Iowa State University's Mike Castellano discusses the factors driving nitrate levels in 2025 and how farmers are collaborating to improve outcomes.

Editor's note: The Spokesman recently sat down with Mike Castellano, Iowa State University (ISU) soil science professor and lead on the Iowa Nitrogen Initiative, to talk about water quality and ISU's efforts to help farmers optimize nitrogen application rates. Following are excerpts from that conversation. Answers have been edited for length. 

Why do we hear about high nitrate levels in water some years — like we are in 2025 — and not in others?

Castellano: We grow crops for nine months of the year. We have several months when there's no plant demand for nitrogen, and if we don't have plant demand for nitrogen, it's susceptible to loss to the environment. 

Wet weather obviously exacerbates that. So when we have no crop demand, if we don't have excess precipitation, we lose very little to the environment, as we saw in the past few (drought) years with relatively little nitrate loss and relatively low nitrate concentrations in the spring. This year's a very wet year, so we've seen higher nitrate levels. 

The other thing I'd mention on this is that while the nitrate concentrations are elevated in the springtime in Iowa, they're very low in the summer.

So if we look at whether they're wet years or dry years, nitrate concentrations by August, September, October generally go down to below a couple ppm (parts per million) in the water because the crops have such great nitrogen demand that they're preventing nitrogen loss downstream. 

Q: How do this year's nitrate levels compare to other years and long-term trends?

Castellano: Certainly the nitrate levels are very similar to last year, for example, and we've had other years in the past where they've been elevated to similar levels as this year. 

Detecting long-term trends in nitrate, whether it's up or down, is extremely difficult ... because in the context of the annual cropping system, it's weather that makes the losses high or low. And so because we have very high year-to-year variability in weather, it makes detecting long-term trends in nitrate loss very difficult.

Increasing spring precipitation has been occurring for about the last 100 years. The United Nations Intergovernmental Panel on Climate Change as well as state monitoring records (indicate) springtime precipitation has increased by about 20% over time, and so has the intensity. 

Greater springtime precipitation, and greater intensity of springtime precipitation, causes more nitrate loss. 

This is why monitoring of not just water quality but conservation practice implementation, things like denitrification wetlands, cover crop acres, no-till acres, is really important for evaluating progress on our nitrate loss reduction goals. 

What kind of progress are you seeing in farmer adoption of those conservation practices? 

The Iowa Nutrient Research Center's Conservation Practice Tracking Dashboard shows remarkable progress in the implementation of wetlands, cover crops and reduced tillage throughout the state.  

Not every practice is going to work in every situation, so we can think about scenarios where certain practices may be more effective than others. In our northern, cool, wet, drained soils, edge-of-field practices are really, really effective. 

This isn't going to happen overnight. Not only will it take time to get those practices on the landscape, but it will also take time to measure those changes in nitrate levels in our streams and rivers because of the strong effect of weather.

Just as we shouldn't celebrate victory in a few dry years when nitrate levels are low, we shouldn't condemn a lack of progress in wet years when nitrate levels are elevated. 

Nitrate levels are going to go up and down for the foreseeable future as we make progress towards implementing conservation strategies. And we've been making continuous improvements in nitrogen fertilizer use efficiency despite an increasingly challenging environment to manage nitrogen fertilizer. 

What role does Iowa's naturally occurring organic matter play in the system?

Iowa's a leader in crop production for two reasons: favorable climate and the world's best soils. 

The world's best soils are the world's best soils because they have roughly 10,000 pounds of nitrogen per acre on average in their organic matter. We can count on roughly 2% to 5% of that turning into ammonium and nitrate every year. 

Unfortunately, we can't always account on the timing of that transformation to be perfectly matched with the timing of demand by the crop. This is where things like cover crops and edge-of-field practices are so important in helping retain that ammonium and nitrate that our organic matter is naturally supplying when the crops aren't growing.

What does your research show about the levels of nitrogen that farmers are applying?

The Iowa Nitrogen Initiative demonstrates that Iowa farmers on average are applying the right amount of nitrogen fertilizer to produce an economic crop.

In crop year 2023, for example, Iowa farmers applied 167 pounds of nitrogen to corn following soybeans and 186 pounds of nitrogen to corn following corn on average — and that's according to the Iowa Nutrient Research and Education Council, which conducts a statistically sound survey. It's a robust sampling of commercial fertilizer application rates in the state. So those numbers come out to about an average of 172 pounds per acre for all the crops in the state, considering the distribution of those two cropping systems. 

That 172 pounds of nitrogen per acre compares to the economic optimum nitrogen rate in crop year 2023 — according to the Iowa Nitrogen initiative — of 196 pounds per acre. So if anything, our data suggests farmers may be applying a little bit below the average. 

What kind of response have you seen from farmers regarding participation in Iowa Nitrogen Initiative nitrogen rate trials?

Our work would not exist without the important partnership of farmers. 

Farmers want to improve their nitrogen use efficiency — there's no question about it — and there's no better demonstration of that interest in improving their nitrogen use efficiency than the participation of farmers in our work.

We had a goal when we started the Iowa Nitrogen Initiative in 2022 of achieving 400 trials somewhere in the future — maybe five, 10 years. In year one, we had over 250 trials. In year two, we had over 450 trials. This year, we're gonna have close to 600 trials in the state. 

It's just a remarkable testament to farmers' interest in working with Iowa State University to improve nitrogen fertilizer management.