August Founder's Note

The Season Is Shaping Up Fast 

From our field observations the last couple weeks, it’s clear that corn is racing to the finish line. July delivered long stretches of consecutive hot days, and growing degree days (GDDs) piled up accordingly. For example, our office in Ithaca, NE, accumulated nearly 1,000 GDDs in July, right around 31 GDDs per day. GDD accumulation by August 4 in this part of Nebraska is typically about 1,854 GDDs. HPRCC had us at 1,997 this year. That would put us about 5 days ahead of schedule; walking fields and based on data in the Sentinel platform (closer to 2100 GDDs) we are about 9 days ahead of schedule.

2026 accumulated GDDs (green) against the 1991–2020 average (pink) near Mead, NE. Source: High Plains Regional Climate Center.

Early isn’t the same as good outcomes. As of the August 3 progress report the national crop was 61% good-to-excellent, down from 73% this time a year ago and the third consecutive week of decline. Dr. Greg Ibendahl's trend-and-condition model puts Nebraska at an 88% probability of finishing below its long-term trend yield, with North Dakota at 83% and Kansas at 73%. Nationally his estimates are tracking roughly a billion bushels behind 2025.

Probability of finishing below long-term trend yield by state, week 31. Nebraska leads the Corn Belt at 88%.

So what does that mean for nitrogen?

Heat Stress and Nitrogen

Above 86°F, corn starts giving up yield potential. Stomata close, photosynthesis slows, kernels abort, and kernels don’t bulk as much. Heat also stresses root function and growth, especially in dry soil conditions, and can degrade pollination quality. Heat and nitrogen interact through these impacts on crop health and performance.

During heat stress, nitrogen movement slows. Nitrate reaches roots through the water they pull in as the plant is transpiring. Close the stomata, limit transpiration, dry down the top of the profile, and nitrogen uptake stalls. A quick note on roots – they are the insurance against heat and moisture stress you bought a month or two ago. Too much moisture too early, and roots never had to grow deeper to find moisture – now they can’t grow quickly enough to get to moisture that may be deeper in the soil profile. Lower moisture conditions early in the growing season can stimulate root growth and build the root system that protects against late season moisture stress. These deeper roots can also maintain N uptake.

Too much nitrogen builds a fragile plant. High N rates build more canopy, and more canopy transpires more water. Plentiful N rates too early can also limit root growth. Excess leaf N is also associated with reduced stomatal conductance and greater drought sensitivity. Work in Scientific Reports found moderately high soil nitrogen depressed grain yield roughly 9% under both limited and full water, and that adding 105 lb-N/ac returned only 20 lb-N/ac of additional uptake.

Too little nitrogen limits plant resilience. When hot conditions combine with dry conditions, N uptake through the roots is limited and plants may scavenge N from the lower canopy to perform key functions like filling grain. Plants need to have assimilated adequate nitrogen before being stressed for this pathway to exist.

When it comes to N’s contribution to heat stress, there is an optimal band that produces a strong and resilient plant. It doesn’t pay to be excessive or insufficient.

Mineralization Is Happening — Where Moisture Allows It

Mineralization needs organic matter, warmth, and moisture. Rates rise roughly fivefold between 40°F and 80°F soil temperature, and July was not short on heat. Moisture is the variable: the microbes converting organic nitrogen to nitrate peak near 60% water-filled pore space, which on a medium-textured soil is roughly 80–100% of field capacity. Mineralization declines in drier conditions. Wetter conditions can result in loss of nitrate to leaching. For well-managed irrigated fields, mineralization conditions have been optimal throughout July. That means we have had a lot of organic N converting to plant available N in the month of July.

One of our CSPs recently captured the potential impact of mineralization powerfully. They pulled samples for a grower trying to decide on a late-season application. Analysis showed 32 lb of nitrate-N and 22 lb of ammonium-N in the top two feet. Based on applied N and anticipated N need to reach their yield goal, at least another 50 lb-N/ac would be required. He then asked the lab to run total nitrogen by combustion. The same two feet held roughly 15,200 lb of nitrogen per acre — better than 99% of it in organic form. Less than 2% of this amount mineralizing can supply all nitrogen needs for a 250 bu/ac corn crop. The actual contribution of mineralization is hard to estimate and measure, but the crop integrates what’s made available. That’s why Sentinel is so valuable. It provides a real-time test for mineralization contributions to N availability. The crop itself is the test.

A different field at ENREEC is showing similar dynamics. This field has an N rate trial with base rates of 30, 60, and 90 lb-N/ac has not triggered for insufficiency through R3. Field checks confirm it. Thirty pounds applied, and the canary has not chirped.

•  Field Recon: UNL Nitrogen Rate Trial at Silking Stage — Mead, NE

•  Field Recon: 90 lb Nitrogen Sector at Silking Stage — UNL Trial Field, Mead, NE

With the yield potential on this field, N contribution from mineralization has been at least 200 lb-N/ac to this point.

Across the platform, most corn fields are estimated to be milk stage (R3) to dent stage (R5). At dough stage (R4) nitrogen applications are no longer advised as response at this stage is unlikely. Here are some key takeaways from the conditions we’ve seen this year.

• Consecutive days of high heat may have accumulated stress conditions in July

• If soil moisture has been sufficient, mineralization conditions have been optimal

• Yield forecasts indicate yield potential may have been limited by conditions, which would limit overall N demand

• Conditions have been optimal for nitrogen savings potential.

  
  

Good luck as the season winds down. For those of you growing crops that are not as close to the finish line, Sentinel will continue to monitor until the crop is unlikely to respond to more N. Lastly, for those of you who are already harvesting, have a safe harvest – we are looking forward to hearing your results.

— Jackson Stansell