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.
Can reducing nitrogen rates save money?
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. 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. If you want details, 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 saving money, 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 However, those figures were already out of date when the Iowa Nutrient Reduction Strategy was released in 2013, and in the decade since, fertilizer application rates have levelled off while the amount of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 reduce rates while saving money, but on average, rates are now below the economic optimum.
Is that good or bad?
I had to puzzle over this for quite a while!
The science looks good, and an honest appraisal of other sources of data leads me to the same conclusion. Practical Farmers of Iowa have done their own trials and found that a majority of their participants were able to save money by reducing rates in an especially dry year, but not in a more typical year.
It’s good that farmers are applying less fertilizer than is required to maximize yield. In that sense they are not over-applying. However, the ISU researchers found that one of the reasons that the optimum rates have gone up is to compensate for a trend of increased nitrogen losses to the air and water.
It’s good that fertilizer application rates have levelled off recently. Farmers are growing more bushels of corn per pound of nitrogen than they were 10 or 30 years ago. That’s something to celebrate, but be skeptical of claims that this has translated into cleaner water.
It’s bad that fertilizer application rates increased before that. There is a non-linear relationship between nitrogen application rates and nitrate concentrations in tile water, so instead of a 10% reduction in water pollution since 2007, we got an 18% increase. Even the 3.8 million acres of cover crops reported in recent years is not enough to undo the damage. After adjusting for streamflow, many Iowa streams show a peak in nitrate concentrations around 2014. I suspect that the increase was partly due to a rise in fertilizer rates and a dip in CRP enrollment and that the decrease is partly due to cover crops.
It is bad that no-one planned for this. All the scenarios in the Iowa Nutrient Reduction Strategy assumed that it would possible to reduce nitrogen application rates (for corn after soybeans) from 151 to 133 lbs/acre. It may not be possible to reach our water quality goals with higher rates.
The Iowa Nitrogen Initiative is a good thing, but not a game-changer. ISU is partnering with farmers to conduct a huge number of nitrogen rate trials, which are being used to power a decision support tool that provides customized nitrogen rate recommendations. It should help farmers dial in rates that maximizes profit. I’m sure we’ll learn a lot from it. But if the average rate is lower than the economic optimum, then there’s no room to improve water quality, is there? Dr. Castellano has made a complicated argument for how the water quality benefits of bringing down the high rates can outweigh the water quality penalties of bringing up the low rates. Okay. Then why aren’t we doing something about manure?
What about manure?
So far, I’ve presented a mix of good and bad news, but nitrogen management for manure is just ugly. If you’ve read anything by Chris Jones, this analysis won’t surprise you.
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 replicated his analysis for several other years and compared it to the amount of nitrogen fertilizer needed to maximize yield. Guess what? Fields that receive manure are getting way too much nitrogen.
I’m showing the agronomic optimum nitrogen rate rather than economic optimum because the economics of manure aren’t the same as commercial fertilizer. Manure has much lower nutrient concentrations 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 concentration, which adds to the uncertainty and makes a supplemental application of commercial fertilizer seem like cheap insurance. If farmers had a strong incentive to make the most of manure nitrogen, nobody would be applying in early fall and we wouldn’t have a cloud of ammonia hanging over the Midwest. However, perhaps there’s a combination of carrots and sticks that could level the playing field and help environmentally-conscious livestock farmers come out ahead.
What about continuous corn?
With all the disruptions to soybean markets, growing corn after corn is probably going to get more popular, but it’s a terrible practice that would not be commercially viable if our society put a dollar value on clean water, stable climate, and public health. That was my interpretation of Figure 5 in the Nature Communications paper. The authors had a different interpretation that I was unable to follow.
Can reducing nitrogen rates save the earth?
If you take the new study in Nature Communications at face value, no. The authors claim 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.” This is because they estimated the environmental impact of reducing rates from an economic optimum of 143/lbs acre for corn after soybeans to an environmental optimum of 116 lbs/acre. Instead of looking at the most recent and most relevant numbers, they took the average for the entire 20-year period. Oops!
Yes, the relationship between fertilizer application rates and water quality is non-linear. That means that getting fertilizer rates under control is even more important than it was at the time the Iowa Nutrient Reduction Strategy was written. Let’s talk about how we can make that happen.
