The economics of nitrogen have changed. Has our thinking?

by Dan Haug | Dec 4, 2025

Revised December 10

It sounds too good to be true, wrote Neil Hamilton in a 2021 opinion piece. Reducing nitrogen fertilizer application rates to the Maximum Return to Nitrogen (MRTN) recommended by Iowa State University promised to save farmers money while keeping nitrate out of the rivers and greenhouse gases out of the atmosphere. In retrospect, it was too good to be true.

Reducing fertilizer rates will cut into profits (for most farmers)

Early this year, ISU researchers published a study in Nature Communications showing that the amount of nitrogen fertilizer required to maximize yield (the agronomic optimum) and maximize profits (the economic optimum) have been steadily increasing, driven partly by corn genetics and partly by weather. The economic optimum is always lower than the agronomic optimum (the revenue from those last few bushels isn’t enough to pay for the fertilizer) but the difference between the two is getting smaller. I wrote about this in July, but since then I’ve had a chance to download and explore the data used in the study. Here are the trends in optimal nitrogen application rates for just the sites in Iowa, compared to actual nitrogen application rates, which I estimated using a combination on IDALS fertilizer sales data and INREC survey data. For more details on the data I used to estimate actual nitrogen application, read this attachment.

The scenarios in the Iowa Nutrient Reduction Strategy were based on data from 2006-2010. At that time, it would have been possible for farmers to reduce nitrogen application rates on corn following soybeans from 151 lbs/acre to 133 lbs/acre while increasing profits, on average. However, those figures were already out of date when the Iowa Nutrient Reduction Strategy was released in 2013, and in the decade since, fertilizer application rates have levelled off while the amount of nitrogen needed to maximize yields or profit has continued to increase. A minority of farmers may still find opportunities to boost profits by reducing nitrogen application, but average rates are now below the economic optimum.

This part of the study looks solid and matches what I’ve seen from other sources. Practical Farmers of Iowa have done their own trials and found that a majority of their participants were able to save money by reducing nitrogen rates in an especially dry year, but in a more typical year only 41% of farmers saw potential for savings.

Reducing fertilizer rates could still have significant water quality benefits

Farmers no longer have an economic incentive to reduce rates (on average) but it’s not hard to imagine policies that could shift the incentives by making it cheaper or less risky to apply at low rates, or more expensive to apply at high rates.

The ISU study includes an “environmental optimum nitrogen rate” that hints at that. The authors used a crop systems model to estimate nitrous oxide emissions and nitrate leaching for different scenarios, assigned a price to the pollution, and calculated the nitrogen application rate that would be economically optimal if those costs were reflected in the marketplace. Instead of evaluating the environmental benefits of reducing rates from current levels, they estimate the impacts of reducing rates (for corn after soybeans) from an economic optimum of 143 lbs/acre to an environmental optimum of 116 lbs/acre. Those are averages for the entire 20 year period and not at all relevant today. Oops! Because of this mistake, they conclude that “a reduction in N fertilizer rate towards improving sustainability will not have the anticipated reduction in environmental N losses because of the nonlinear relationship between N rate and N loss.”

Actually, the non-linear (curved) relationship between nitrogen application rates and nitrate pollution implies that rate reduction will have bigger benefits now than it did when rates were lower. The figures below contrast some outdated assumptions with new reality.

The increase in fertilizer rates has been bad for water quality

In most presentations and interviews about the Iowa Nutrient Reduction Strategy, ISU faculty correctly point out that fertilizer management alone is not enough to meet our water quality goals and emphasize the need for a variety of conservation practices. However, every scenario in the INRS assumed that fertilizer application rates would go down. It may not be possible to meet our goals now that fertilizer rates have gone up.

Based on the increase in fertilizer rates for corn after soybeans (from 151 lbs/acre in 2007 to 173 lbs/acre since 2017), we would expect a 16% increase in nitrate concentration in drainage water. The 3.8 million acres of cover crops reported in recent INREC aren’t enough to undo the damage. Nitrate in streams is also affected by weather, changes in land use, and other practices not modeled here, but fertilizer rates and cover crops do help to explain why nitrate concentrations in many streams peaked between 2013 and 2015 and have fallen since.

What about continuous corn?

