Becoming better nitrogen (N) managers in corn production starts with planning.
There are many good ways to manage N now, but we often spend more time trying to figure out when to apply it than determining how much to apply based on yield goals and soil reserves.
Soil sampling often assesses nitrate and doesn't account for other residual forms such as ammonium and mineralizable N. This ignores a potentially large N credit. Ammonium N is available, but it's fleeting because it converts readily to nitrate, so we often assume that this form is negligible. However, there is potentially a huge pool of mineralizable N (Nmin) from organic matter that isn't accounted for.
This summer we collected soil samples in June from 16 cornfields and at three depths: 0-12 inches, 12-24 inches and 24-36 inches. We ran soil nitrate, organic matter (OM), Woods End Solvita soil respiration and SLAN (Soil Labile Amino Nitrogen) mineralization tests in each field and at each depth. My goal was to assess the total N pool in the soil as contributed by nitrate and Nmin.
As you would expect, nitrate, OM, respiration and Nmin declined with depth. At 24 to 36 inches, respiration drops considerably as does Nmin -- in some fields to near zero.
Nitrate amounts averaged 78, 68 and 52 pounds per acre at 12-, 24- and 36-inch depths for a total of 198 lbs./acre N through the profile. This large pool of residual nitrate is available for uptake and, unfortunately, is also vulnerable to leaching and denitrification. We can only hope that the crop takes most of it before excessive rainfall events remove it from the pool.
Mineralizable N averaged 153, 88 and 55 lbs./acre at 12-, 24- and 36-inch depths for a total of 296 lbs./acre through the profile. Add the two pools (nitrate and Nmin) together and there was 500 lbs./acre of potential N available. When you average across all fields and depths, nitrate and Nmin contribute almost equally to the N pool, 48% versus 52%, respectively. However, the Nmin pool releases its N slowly depending on soil biology, soil health, soil temperature and moisture. While there might be 300 lbs. of Nmin in the pool, it is not like a bank that you withdraw it all at one time.
How did soil depth influence the pool? At 12 inches, about 40% of the N came from nitrate and 60% came from Nmin. At 24 inches, 45% came from nitrate and 55% from Nmin. And at 36 inches, 60% came from nitrate and 40% from Nmin. It is not a surprise that as you go deeper in the soil, respiration and Nmin decline and nitrate constitutes a larger portion of the pool.
So why did I go to all this work and investment to sort this out?
Tracking nitrate before planning a sidedress recommendation is important because you need to know how much N is there and how much you've lost before deciding how much to apply.
Second, Nmin is rarely counted when making N recommendations. Sure, some of us have used the 10 lbs. N/%OM in writing a recommendation, but this isn't accurate and totally underestimates that pool of Nmin that will become available. As growers build their OM and strive for healthier soil, the Nmin contribution will become larger and needs to be taken advantage of.
I was surprised by the high levels of mineralizable N in these soils, being almost equal to nitrate nitrogen in scope. I believe that our N recommendations are often too cautious and conservative. Fortunately, this pool of mineralizable N kicks in and can easily make up for any shortcomings.
As we move toward following tighter N management strategies, we ought to consider tracking both nitrate N and mineralizable N and use the two to make N recommendations for corn.
Dan Davidson can be reached at firstname.lastname@example.org
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