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Issue No. 12, Article 8/June 12, 2009

How Much Nitrogen Do I Need?

I have received a lot of inquiries about nitrogen loss from corn fields that have received too much water this spring. The question in everybody's mind is, Do I need more? The answer unfortunately is not a simple yes or no. Even more complicated to answer is the subsequent question: How much N should I apply?

Answering these questions is challenging because the process of N loss is complex, and many variables can affect N loss at different field locations. Despite these complications, farmers still need to make informed decisions in the attempt to ensure that crops will have what they need to produce high yields.

In considering whether to apply N and, if so, how much, you need to understand loss mechanisms, rely on your experience with the particular field in question, and put the two together in the framework of this spring's weather conditions. For details on N loss mechanisms, see my earlier articles in the Bulletin on N loss (issue 7, May 8) and N management (issue 10, May 29).

Key factors in determining whether additional N is needed are soil drainage, N source, time of application, and amount of precipitation in relation to application. Consider the following suggestions:

  • In silt-loam or fine-textured fields with poor drainage, if you had excessive rain (and water sat on the field long enough to kill the crop) about 2 weeks after applying UAN or 4 or more weeks after applying anhydrous ammonia, you might consider applying 50 to 100 pounds of N per acre for the new corn crop. This situation occurs most often in low areas of a field.
  • In sandy or light-textured soils, if more than 7 or 8 inches of rain infiltrated the soil (as opposed to running off the soil surface) 2 weeks after applying UAN or 4 weeks after applying anhydrous ammonia, it is likely that a substantial part of that nitrogen was leached out of the root zone. In this situation you might also consider applying between 50 and 100 pounds of N per acre.
  • In silt-loam or fine-textured soils with poor drainage where a large rain event caused water to be ponded for 1 to 3 days and UAN was applied at least 2 weeks before or anhydrous ammonia at least 4 weeks before the time of waterlogged conditions, you might consider applying 30 to 50 pounds of N per acre.
  • In fields where the chance of N loss is low (less than 30 pounds of N per acre), there is no need to worry about applying more. This would include fields where excess soil water was present for 1 to 3 days within a week after applying UAN or urea or where anhydrous ammonia was applied less than 3 weeks before soils were waterlogged. Another situation where N loss potential is low would be light-textured soils where infiltrated rain was less than 4 inches and most of the applied nitrogen was not in nitrate form.
  • What if you sidedressed with UAN or urea and had heavy rains the next day? This is a concern only if you have sandy soils. The amount of additional N needed will depend on how much rainfall occurred. If more than 7 or 8 inches of rain fell, much of the N was likely leached out of the root zone. If rainfall was 4 to 7 inches, some of the N probably leached out, and you might consider applying 30 to 50 pounds of N per acre. If there was less than 4 inches of rain, most likely additional N is not needed.

As I mentioned, these are suggestions to help you make a decision. In determining the need for additional N, make sure to prioritize which fields will need it the most. Whatever you consider most appropriate for your field, the best measure of whether enough N is available is the response of the crop. To measure this, there is no substitute for looking at the crop as it develops. One simple way to test whether the crop has sufficient N is to establish a reference strip. If you are planning to apply additional N, an easy way to do this is to apply a higher rate in one strip in each field. If you can see differences between the strip and the rest of the field, it likely indicates that more N is needed. If you determine that additional N is not necessary in your field, it might be worth your time to apply some additional N in a small area just to doublecheck. If you don't see differences, it will indicate that you have made a correct decision.--Fabián G. Fernández

Fabián Fernández

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