data collected goes beyond yield response. “We’re looking at different management systems, residue management, harvest, tillage, cover crops and drainage,” Miller explains. Many factors influence nitrogen mineralization rates.

Take residue harvest as an example. In a field with less residue, soils will warm up and dry out faster, Miller explains. That increases the mineralization rate.

Discover your optimum nitrogen rate

Finding optimum rates is relatively simple. Navigate to the N-FACT webpage and select “Discover your optimum N rate.” From there, you can customize results by supplying the following information about your field:

  • Crop rotation
  • Expected precipitation for the coming spring and summer
  • Post-harvest soil nitrate
  • Intended or actual planting date
  • Nitrogen source and price
  • Market price for corn

Anyone can access this tool and put it to use for their farm — no username or password required! It’s also easy to explore trial results and compare the outcomes with your own fields.

User-friendly results

The results themselves are interactive and easy to follow. Users can quickly see their optimal rates and likely yields based on the farm specifications supplied. It’s simple to manipulate various fields to see how changes in application rates, nitrogen source, rainfall and other factors might impact the projections.

Miller advises using the results as a benchmark or baseline. “The model doesn’t have all the information about your farm,” Miller says. “Every acre and farm are unique and so are your N needs.”

“Take the range result you get from N-FACT and pair that with what you know about your field. Work with an agronomist and do some soil nitrate testing,” Miller recommends. “Putting all that information together can help you make an informed decision about the best nitrogen rate for your ground.”

An ongoing effort to improve nitrogen use

Melissa Miller, project director for the Iowa Nitrogen Initiative, says farmer interest in the nitrogen trials has been strong.

Any farmer within Iowa who can apply nitrogen using variable rate prescription and has a yield monitor can participate in these on-farm nitrogen trials. As more data flows in, new versions of the N-FACT tool will be released to refine nitrogen use recommendations based on a farm’s unique needs and circumstances.

“In the majority of our trials, farmers have a nitrogen use efficiency ratio below 1 lb. per bushel,” Miller says. “We’re seeing how efficient Iowa farmers already are, and there are still ways we can get even better,” she continues.

“If we can pinpoint that mineralization rate and when it’s happening, we could hit the nail on the head in terms of optimal nitrogen use,” Miller says. The trial data and the decision support tool are moving the industry closer to that goal.