Are We Fertilizing for a Nitrogen Shortage That Doesn’t Exist?
- 2 days ago
- 4 min read
Updated: 4 hours ago
A few weeks ago, my friend Ben, Chair of the North Carolina Soil Health Coalition, gave me a call. Their coalition, inspired in part by the South Dakota Soil Health Coalition, has been thinking hard about nitrogen management heading into 2026. With nitrogen prices where they are, that’s not surprising.
Ray Archuleta had recently spent time with their group, and as often happens after a Ray visit, something shifted. The conversation moved from how much nitrogen to apply to a deeper question: what is the soil already supplying?
Ben asked if I’d be willing to help think through some nitrogen work on a few South Carolina farms. That conversation took me back to a set of experiments (2 experiments of 20 plotes each) we ran in 2016 and 2018 as part of a four-year corn–soybean–wheat rotation study.
We weren’t trying to prove anything grand. After applying a base rate at planting, we simply varied nitrogen side-dress rates and watched what happened. What we saw has stayed with me.
The Curve That Isn’t Linear
In 2016, the response looked familiar at first. As nitrogen increased, yields went up. But not evenly (Image below).
The first units of nitrogen did most of the work.
By the time we approached what would be considered a 70% of the standard recommendation of 140 units, the curve had already started to flatten. Adding another 40 pounds of nitrogen only gave us few more couple of bushels.
When we look at it as a percentage of maximum yield, the picture becomes clearer:
You can see it plainly—the climb is steep early, then it levels off.
Corn yield response to nitrogen: steep early gains, then diminishing returns—and in dry conditions, even reversal.
2018: When Water Has the Final Say
In 2018, we ran two experiments—one on heavier ground and one on lighter soil (Image above). Same approach. But that year, the rain shut off in late July. Our collaborating farmer, Carl Coleman, later told me, “We were about one rain short of a really good crop.”
That one missing rain showed up clearly in the data.
On the heavier ground in EXP1, yields plateaued early
On the lighter ground in EXP2, something more striking happened: the higher nitrogen rates produced more vegetative growth in the early part if the season, but when grain fill came, the system ran out of water. And the corn that had received less nitrogen performed better.
Not because it had more nitrogen, but because it demanded less water.
A Revelation About Nitrogen
Years ago, Rick Haney shared something that stuck with me. He said it was a revelation to him that only about 30% to 60% of applied nitrogen is taken up by the crop.
Think about that for a moment.
That means 40% to 70% of the nitrogen the crop uses comes from somewhere else.
From the soil, from biology, from organic matter cycling.
The Nitrogen We Measure—and the Nitrogen We Don’t
Most of us are familiar with the PSNT—the pre-side dress nitrate test. It measures nitrate nitrogen in the soil.
That’s useful.
But it’s only one pool of N.
In our plot work (20 plots x 2 experiments), we typically saw something like:
~6 lbs/ac of nitrate-N
~30 lbs/ac of organic, potentially mineralizable N
Five times more nitrogen in forms that the PSNT doesn’t capture.
There are tests that try to get at this:
The Illinois Soil Nitrogen Test (amino sugar N)
Solvita’s labile amino nitrogen
The Haney test (water-extractable organic N and microbial activity)
Estimated Nitrogen Release (ENR) values (often ~20 lbs N per 1% organic matter per year)
Each of these is looking at a different part of the same system, and while none of them are perfect, they all point to something important:
The soil is not an inert medium. It is an active supplier of nitrogen.
So What Are We Really Managing?
If a crop needs, say, 140 pounds of nitrogen, it’s tempting to think we need to apply 140 pounds.
But that’s not how it works.
Some of that nitrogen is coming from:
Organic matter mineralization
Microbial turnover
Residue breakdown
Carryover from previous seasons (including, e.g., a soybean N credit)
And in a year like 2018, the real limitation wasn’t nitrogen at all, it was water.
A Different Question for 2026
So here’s the question I’ve been sitting with since that call with Ben:
Are we trying to grow the biggest crop possible?
Or are we trying to make the best nitrogen application/side dress decision with the system, the weather conditions, and the input prices we actually have?
Because those are not always the same thing.
If nitrogen is expensive—and it is—then it may be worth asking:
What is my soil already supplying?
Where does my response curve flatten?
And what is truly limiting yield this year?
Closing Thought
We often fertilize for a nitrogen shortage that doesn’t exist while overlooking the water and soil conditions that actually limit our yield.
And sometimes, the most profitable pound of nitrogen may be the one we never apply.
Visit these “Growing Resilience Through Our Soils” information pages:
1. Podcast page for drought planning fact sheets, Q&As, news, podcasts, and more.
2. Video page to watch videos of other ranchers’ journeys toward improved rangeland/pasture.
3. Follow Growing Resilience on social media:
4. Our homepage: www.growingresiliencesd.com