Source: Penn State University
By Charles White
There are many environmental factors that can lead to nitrogen losses, and nitrogen that is applied to a field significantly earlier than it will be utilized by the crop is potentially susceptible to leaching and denitrification losses. Leaching losses occur when water percolating through the soil profile carries nitrate below the rooting zone. Denitrification losses occur when anaerobic soil conditions cause microbes to convert nitrate to nitrous oxide or di-nitrogen gases which escape from the soil into the atmosphere.
For either of these processes to happen, nitrogen in the soil must first be in the nitrate form. Nitrate can be added directly as a fertilizer. For instance, 25% of the total nitrogen in UAN solution is in the nitrate form. Nitrogen added as organic N (such as in manure), urea, or ammonium, will ultimately be converted to nitrate through microbial processes, but these processes take some time and are temperature dependent. The cooler the temperature is, the slower the conversion from ammonium to nitrate will occur.
Whether or not the nitrogen you applied to your field has been lost through leaching or denitrification will depend many site-specific factors, such as the form of fertilizer you used, when you applied it, the precipitation you received since then, the temperatures, and soil characteristics such as rooting depth and texture. Because all these factors interact to affect nitrogen losses, it is difficult to make a broad generalization about whether your nitrogen has been lost. One set of tools that have recently become available to help answer this question based on site-specific conditions are nitrogen simulation models.
Two nitrogen modeling platforms that are commercially available in Pennsylvania are Adapt-N (owned by Yara International) and Encirca (owned by DuPont Pioneer). Climate FieldView (owned by Monsanto) also provides a nitrogen modeling service that is available in mid-West states with plans to expand into Pennsylvania in the future. I used the Adapt-N and Encirca nitrogen models to see what they said about nitrogen losses from a field I am managing near our Agronomy Research Farm at Rock Springs. The field was planted to corn on May 9, 2018 and 25 gal/ac of 30% UAN solution (82 lbs N/ac) was used as a carrier for the herbicide applied that same day. As of writing this (June 5, 2018), the field had received 4.1” of rain since planting and the N application. Both modeling platforms provide a base map of soil types for the field and I could manually enter the rooting depth, which I set to 24 inches. You can also set a yield goal, which is used to determine the total N requirement for the crop and is used to calculate recommended sidedress N rates and predict N deficiencies. I entered a 180 bushel/acre yield goal for the field.
Both Adapt-N and Encirca provided a variety of detailed reports about the status of the nitrogen in soil over time. Because of reporting formats and starting assumptions unique to each platform, it wouldn’t be appropriate to directly compare numbers between the two platforms. But by looking at the patterns over time, both Adapt-N and Encirca suggested that the large majority of the N applied on May 9 was still in the rooting zone of the crop. Adapt-N provided a detailed reporting of the types of N losses that occur, and it suggested that since May 9th less than 5 lbs/ac N had been lost to leaching and about 10 lbs/ac N had been lost to gaseous losses, though it did not specify ammonia volatilization versus denitrification as the type of gaseous loss.
I was also curious to see what the simulation models predicted if I had applied the nitrogen a month earlier than planting, on April 9. Between April 9 and May 9, the site received 4” of rain, for a total of 8.1” between April 9 and today. The Encirca model did not predict any greater loss of N due to the month earlier application date and Adapt-N only predicted 1 extra pound of nitrate leaching and 7 extra pounds of gaseous losses.
Whether or not these models are absolutely accurate in their predictions remains to be seen, but they are one of the most comprehensive attempts by academics and industry to integrate all of the complex factors that regulate nitrogen losses in a site-specific way. These tools are certainly worth considering as an additional piece of information that can go into your nitrogen management decisions. In this case, it made me think twice about whether the nitrogen we applied a month ago had really all been lost. The models suggested much of it may still be available in the root zone. So, for those of you looking at yellow corn seedlings in your field and wondering whether your nitrogen was lost, perhaps give the crop a week or so to see if it catches up with some N that might be sitting a few inches deeper than normal in the root zone. Other tools that you could consider using later in the season to help make decision about sidedressing rates are the PSNT test and the Chlorophyll meter test. And given the potential risk for early season N losses, you should always consider splitting your N applications between at planting and sidedressing. Keeping nitrogen in the bin, bag, or tank until your crop needs it will always be the best way to reduce the risk for losses.