Nitrogen application rates for corn after soybeans have gone up, but application rates for continuous corn may actually have gone down. I say “may” because we didn’t have good baseline data. Because corn stover ties up a lot of nitrogen as it decomposes, growing corn after corn requires higher nitrogen application rates to achieve the same yield. The confusing Figure 5 in the Nature Communications paper looks at the yield penalty for reducing nitrogen rates from the economic optimum to the “environmental optimum,” which is what it would make economic sense for farmers to apply if the societal costs of pollution were reflected in the marketplace. The authors concluded that reducing nitrogen application rates past the economic optimum would have unacceptable consequences for grain markets and food security, especially for continuous corn. I looked at the same figure and concluded that growing corn after corn would not be commercially viable in a society that valued clean water, a stable climate, and public health.

Your mileage may vary

My biggest takeaways from both the Iowa State University research and the Practical Farmers of Iowa research are how much the optimum nitrogen rate varies from year to year and place to place. One farmer in the PFI study saved money by reducing rates from 150 to 100 lbs/acre, while another lost money by reducing rates to 246 to 200 lbs/acre.

The ISU study includes nitrogen rate trials for seven sites in Iowa and six years since the Iowa Nutrient Reduction Strategy was released. If you had followed the recommendations from the old nitrogen rate calculator and applied 140 lbs/acre to corn after soybeans, 62% of the trials would been at least 10 lbs/acre below the economic optimum. But even in the most recent year, there were 2 sites where that would have been at least 10 lbs/acre above the economic optimum!

The Iowa Nitrogen Initiative addresses this problem through an expanded program of nitrogen rate trials and a decision support tool that can provide customized recommendations by county given assumptions about rainfall, planting date, and residual soil nitrate. Using the new information, some farmers will find an opportunity to increase profits while reducing nitrogen rates. A larger group of farmers will find opportunities to increase profits by increasing nitrogen rates. Dr. Castellano has made a complicated argument for how the water quality benefits of bringing down the high rates can be greater than the water quality penalties of bringing up the low rates. Great. Please apply that logic to manure.

What about manure?

Since 2017, the INREC survey report has asked farmers what percent of fields receive manure application (about 20%), how much commercial fertilizer is applied to fields that do not (174 lbs/acre for corn in rotation and 199 lbs/acre for continuous corn), and what proportion of cropland is planted to continuous corn (about 12%). Manure expert Dan Anderson recently did some algebra to see what that implies about nitrogen application rates for fields that do receive manure, and came up with 342 lb N/acre on corn-after-soybean and 391 lb N/acre on continuous corn. I used slightly different assumptions and came up with lower numbers, but they’re still much higher than needed to maximize yield. If you’ve read anything by Chris Jones, this won’t come as a surprise.

I’m showing the agronomic optimum rate rather than economic optimum because the economics of manure aren’t the same as commercial fertilizer. Manure has much lower nutrient content and is much more expensive to haul. Manure pits fill up and there’s often a time and labor crunch to get it applied. Manure has highly variable nutrient content, which adds to the uncertainty and makes a supplemental application of commercial fertilizer seem like cheap insurance. If farmers had a strong economic incentive to make the most of manure nitrogen, nobody would be applying it in early fall and we wouldn’t have a cloud of ammonia hanging over the Midwest. There are also some farmers who are doing an exceptional job of conserving soil and water by feeding cover crops, small grains, or forage to livestock, and we should figure out how to level the playing field to make it easier to replicate their model.

Are these changes in nitrogen management good or bad?

It’s a mixed bag. I had to puzzle over this for quite a while!

The increase in the economic optimum nitrogen rate is partly due to good things (improved corn yield response) and partly due to bad things (increasing nitrogen losses to the air and water).

It’s good that nitrogen fertilizer use has gotten more efficient. Farmers can grow more bushels per pound of nitrogen than they used to. It’s bad that manure management plans still allow nitrogen to be applied at a rate of 1.2 lbs per bushel of potential yield.

It’s good that fertilizer rates for corn following soybeans have levelled off recently. It’s bad that nitrogen fertilizer rates went up in the early 2000s.

It’s good that nitrogen application rates for continuous corn have fallen. It’s bad that farmers are planting corn after corn.

It’s good that farmers are now applying less commercial fertilizer (on average) than required to maximize yield. It’s bad that farmers are over-applying manure.

