Nitrogen - What is the correct rate?

What is the correct rate of nitrogen to apply to raise corn in Iowa? It depends on several factors:

• Source of Nitrogen

• Commercial fertilizer

• Manure’s -- what species and what form

• Method of application

  • Incorporated or applied on the surface

• Weather

  • Is it going to be cool and wet or hot and dry?

Several misperceptions regarding nitrogen rate guidelines over the years have resulted in discussions at many levels. Older guidelines have used major land resource areas and nitrogen requirement per bushel. As research and cropping methods have evolved it has become apparent that yield is not a linear equation to the amount of nitrogen applied. In other words, if you apply 200 pounds of nitrogen you are not guaranteed to get 167 bushels of corn -- if you are in the area of the state that the 1.2 pounds of nitrogen per bushel is recommended. 

Since 1997 Iowa State Extension has been recommending a range approach to nitrogen. See PM 1714 “Nitrogen Fertilizer Recommendations for Corn in Iowa”.   http://www.extension.iastate.edu/Publications/PM1714.pdf This guideline has primarily been used by producers using commercial fertilizers, however does apply to manures. Iowa State Extension Publication PM 1811 “Managing Manure Nutrients for Crop Production” http://www.extension.iastate.edu/Publications/PM1811.pdf  two methods are suggested for nitrogen recommendations. The use of major land resource areas, proven yields, and any nitrogen credits are taken into account when making the decision of how much nitrogen is needed for crop production is one method. The other method is taking advantage of the Late Spring Soil Nitrate Test. The initial amount of manure to apply should be method one or less, and then do the Late Spring Soil Nitrate Test to determine how much nitrate is in the soil for plant use.

We need to manage the manure nitrogen in same way we manage commercial sources of nitrogen. Some producers only apply 125#N with anhydrous ammonia but apply plant available nitrogen with manure at greater levels. 

Why do they do this? 

• The need to get rid of the manure.

• Not enough land for the manure.

• It is being hauled and applied by custom applicators that get paid by the gallon and not by the acre.

• Unsure or not confident in the plant availability nitrogen of the manure source.

A couple of ways to help with the confidence in the manures is to do replicated strips of different rates to find out what is the best rate and evaluate the nitrogen application by using the Fall Stalk Test to determine the amount of residual nitrates in the stalk. High levels mean that there was more that enough for this year. Remember, this test evaluates the nitrogen use given this years’ conditions.

Several Universities across the Midwest have come together to give us some newer guidelines. PM 2015 “Concepts and Rationale for Regional Nitrogen Rate Guidelines for Corn” http://www.extension.iastate.edu/Publications/PM2015.pdf  this publication takes a hard look at the economics of nitrogen application rates. 

The term Maximum Return to Nitrogen (MRTN) is referred to extensively. A spreadsheet has been developed and is available on the web http://extension.agron.iastate.edu/soilfertility/nrate.aspx the spreadsheet helps determine what the economic rate of nitrogen is given the price of corn and the price of nitrogen.

I’m going to stop here and see what kinds of questions are generated.  Please read PM 2015 if you get a chance, a lot of questions are answered in this material.  As always if you have questions let me know.

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