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:

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

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