It’s bad that we don’t have a plan to reach the goals of the Iowa Nutrient Reduction Strategy without rate reduction, and it’s bad that the price tag of reducing rates (either to farmers, the public, or both) is higher than we previously assumed. However, it might still be a better deal than other conservation practices. It’s bad than more people aren’t talking about this.

In case you missed it – 2025 event recap

by Dan Haug | Oct 24, 2025

Signature Events

Our third annual Monarch Magic was held at Ada Hayden Heritage Park in Ames on September 6th. 37 families brought their kids to learn about butterflies and nature with hands-on activities. Normally we tag butterflies but didn’t this year because of concern about their numbers. Thanks to Friends of Ada Hayden Heritage Park, Raising Readers in Story County, The Outdoor Alliance of Story County, Trees Forever, Iowa Chapter of the Sierra Club, Story County Conservation, Boone County Conservation, Greater Des Moines Botanical Gardens, Big Bluestem Audubon Society, Pollinator Friendly Ames, Iowa Monarch Consortium, City of Ames (Parks and Rec and Smart Watersheds) for participating!

Our sixth creek cleanup, which we’re now calling PACRAT (Paddle and Cleanup Rivers Around Town) was held on May 3rd. Thirty-three volunteers removed 3,200 pounds of trash from Ioway Creek! Thanks to the City of Ames, Story County Conservation, Outdoor Alliance of Story County, and Skunk River Paddlers for helping make it happen!

PACRAT 2025 creek cleanup, launching canoes in Ioway Creek.

PACRAT 2025, collecting trash from a log jam

Volunteer water monitoring

We continue to experiment with the format for seasonal volunteer monitoring events. In May (as described in another article), we coordinated our event in Story County with Polk County Conservation and created a map showing water quality across central Iowa. On October 11, eight volunteers joined us bright and early so we could test dissolved oxygen at the low point in its daily cycle, for an event we billed as a “water quality breakfast.” All but one of the sites we tested had dissolved oxygen levels of 8 mg/L or higher, which means we weren’t able to narrow down the cause of the problems we’ve seen in past years, but it’s good news for fish and other aquatic life! The lowest reading was College Creek at the ISU Arboretum (6 mg/L), where the water was barely flowing.

Fall is also a good time for biological monitoring. On October 3, an Ames High School environmental science class helped me survey aquatic invertebrates at Brookside Park in Ames. We found enough insects to make it interesting for the students but not a wide enough variety to indicate a healthy aquatic community–the overall score was “fair.” In one picture below, you can see a new cross-vane structure which redirects the creek’s flow to the center and away from eroding banks. It’s one of several interventions the city is making that we hope will reduce erosion and improve aquatic habitat.

A new volunteer tests temperature and dissolved oxygen

Ames High School students catching invertebrates in nets

Presentations

Dan and Katelyn are always happy to talk about water quality, pollinators, or land stewardship. Our speaking engagements this year included:

A sustainable agriculture class at Iowa State University
4H youth fishing club, by Zoom.
Kirkwood Women in Natural Resources Club
An evening program in Mason City for Iowa Project A.W.A.R.E.
Central Iowa Beekeepers Association winter seminar, by Zoom
A Story County Master Gardeners meeting in Ames
A joint meeting in Stanhope of the Ioway Creek and Headwaters of the South Skunk River Watershed Management Authorities
A panel discussion at a statewide gathering of watershed management authorities in Cedar Rapids
A non-partisan rally organized by the Iowa League of Women Voters

We also hosted a webinar about the importance of native habitat featuring speaker Sarah Nizzi of Xerces Society.

Tabling

Thanks to the following organizations for giving us an opportunity to set up a display and meet members of the community.

EcoVision at the Ames Public Library
STEAM Around the World, organized by the Nevada PTA

We’re looking forward to more opportunities to engage with communities in central Iowa and beyond in what’s left of 2025, and next year!

Pollinator booth Nevada PTA event 2025 scaled

Amending the Clean Water Act

by Dan Haug | Aug 28, 2025

When the House of Representatives reconvenes in September, they will likely vote on a collection of amendments to the Clean Water Act: H.R. 3898, which they’re calling the PERMIT Act (Promoting Efficient Review for Modern Infrastructure Today). It’s a bad bill and you should urge your representatives to vote against it.

