Iowans have a lot of questions about nitrogen and the impact it is having in our state. In an effort to provide science-based information on the subject, Iowa State University's Nutrient Management Workgroup, has compiled the following FAQs. The Workgroup is made up of professors, research scientists and Extension professionals from the College of Agriculture and Life Sciences.
FAQ 1: What is the relationship between nitrogen application rate and nitrates in our rivers?
In general, as the nitrogen application rate increases for corn, the nitrate concentration in water moving below the plant root zone increases. However, it’s not a straight-line relationship. Instead, increasing nitrogen fertilizer doesn’t cause a large loss of nitrate until crop demand is met. Once crop demand is met, higher fertilization creates a significant chance of nitrate loss. This means that getting the application rates correct is important. On the flip side, if plant nitrogen needs are not fully met, yields drop, so farmers have a large incentive to ensure their fields are not short of nitrogen.
Fertilizer is not the only source of nitrate: mineralization of organic matter is a major source, and mineralization increases in warming and wet springtime soils (before crops are planted) or in unusually warm late falls (like 2025). In rich Iowa soils, mineralization can release anywhere from 100 to 300 pounds per acre per year of nitrate. The problem is that this release is often out of synch with crop needs. This makes it harder to discern the difference between nitrate lost from excess fertilizer application and that coming from mineralized organic matter.
When fertilizer is added beyond what turned out to be needed, farmers lose money, but those losses are far less than the economic loss (due to yield loss) of having too little nitrogen. Furthermore, out-of-growing-season mineralization is not readily used by the crop.
The contrasting behavior of yield and nitrate loss creates a tension between agronomic and environmental goals; farmers face productivity uncertainty that encourages higher nitrogen use. At the same time, water quality is improved by nitrogen being applied slightly below that tipping point. The balance point is sought (and there is good evidence that farmers mostly operate at this point), but that balance point is never fully knowable before the season because it depends on the weather.
Putting some numbers on it: In a corn-soybean system, even with zero nitrogen applied to the corn, we have observed nitrate concentrations of about 8 ppm (parts per million, which is also the same as 8 mg/L) – about 2 ppm less than what has been seen with fertilized corn. Soybeans, which generally don’t receive nitrogen fertilizer because of their ability to biologically “fix” nitrogen, have similar nitrate losses as from corn, emphasizing the substantial contribution of soil organic mineralization to nitrate losses.
Here’s an analogy: Imagine you’re going wilderness backpacking with a friend who likes to plan trips ranging from 1 to 2 weeks, but who doesn’t tell you how long exactly the trip will be until you’re out on the trail. How much food do you bring? There’s an “economic optimum” based on the typical trip duration (e.g., 10 days, so you bring enough food for 30 meals), and if your backpack is overfilled, you might pack just that much or even a bit less. Because the downside risk of running out of food is large, most people would pack for 14 or 15 days. Yes, they will spend more on food and carry some extra weight if the trip is short, but avoid the very high “cost” of running out of food on the trail if the trip goes long.
Mineralization is like someone giving you some extra food just as you arrive at the trailhead (before your hike starts, but after you already packed) or along the way (which could be useful), or on your drive home (no use). If you could predict mineralization accurately, you could pack (and waste) less food!
FAQ 2: What is the biggest challenge to decreasing nitrate levels right now?
In a nutshell, cost. Let’s expand a bit: We have three major approaches to reducing the nitrate in Iowa’s surface waters, as follows:
- In-Field practices, such as fertilizer management, annual cover crops, etc.
- Edge-of-Field practices, such as constructed wetlands, nitrate bioreactors, saturated buffers, etc.
- Land-use changes that involve removing land from normal row-crop production, such as Conservation Reserve Program (CRP), pastures, prairie strips, small-grain rotations, etc.
The economic impacts of implementing nitrate-loss-reduction practices vary by farmer. Some report positive impacts on their net farm income, others zero, and still others negative impacts. Approaches that led to positive change in farm income include taking low-yielding acres out of production and converting them to perennials, adding a third crop such as small grains or alfalfa, and grazing cover crops. However, not all farmers have access to these pathways, and many of the most widely and rapidly applicable practices have costs (either direct or indirect via yield loss) in the range of $40 per acre per year or more.
With Iowa farmland selling for $10,000 to $20,000 per acre, and yielding corn worth $800 to $1000 per acre per year, $40 per acre per year doesn’t sound like much. The problem is that farmer margins are such that many farms are currently operating in the red (-$150 per acre per year) or only slightly in the black (less than $50 per acre per year). With such tight margins, $10 per acre per year matters, and $40 per acre per year is huge. Iowa State University research continues to seek ways of driving these costs down, while key stakeholder partners of ours consider ways of incentivizing nitrate mitigation approaches.
FAQ 3: Is water quality in Iowa improving?
In some places and at small-scale, yes—water quality is getting better. Certain practices (like wetlands, cover crops, and bioreactors) reduce nitrate loss, and they’re being used more than they were 10–15 years ago. However, because Iowa’s ag land is so large, the practices have not yet hit a sufficiently large scale to reduce statewide nitrate loads, so measurable water quality improvement hasn’t yet occurred.
Iowa’s Nutrient Reduction Strategy tracks progress against a 1980–1996 baseline, a period when the state had 10 - 15% fewer row crop acres, had 35 - 40% lower nitrogen inputs total, but applied approximately 10% more nitrogen per bushel. The increases in cropland and nitrogen use increase the potential for nitrate loss. Since the Nutrient Reduction Strategy began, Iowa farmers have made meaningful gains in conservation adoption—cover crops now reach about 3.8 million acres, and edge-of-field practices treat roughly 163,000 acres. With 24 million acres of cropland in Iowa, these practices have not yet scaled to what’s needed for statewide nitrate load reductions, and as a result, the nitrate N load per inch of water leaving the state has remained essentially unchanged for the past two decades.