Q&A with a CCA: Corn nitrogen management


…With Kevin Otte, Otte AG, LLC, Maria Stein

Q: How much nitrogen (N) do I need to supply my corn crop?

A: Depending on your efficiency factor of nitrogen, you can figure from 0.8 to 1.2 pounds of nitrogen per bushel to be supplied to the crop. You can enhance your nitrogen rate determination by utilizing an economic return to nitrogen model. These models consider the price of nitrogen and the price of corn and give a range of nitrogen rate that will return most dollars per acre.

Kevin Otte

Q: Should I include a stabilizer with my N source?

A: Anything that can help keep the nitrogen in the field should be looked at. Stabilizers offer protection from nitrogen losses and there are a number of different stabilizer products to choose from. If your nitrogen can be split applied, this can reduce the potential need of a stabilizer. 

Q: The price of N is high. Do I cut back rates to save money?

A: With today’s high commodity prices, you do not want to shortchange yourself on yield and higher returns. It is a balancing act and the economic nitrogen models can help in answering this question.

Q: I use manure for crop nutrient needs. Can I account for the N?

A: Some may discount nitrogen from manure altogether, while some give it full credit according to a manure analysis. Well, it is best to figure the answer lies somewhere in the middle.

Manure nitrogen exists in three different forms: organic, ammonia and (rarely) nitrate. Plants use mostly nitrate nitrogen. There are nitrogen availability tables that generally show 33% of organic nitrogen and 100% of the ammonia and nitrate nitrogen being available. However, there are factors that can adjust availability to account for losses. For example if you smell ammonia, you are losing nitrogen.

Q: How can I make the best use of N?

A: A corn crop responds to nitrogen best when supplied at different growth stages. The easiest method is to apply 20% to 30% of the nitrogen need at planting and the remaining 70-80% at sidedress. Some may include some nitrogen with their pre-emerge herbicide and others may apply some late season nitrogen. If sidedressing is not an option, a grower may choose to utilize a slow-release nitrogen source that can be applied prior to or following planting to supply nitrogen for the entire growing season.

Q: If a corn crop uses mostly nitrate and manure has very little, where does it come from?

A: The soil is very dynamic and the microbes that reside in the soil are the nitrogen gatekeepers. The activity of the microbes in the soil is impacted by several factors: soil temperature, moisture, organic matter, pH, crop residues, other nutrients, etc. Healthier soil has greater microbial activity. The microbes are continuously converting nitrogen back in forth between organic and inorganic (ammonia, nitrate) forms. 

Q: How do I know for sure how much manure has been supplied from manure?

A: This is where a soil test can be utilized to determine the value of the manure applied. The PSNT (pre-sidedress nitrate test) has been utilized and is still used today to determine the soil nitrate levels just prior to side dressing. Over the years an ammonia analysis has been added to the testing protocol so as now the PSNT is referenced as pre-sidedress nitrogen test. I have used this test for 20+ years and it has served its purpose to account for manure nitrogen and better equip growers with a sound nitrogen recommendation agronomically, economically and environmentally.

Q: Is there something better than a PSNT?

A: Since we know microbial activity is diminished at lower temperatures which will affect the results of the PSNT, is there a way to evaluate for nitrogen availability that is not temperature dependent? Through a cooperating laboratory and Brookside Labs and my consultant mentor, a new nitrogen test method was developed. Through soil incubation trials and field calibration, this “mineralized nitrogen” analysis has been a game changer. We still analyze for nitrate and ammonia and include the mineralized nitrogen results to generate a sidedress recommendation for our growers. 

The uniqueness of the mineralized nitrogen test relieves us from making guesses on nitrogen availability from previous legume crops, crop residue and organic matter. It has been a staple in our nitrogen sampling program for over 10 years now. 

The PSNT test is very accurate. It is a “snapshot” of what the microbes have provided us the day of sampling. The day of sampling is key. To get the best snapshot of nitrogen availability the sampling needs to occur as close as possible to when the crop will be sidedressed. This presents some limitations. Ideally corn should be sidedressed at 8 inches to 12 inches in height. However, this does not always happen based on acres to cover, equipment rental and weather just to name a few. This pushes to get sampling done sooner in the season and PSNT results typically are lower compared to later season sampling (ideal corn sidedress height).

Q: Can the PSNT and mineralized nitrogen test be used on non-manure ground?

A; They certainly can. The results typically will not be as dramatic as results from manured fields, but it does take some of the guess work out of determining the ideal rate of nitrogen. For fields with a history of cover crops, higher organic matter and/or great soil health, these tests are great tools to be included in your nitrogen management toolbox.

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