The Clean Water Act no longer works well in Iowa, because our largest remaining source of pollution is agricultural stormwater runoff, which has a special exemption. The Iowa Department of Natural Resources has been reluctant to put waters on the impaired list and write cleanup plans because they can’t enforce them. They have been reluctant to update standards because they think it will result in higher costs for small town sewage treatment plants without making any noticeable difference for water quality in rivers. I have a whole presentation about impaired waters if you’d like more details, but that’s the situation in a nutshell.

HR 3898 makes this worse. It expands exemptions for agricultural stormwater and pesticides without any provisions to encourage conservation. It mixes economic considerations into the process for setting standards and evaluating impairments. If there’s no widely available and cost-effective way to get a pollutant down to the level that science says is needed to protect aquatic life or drinking water, then the DNR would have to set a weaker standard, or delay setting standards. This would leave the public in the dark about the condition of our lakes and rivers.

Oxbow wetlands have lost protection because do not have a "relatively permanent" surface water connection to a navigable water.

How often does a stream need flowing water to be protected?

There has been a long-running controversy over the extent to which the US Army Corps of Engineers and EPA can regulate construction in wetlands and waterways, which hinges on the definition of phrase: “Waters of the United States.” The Supreme Court recently ruled that many kinds of wetlands aren’t covered. This bill would exclude ephemeral streams, which had previously been considered on a case-by-case basis. I would be less worried about the impacts of these decisions if Iowa had state law requiring developers to avoid or mitigate wetland fill, like some of our neighbors do.

HR 3898 goes beyond clarifying a few exceptions to roll back what many people see as federal overreach. It also raises the threshold for general permits, making it easier to fill small wetlands even if they do have an obvious connection to navigable waters. It makes it harder for states to block pipeline projects. Amendments by Democrats to study the impacts of the bill or mitigate them with a no-net-loss policy were rejected and the bill passed out of committee on a party line vote.

The Cuyohoga River in Ohio caught fire nine times before Congress took serious action to address water pollution

The one good thing about the bill is it’s a reminder that Congress does have the ability to fix outdated laws. What we now call the Clean Water Act are the 1972 and 1977 amendments to the 1948 Federal Water Pollution Control Act. Those amendments made a big difference for waters that were once polluted by sewage and industrial waste. This summer, I paddled on a beautiful and relatively clean stretch of the Shell Rock River with a canoe partner who grew up in the area and recalled how foam used to float down the river from meatpacking plants in Minnesota. The law was amended again in 1981, 1987, and 2014. It could use another amendment to cut red tape while better addressing today’s water quality challenges, but this isn’t it!

How I Spent My Summer Glaciation

by Dan Haug | Aug 25, 2025

If I was in elementary school, this is what my back-to-school essay would look like.

I’m sharing some vacation photos, taken at the Athabasca Glacier in Alberta, Canada, because it’s a nice window into Iowa’s icy past and its warming future. Earth science concepts like glacial moraines and watersheds and climate change that can be subtle or hidden in Iowa are plain to see in the mountains!

1. Glacial Ice

This is the Athabasca Glacier. It’s a slow moving river of ice 4 miles long and 300-980 feet thick, sliding off the larger Columbia Icefield in the Canadian Rockies. It looks big from a distance, and it feels even bigger when you’re walking across it on crampons and aren’t used to the altitude! The depth of the ice became apparent when we looked into a crevasse.

All of Iowa looked like this at some point during the last 2 million years, except the ice was even thicker. The Des Moines Lobe of the Wisconsinian ice sheet melted from northern and central Iowa just 12,000 years ago, leaving behind flat, marshy land. Other parts of Iowa have been ice-free for longer and had more time for wind and water to shape the hills and valleys.

Ground moraine and a lateral moraine deposited by the Athabasca glacial

2. Glacial Till

A sheet of ice hundreds of feet deep is heavy, and pushes against the earth with tremendous force as it slides slowly downhill. This photo shows both the scratches on the bedrock made by the glacier and the glacial till that it leaves behind–an unsorted mix of sand, silt, gravel, and boulders. You don’t have to travel to the Canadian Rockies to see Canadian rocks; during the ice age, the glaciers carried some all the way to Iowa!

Like a bulldozer, glaciers smooth out the ground underneath and leave a big pile of material at the edge when they stop. That’s why there’s a line of hills near Des Moines and a lot of flat land north of there. You can see a flat ground moraine in the foreground of this picture and a steep lateral moraine in the background.

