A cooler than average start to the growing season has added some uncertainty to post-plant nitrogen fertilization strategies for Ontario corn producers. Soil supply of nitrogen to the corn crop is likely lower than normal due to reduced N mineralization from both soil organic matter and manure, and the Maximum Economic Rate of Nitrogen (MERN) for a given field may be somewhat higher than an average year.
Summary:
- Extended cool temperatures through spring 2025 have likely reduced soil nitrogen supply to the corn crop.
- Net soil N supply is likely to be lower than normal.
- N mineralization is likely reduced compared to average.
- However, N losses through the early spring season are likely slightly lower than normal.
- Soil N supply can be highly site-specific, as differences in soil organic matter, inputs such as fertilizer and manure, precipitation and temperatures vary substantially.
- Pre-sidedress nitrate tests (PSNT) may give the best indication of soil N supply on an individual farm.
- If in-season application is at risk of nitrogen loss, it may pay to take advantage of N stabilizers.
- N demand (driven by yield) is also an important driver of optimum N rates, but is likely too early to adjust for at traditional sidedress timing (e.g. knee high corn stage)
Cooler Spring Has Slowed Soil Nitrogen Mineralization
Soil microbes convert organic nitrogen into plant-available ammonium and then to nitrate. This process, known as mineralization, is highly temperature dependent. The cooler spring temperatures, especially more days with night-time temperatures below 10oC, across much of Ontario in 2025 have slowed microbial activity, delaying the release of nitrogen from soil organic matter, crop residues, and organic amendments like manure.
This means that early-season nitrogen availability may be lower than normal, particularly in fields relying heavily on manure or other organic amendments as a primary N source.
Cool Temperatures Have Reduced N Losses Thus Far
Although much of the corn crop showed slow growth and stark yellowing in the field early in its growth in 2025, this was due to the temperature stress more so than N deficiency. Crop Heat Units (CHUs) across southern Ontario are behind 10-year averages, and well behind accumulation from 2024, as shown in Table 1 at the end of the article. Warmer temperatures lately have contributed to a healthier colour as well as vigorous growth in most fields.
Frequent rains have likely reduced volatilization losses of surface applied N by providing sufficient movement of the N into the soil profile, and lower temperatures have also reduced loss potential. While denitrification may be a concern on heavier soils where water has ponded, actual losses from these areas are likely relatively small due to cool temperatures. Lower temperatures decrease the conversion rate of ammonium to nitrate by bacteria, meaning less potential for losses of the nitrate form by conversion to nitrogen and nitrous oxide gas (denitrification).
Implications for Nitrogen Fertilization Rates
From a nitrogen supply perspective, optimal N rates for corn may be slightly higher on average in 2025.
However, Ontario research suggests that getting within 25 lb-N/ac of the optimum rate is all that’s necessary to be within $10/ac of the MERN. If you are already planning to apply a bit of “insurance” N, it’s not likely that increasing that rate will bring higher profitability.
Producers should consider the following when adjusting in-season N applications:
- Soil nitrate testing: Pre-sidedress nitrate tests (PSNT) are an excellent tool to estimate differences in soil N supply year to year. However, OMAFA PSNT recommendations are only calibrated for fields with 30 lb-N/ac or less banded as a starter. Handling of samples is critical, as samples need to be refrigerated (but not frozen) to prevent further mineralization before sending to a lab.
- Use of N stabilizers: Assess the weather forecast for the next couple of weeks after your intended application. If forecasts show very little rainfall in the forecast, and your nitrogen is sitting on the soil surface, there is potential of volatilization loss. Conversely, if forecasts show heavy rainfall is imminent, there is potential of leaching loss on coarse soils and denitrification loss on fine-textured soils. The higher the risk of loss, the greater the benefit that urease inhibitors (for volatilization) and nitrification inhibitors (for denitrification) provide. In many cases, adding a N stabilizer to the application may eliminate the need to increase total N application rate.
- Testing rates on farm: While it doesn’t help with fine-tuning N rates in the current season, testing higher and lower sidedress rates can be used as guidance for decision making in future years. Test 30 lb-N/ac more and less than your current rate, or try a 0N block or strip to give an indication of true soil N supply.
Crop Demand Also Impacts Optimum Nitrogen Fertilization Rates
It’s also important to consider that soil nitrogen supply is only one part of the nitrogen puzzle. Nitrogen demand (as driven by yields) also varies year-to-year. Research at University of Guelph has shown that MERN can also be strongly driven by yields. At Elora, this was strongly associated with rainfall from mid June to mid July – the more rainfall during this period, the higher the yields, and the higher MERN. For traditional knee-high sidedress timing, it may be difficult to incorporate adjustments for expected demand beyond gut feel for yield potential at this time and the dominant precipitation trend of the long-term weather forecast. Later nitrogen applications (e.g. late June or early July) may offer more ability to incorporate N demand adjustments into a final N rate decision.
Please note that this article is meant as a relative indication of soil nitrogen supply only, and should not be used as a sole source for adjusting N application rates on any given farm.
Table 1. Accumulated Crop Heat Units as of June 12, 2025 (May 1 start date)
| Location | CHU – May 1st | ||
| 2025 | 2024 | 10 Yr Average | |
| HARROW | 695 | 908 | 822 |
| RIDGETOWN | 644 | 845 | 755 |
| SARNIA | 570 | 809 | 702 |
| LONDON | 650 | 844 | 742 |
| BRANTFORD | 637 | 803 | 725 |
| WELLAND | 658 | 836 | 736 |
| GODERICH | 548 | 706 | 618 |
| ELORA | 565 | 733 | 637 |
| MOUNT FOREST | 556 | 713 | 631 |
| BARRIE | 570 | 701 | 615 |
| PETERBOROUGH | 610 | 719 | 652 |
| KEMPTVILLE | 659 | 790 | 690 |
| SUDBURY | 530 | 640 | 557 |
| EARLTON | 500 | 645 | 516 |
| SAULT STE MARIE | 437 | 540 | 444 |
| THUNDER BAY | 456 | 473 | 426 |
| FORT FRANCES | 589 | 539 | 525 |