By Charles White
This article is this week’s Agronomy Highlight, recorded live on Fridays at 9:00 am. The Agronomy Highlight is an opportunity for readers to ask the author questions and hear updates from around Pennsylvania. Join the Agronomy highlight live on Facebook or zoom or join by calling +1 646 876 9923, and when prompted enter the webinar ID: 946 6516 7271.
Nitrogen (N) management is one of the most challenging aspects of agronomy because there are so many different loss pathways for N, and these loss pathways are all affected by the weather. The weather can also affect the rate of N mineralization, an important source of available N from soil organic matter, manure, and cover crop residues. Finally, you can factor in changes in the cost of N fertilizer and the value of the crop to adjust application rates to be as economically efficient as possible.
If you averaged out the temperature and moisture swings we’ve experienced this spring, we might be on track for an “average" weather year. But underneath this average are several weeks of hot and dry weather followed by the recent week of cool and wet weather. Has this variability affected your early-season N applications? And do the high prices of N fertilizer and corn grain warrant an adjustment in N application rates? With sidedress time quickly approaching, now would be a good time to take stock of the season so far to determine if any adjustments are needed.
The warm dry weather we experienced early in the growing season may have affected N in various ways. If you applied a urea containing fertilizer (straight urea or UAN solution, of which half of the N is in the urea form) during the dry period without a urease inhibitor and without incorporation into the soil, the risks for N losses through volatilization are relatively high. Under worst case scenarios, about 30% of untreated, surface-applied urea can volatize into the atmosphere. UAN that is applied in a concentrated band usually soaks into the soil and has less risk of volatilization. UAN applied as droplets in a spray, on the other hand, is at very high risk of volatilization without a urease inhibitor. The urease inhibitors usually protect the urea from volatilization for about 2 weeks, during which time you hope to receive at least a half-inch of rain to soak the urea into the soil. Even if you treated urea or UAN with a urease inhibitor, if you did not receive a half-inch of rain within two weeks of application, then the risk of N volatilization would start to go back up again.
The dry weather may have also affected N mineralization rates. Soil that dries out significantly will start to have reduced microbial activity and slower mineralization rates. However, this slowdown from a moisture deficit could have been counteracted by the warmer weather, which speeds up mineralization rates. Farms that were able to maintain soil moisture through crop residue cover may have benefitted from overall increased mineralization rates during the early part of the season due to the warmer temperatures, while farms that had soils drying out probably experienced no significant difference from normal mineralization patterns.
The recent week of cool, wet weather has probably had a negligible overall effect on N mineralization and N losses compared to normal. Although cooler weather slows down microbial activity, soil temperatures are somewhat buffered from swings in air temperature due to the thermal mass of the soil. The rain accompanying the cool weather also likely stimulated microbial activity after the dry period. So again, these counteracting effects probably have us on track for normal N mineralization rates in recent weeks. Despite some localized heavy rainfall last week, it is unlikely to be enough to cause significant N leaching or denitrification losses.
Next, you may be wondering how the high crop and nitrogen fertilizer prices might be affecting the economics of N fertilization. In Pennsylvania, our N recommendations given in the Agronomy Guide do not factor in real-time variation in corn grain prices or fertilizer prices, as is done in many mid-western states. One of the reasons for this is because we do not have as rich of a database of N response curves to develop recommendations from, and our N response curves tend to be more variable than in the mid-west due to greater variability in soil types and farming systems. In analyzing a few N response curves from recent corn studies in Pennsylvania, however, I found that updating the price of grain and N fertilizer from last year to this year reduced the economic optimum N rate by about 10 lbs N/ac in most of the locations. Given the margin of error in our N recommendations, this level of reduction in N fertilizer suggested by current prices is probably not significant enough to act on. It also tells me that if you suspect you experienced any early-season N losses from volatilization, unless those N losses were extremely major, it probably won’t pay to try and replace those losses with any additional N beyond what you would typically use.
So, despite the gyrations in weather and the crazy behavior of the markets, from an N management perspective, I would encourage you to stay the course with your normal N management program while striving for overall improvements where possible. This includes using N stabilizers like urease inhibitors and nitrification inhibitors when it makes sense, which you can learn more about online . You should also consider improving N application timing in corn production by sidedressing the majority of N, which allows you to use adaptive tests like the pre-sidedress soil nitrate test and the chlorophyll meter test, which I wrote about in more detail at this time of the season last year . And at the end of the growing season, consider using the late-season corn stalk nitrate test to determine if your N applications were too little, too high, or just right, which provides valuable feedback on how to improve management in future years.
So, although my recommendation at this point of the season is to stay the course with your normal N program, it’s not because I don’t think you should be positioning yourself to be as adaptive as possible in your management. It’s just that being adaptive sometimes means that when situations are normal, or in this year’s case when extremes might act to cancel each other out resulting in something not too different than “normal", continuing to do what you’ve learned works well for your system may still be the best option.
Source : psu.edu