Purple saxifrage flowering at the edge of a glacier

3. Soil Formation

Despite the harsh conditions at the edge of the glacier, a few tiny flowers were blooming! This plant is a Saxifrage, “stone-breaker” in Latin. Plant roots and associated fungi speed up the process of breaking down rock and turning it into soil.

It’s still a slow process! It took thousands of years for prairie roots, microbes, and burrowing animals to turn lifeless glacial till into Iowa’s rich black topsoil, but only 160 years of tillage to lose half of it. Fortunately, there are ways to grow crops without degrading our soil. As Practical Farmers of Iowa puts it, “don’t farm naked!”

Aerial photo showing continental divide at Columbia Icefield

4. Continental Divide

The Columbia Icefield sits at the junction of two continental divides. Meltwater from the Athabasca Glacier joins the Saskatchewan River, which ultimately makes its way to Hudson Bay. However, meltwater from the Stutfield Glacier on the other side of the mountain ends up in the Arctic Ocean, and meltwater from the Columbia Glacier goes to the Pacific Ocean.

In the Canadian Rockies, the watershed boundaries are impossible to miss: lines of awe-inspiring mountain peaks on either side of a river valley. You might be driving along the Bow River until you reach it’s headwaters, and then you come to a mountain pass, your ears pop, and… ta da! You’re in a new watershed, following the Mistaya River downhill.

In Iowa, the dividing lines between land that drains to the Mississippi River and the Missouri River (or any other pair of rivers) are hard to find without a topographic map. We’ve put up watershed signs around Story County to make it more obvious.

Why does is matter what watershed you’re in? Well, knowing which land drains to which river can tell you something about the water quality in the river, and who to talk to if you want to improve it.

Fine silt in glacier meltwater makes for beautiful blues and greens

5. Meltwater

Our guide refilled our water bottles from a stream of glacial meltwater. I felt comfortable drinking it without boiling or filtration because nothing lives on a glacier. Elsewhere in the mountains, there’s a risk that wild animals living in or near the water might have left behind a little present containing a parasite like Giardia. Fecal contamination tends to get worse as you move downstream into areas with a lot of cows or a lot of people, and Alberta has its own set of water quality issues associated with mining and oil extraction. However, there’s a notable difference from Iowa: when natural resource agencies issue a beach advisory because of E. coli, it’s unusual enough to make the news!

Lakes can be bluish green from minerals or from algae

The water from the glacier was refreshing but tasted a little… dusty. Like the taste in your mouth when you’re standing on a gravel road and truck passes by. Ames tap water is better!

However, the glacial dust (rock flour) reflects the light, turning mountain lakes a lovely shade of blue, turquoise, or green. In Iowa, green or blue-green colored water is usually a sign of an algae bloom, unpleasant to swim in and possibly toxic!

Changes in the Athabasca Glacier from 1917 to 2011

6. Climate Change

We made sure to include a visit to the glacier in our vacation plans because there may not be many more opportunities to see one. In a stable climate, a glacier would melt a bit in summer and grow back with winter snows. In a warming world, glaciers are melting rapidly.

Driving from the visitor center to the trailhead, we passed a series of ridges (terminal moraines) marked with dates. Since 1844, the ice has been receding and the pace of melting has accelerated in recent years. None of my pictures capture this, so I will screenshot an a pair of images from the Mountain Legacy Project and point you to Amy Snider for more art, maps, and photos about the Athabasca Glacier.

Climate change has been less noticeable in Iowa than in other parts of North America because of the moderating influence of corn sweat (evapotranspiration), but that won’t last. Rising temperatures, more intense spring rains, and greater weather variability are projected to lower yields and undermine what little progress we’ve made in addressing water quality. Alberta’s economy is dependent on fossil fuels, but Iowa has a lot of gain from a transition to green energy, and a lot to lose if we don’t make the switch.

Trilobite fossil in a rock dislodged by the Athabasca Glacier

7. Fossils

At the edge of the glacier, we saw a trilobite fossil in a 510-million year old rock, a relic of a time when erosion rates were even higher than they are now because there weren’t any plants yet. The broken shells, sand, and silt that covered the seafloor has since turned to rock and been pushed up to form new mountains.

The trilobites are long gone, wiped out during another episode of global warming and ocean acidification, this one triggered by massive volcanic eruptions 252 million years ago. It was the biggest mass extinction in Earth’s history, but after fifty million years or so, the land, seas and sky were teeming with new species.

