By Peter Kyveryga, ISA Director of Analytics; Allie Arp, ISA Research Communications Specialist

While many farmers are harvesting this week they are probably thinking about what’s next on their post-harvest to-do list. For some farmers that list will include applying nitrogen (N).

Debating about fall vs. spring application is not new. This article hopes to provide farmers with the information they need to make the best decision for their operation this fall.

Fall application

  • Removes logistical constraints by shifting application to the fall, creating a wider time window
  • Provides less soil compaction compared with split application if soil conditions are suitable in the fall
  • Reduces the risk of not applying N before planting during wet springs because fall weather is generally more favorable to application than spring weather
  • With the proper equipment, allows dry phosphorous and potassium fertilizer applications at the same time
  • Historically, anhydrous ammonia (NH3) prices have been cheaper in the fall compared to spring or summer
  • There are fewer options, only NH3 can be used in the fall because it is positively charged so it attaches to the negatively charged soil particles and is less likely to leach
  • When application is 6-7 months before corn is planted there is a higher likelihood of loss
  • More horse power is required to inject NH3 deeper in the soil

Anhydrous NH3 

  • Anhydrous NH3 is a mixture of liquid and gas so it is highly concentrated, 82%, and therefore less physical weight to transport and carry over a field
  • Unlike NO3 that leaches easily in the soil profile, anhydrous NH3 increases soil pH around the band, which in turn, reduces rates of nitrification and potential nitrate loss.
  • Applications are less accurate with anhydrous NH3 than with other forms
  • Handling anhydrous NH3 requires following special safety rules

Once the decision is made to proceed with fall application it should be delayed as long as possible for two reasons. The first is simple: the earlier an application is put out, the longer it is on the ground before a crop is able to use it. Waiting decreases the opportunities for loss. 

The second reason to delay fall application is based on the speed the soil bacteria convert NH3 to nitrate. Iowa State University, for example, recommends farmers wait until the temperature is consistently below 50 F so the conversion of NH3 to nitrate is lower. 

“While it is well known that the relative high soil temperature in the fall (> 50 F) drives nitrification rates of fall-applied ammonia, other factors, especially soil pH, greatly impact NH3 nitrification rates,” said Peter Kyveryga, agronomist and Iowa Soybean Association (ISA) director of Analytics. “The higher the soil pH, the faster ammonia converts to nitrate in the fall or early spring. With each additional half unit of pH increase, the nitrification rates increase from 5-15%.”

The yield advantage of fall vs. spring N applications is difficult to assess because the weather has such a large influence over the effectiveness of applications. Major differences in application timing success are generally seen in wet years, while during a normal or dry year there is generally not a noticeable difference. This has resulted in limited data comparing fall vs. spring N during the last decade of research. 

The ISA On-Farm Network and Environmental Programs and Services conducted a survey of 3,500 corn fields across Iowa over the last 10 years that suggests fall applications of fall NH3 require slightly higher N rates, for reaching the optimal status, than spring applications. However, the risk of N deficiency status is often higher with spring and sidedress applications, especially with liquid urea ammonium nitrate (UAN) application. More information about the survey results and risks associated different N forms and timing can be found here.

Nitrification inhibitors (the most common one being nitrapyrin) are often used with fall NH3 applications as an insurance against N loss. The economic benefits and amount of N saved by using nitrification inhibitors depends on the combination of N rate used by the farmer, the rates of nitrification of fall-applied NH3 and the timing and amount of early season rainfall. If a farmer uses above-optimal N rates, the yield benefits from nitrification inhibitors are unlikely.  

Farmers wanting to apply N this fall should consider the following tips to reduce N loss:

  1. Apply less than the full N rate or consider split applications with some N applied in spring or sidedressed.
  2. Consider soil pH, especially in central Iowa. Fields that have large areas with high soil pH (more than 7) will respond better to fertilizer in the spring.
  3. Use in-season soil tests (late-spring soil nitrate test), tissue (chlorophyll meter) testing or active corn canopy sensor technology (OptRx or GreenSeeker) to decide whether additional N is needed on top of fall-applied NH3.
  4. Use one of the commercial N predictive tools such as Adapt N, Climate-Pro or N-Circa to maximize N efficiency.
  5. Leave replicated control strips within the field, with a reduced or higher N rate, to quantify potential losses from fall N or efficiency of fall N compared with spring or sidedress applications.
  6. Use late-season aerial imagery of the corn canopy and the stalk nitrate test to identify the corn N status and effect of rainfall and management.

The ISA On-Farm Network has published many articles about N research. including topics such as the importance of nitrogen research, how to effectively use nitrogen, the importance of annually updating nitrogen management plans and even includes a manual titled Adaptive Nitrogen Management