If you take a long enough view, arable soil, clean water, and biodiversity are renewable resources. On human timescales, they’re definitely not. Agriculture and civilizations developed during a time of stable climate, and they may not last much longer if we don’t take more aggressive action to limit greenhouse gas emissions. The Earth will go on without us.

My Geology Summer Reading List

Landforms of Iowa, by Jean C. Prior

Timefulness by Marcia Bjornerud

Otherlands by Thomas Halliday

Progress Tracking: Why It’s Lacking

by Dan Haug | Jul 31, 2025

If the Iowa Nutrient Reduction Strategy had a United Way style progress banner

You know those United Way posters with a thermometer showing progress toward a fundraising goal? The image above is my attempt at a similarly easy-to-understand progress meter for the Iowa Nutrient Reduction Strategy (INRS). Based on changes to farming practices in the decade since the Iowa Nutrient Reduction Strategy was released, we should have met our goal for phosphorus but have only reduced nitrogen losses by 2%, not enough to undo the increases of the previous decade. These are best-case scenarios which do not account for manure, legacy sediment and nutrients, and interactions between practices.

Iowa State University is responsible for tracking the progress of the INRS, and has created a set of data dashboards covering dollars spent, minds changed, conservation practices on the land, and water quality trends in the rivers. These were updated in May of 2024. There is no top-line summary of where we stand relative to our goals, but wasn’t hard to make one. I just added up the numbers in a pair of tables labelled “change in modeled nitrogen/phosphorus load for practices since the baseline period.”

Change in modeled nutrient loads, from ISU dashboard

Baselines, Goals and Timelines

The timeline requires some explanation. The INRS was released in 2013, but the baseline period is 1980-1996. That’s because the Hypoxia Action Task Force was formed in 1997. The task force set a goal of reducing the size of the dead zone in Gulf to 5000 square kilometers (1,930 square miles). The target date for meeting that goal has been pushed back several times. Despite several dry years, the five-year average extent of the dead zone is still twice as big as the target. Last year it measured 6,705 square miles, larger than Connecticut or Hawaii.

To meet the goals for shrinking the dead zone, each state would need to reduce the amount of nitrogen and phosphorus lost down the Mississippi River by 45%. The Iowa Department of Natural Resources determined that mandatory upgrades for 102 municipal wastewater treatment systems and 28 industrial facilities could reduce state’s overall nitrogen load by 4% and phosphorus load by 16%. The remaining 41% reduction in nitrogen load and 29% reduction in phosphorus load would need to come from controlling non-point sources of pollution like agricultural runoff.

Iowa State University was tasked with answering the question: “based on what we know about the performance of various conservation practices, how would it be possible to achieve these goals?” The answer was “only with a combination of practices, and only if every farm uses at least one of them.” This research began in 2010, using data about land use and farming practices from the previous five years (2006-2010), so the load reduction scenarios are relative to that “benchmark” period. The researchers later issued a supplemental report to compare the benchmark period to the baseline. Phosphorus went down due to changes in tillage but nitrogen went up due to an increase in corn and soybean acres and an increase in fertilizer application rates.

Using the information in ISU’s reports and dashboards, I’ve attempted to summarize our progress relative to both the baseline period (before the formation of the Hypoxia Action Task Force) and the benchmark period (before work began on the Iowa Nutrient Reduction Strategy).

Table showing progress of INRS relative to two periods

Modeling vs. Monitoring

As I said, these are best-case scenarios. While we can certainly expect a big reduction in phosphorus pollution from the growth of no-till and cover crops, ultimately, the only way to be sure that water quality is improving in our rivers is to test it! If there’s a discrepancy between what you expect (a big reduction in phosphorus load) and what you measure (no clear trend in flow-weighted, five-year-moving-average phosphorus loads) that’s a sign that you left something important out of the spreadsheet! The most likely suspects are legacy sediment in the river valleys and livestock manure.

On the other hand, if your water quality data is incomplete or inconclusive or just hard to explain, it’s worth stepping back and asking yourself: “how big a reduction in nitrogen can I reasonably expect, based on the conservation practices installed so far?” If the answer is “1 or 2 percent,” there’s no need to wait for the results of a long-term study to acknowledge that your strategy isn’t working.

I remember doing that kind of reality check in 2019 for the Ioway Creek Watershed Management Authority, at the end of a four-year demonstration project. I did not enjoy being the bearer of bad news then, and I’m not enjoying it now. I remember how much work my colleagues put into those field days, and the ingenuity and vision displayed by the farmers who hosted them. I was there when the woodchips were poured into the first bioreactor in Boone County. So I totally understand the impulse to brag that Iowa now has 300 bioreactors, 3.8 million acres of cover crops, and 10.2 million acres of no-till. Many people worked hard to achieve those numbers! It is real progress! It’s kept nutrients and sediment out the water! It proves that there are many farmers who care about soil and water and are doing something about it! But it doesn’t prove that the Iowa Nutrient Reduction Strategy is working. For nitrogen, it definitely isn’t.

A field day in 2017 for the Ioway (Squaw) Creek Watershed Project

Conservation efforts have been partially offset by increased fertilizer use

I knew that we needed a lot more cover crops, wetlands, and saturated buffers to reach our nitrogen goal. Iowa Environmental Council has made this point with infographics. What I didn’t realize was the extent to which increases in fertilizer application rates have offset the hard-won gains that we’ve achieved so far.

Nutrient rate management was supposed to be the low-hanging fruit the Iowa Nutrient Reduction Strategy. At the time the INRS was written, the Maximum Return to Nitrogen (the point at which the yield bump from an additional pound of fertilizer doesn’t generate enough revenue to cover the costs) was 133 lbs/acre for corn in rotation and 190 lbs/acre for continuous corn. Reducing rates from 150 lbs/acre for corn in rotation and 201 lbs/acre seemed like an easy way to reduce nitrogen in the rivers by 10% while actually saving farmers money. Win-win! Instead, rates for corn in rotation averaged 173 pounds/acre for the past several years, which would raise nitrate levels in tile water by 18%! Rates for continuous corn went down just a tiny bit, to 199 lbs/acre. What happened?

Effects of nitrogen fertilizer rates on water quality. Corrected 2025-10-16.

Several of the authors of the INRS science assessment just co-authored a new paper in Nature Communications which provides an answer. 133 lbs/acre was economically optimal under outdated assumptions about corn genetics and yield response, fertilizer and corn prices, and precipitation. Factoring all that in, the optimal rate for corn in rotation actually rose to 187 pounds/acre in 2020, leaving no opportunity for a win-win. Farmers are acting in their rational economic self-interest and applying as much nitrogen as is required to get good crop yields after a heavy spring rain. (Huge caveat: this doesn’t factor in manure). The researchers acknowledged that this is bad for water quality, as measured by a growing gap between the economically optimal rate and an “environmentally optimal rate” with some externalities priced in. Another way to say that: the fraction of agribusiness profits which come at the expense of other industries (i.e. commercial fishing in the Gulf, tourism for communities on polluted lakes) and the public (i.e. utility bills for customers of the Des Moines Waterworks, hospital bills for cancer patients) has been growing.

The companion piece to this paper is a new calculator. N-FACT uses data from on-farm nitrogen rate trials to factor in location, planting date, residual soil nitrogen, fertilizer price, and your best guess about corn prices and rainfall to provide a customized economically optimal nitrogen rate. The research that went into it is very impressive, and gave me a new appreciation all the factors that can influence agronomists’ recommendations and farmers’ decisions. I hope it will lead to reduced nitrogen losses by helping farmers improve their forecasting, but unless you’re doing a split application and soil or corn stalk testing, it seems like there’s still a lot uncertainty. That’s why I’m more excited about Practical Farmers of Iowa N Rate Protection Program, which directly addresses uncertainty and risk.

What we really need is a calculator to answer the following questions about the Iowa Nutrient Reduction Strategy:

How much voluntary conservation will it take to offset future profit-driven changes in the industry?
How soon can we expect to see meaningful reductions in nitrate in our rivers and drinking water?
How many people will get preventable cancers in the meantime?
How long is the public going to put up with this before we demand a change in strategy?

What do I mean by “a change in strategy?” Making policy recommendations isn’t our wheelhouse, but if you’re looking for ideas, you might start with this 2020 op-ed by Matt Liebman, Silvia Secchi, Chris Jones, and Neil Hamilton, or this 2022 report by the Iowa Environmental Council.

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