Precipitation in the southern Great Plains is never something you take for granted. As I write this blog I am just wondering when it will be dry enough for long enough to finishing sowing my wheat, but I also remember just how dry it was last winter. The last three months, Aug-Oct rank as one of wettest in the states recorded history. Below are the 30, 60, and 90 day rain fall totals (as of 10.26.18) from Mesonet. By the 60 day map most the wheat belt is showing double digits and the 90 day maps shows a lot of our graze out wheat regions in the 20+ inch realm.

30 Day rainfall totals retrieved from Mesonet on 10.26.18.  Putting recording window from Sept 26-Oct 26. http://www.mesonet.org/index.php/weather/category/rainfall

60 Day rainfall totals retrieved from Mesonet on 10.26.18.  Putting recording window from Aug 27 – Oct 2 http://www.mesonet.org/index.php/weather/category/rainfall

90 Day rainfall totals retrieved from Mesonet on 10.26.18. Putting recording window from July 28-Oct 26 http://www.mesonet.org/index.php/weather/category/rainfall

I bring up graze-out wheat for a reason, to get as much forage as possible it is planted as early as possible. I know of fields that were seeded in July and early August. And to produce this great quality forage, nitrogen fertilizer is applied pre-plant. It just so happens that this July more fertilizer was sold than any other month since I have been in Extension. In July producers bought nearly 1/3 of totoal tons of fertilizer what is typically sold in a single year. While a portion of this may have been pre-purchased for later delivery, I know a lot of it made it to the field. To see why this matters, lets take a look at the nitrogen cycle.

Complete Nitrogen Cycle. http://psssoil4234.okstate.edu/lecture

The nitrogen cycle is made up of a central component (Organic Matter), three N sinks (Microbial/Plant, Atmosphere, Nitrate {NO3}), four loss pathway (Ammonia Volatilization, Leaching, Plant Loss, Denitrification), and five additions (N2 Fixation, Fertilization, Lightning/Rainfall, Industrial Fixation, Plant/Animal Residues). We are going to spend the next bit talking about what is happening in the bottom right corner and left hand side.

When we put anhydrous ammonia (NH3) in the soil it pulls a hydrogen (H) from water and turns in to ammonium (NH4). Urea goes through a similar process but has to first be converted to NH3 by the enzyme urease.  Ammonium is important because it is a positively charged ion (cation) which will be fixed on the cation exchange sites. This means is it not going to move around in soil, but is readily available for plant uptake. However when NH4 is in a soil with temperatures above 50 degrees and in the presence of oxygen the two bacteria nitrosomonas and nitrobacter convert it to NO3. Given warm soils and our good soil moisture levels it very likely that any N applied in July or August would have converted 50% or more of its NH4 into the NO3 form by this point.

Nitrification portion of the Nitrogen Cycle. Complete Nitrogen Cycle. http://psssoil4234.okstate.edu/lecture

Nitrate is a negatively charged ion (anion) which is repelled from the negatively charged soil. This is beneficial for plants as when they take up water, NO3 is taken up though mass flow. The downside is that since NO3 is in the soil solution, where ever the solution goes so does the NO3, that is called leaching. So in well drained soils the recent rains will have caused a fair amount of leaching.  For some areas the NO3 that is leached below the root zone and could potentially be drawn back up as the soils dry. But there are going to more scenarios in which the N is gone, or at least gone elsewhere. In a sloping field the soil water will hit a limiting layer or clay increase layer and move down slope. I have already seen many wheat fields that are showing yellowing on side slopes.

Unfortunately leaching isn’t the only way we are losing N during this wet cycle. Denitrification occurs when the soil is saturated and oxygen (O) levels are depleted.  In anaerobic conditions, microbes strip O from NO3 reducing it gaseous forms. Typically it takes about one week of standing water to start seeing high levels of denitrification.

Nitrate loss pathways of the Nitrogen Cycle.
Complete Nitrogen Cycle. http://psssoil4234.okstate.edu/lecture

What does this all mean? Conservative guess is that for July or early August applied N we could be looking at losses of 50% or more.  This is a rough guesstimate of course, a fields soil texture, slope, soil type, tillage etc will all impact the loss amount.  As the date of application moves closer to Oct there will have been less nitrification and less total rainfall. What I can say with 100% certainty is that if N fertilizer was applied any time from July through early September, N has been lost.

So whats my N manage recommendations? First, foremost, and always This is the perfect scenario where N-Rich Strips will pay off! (Here’s a blog on N-Rich Strips https://osunpk.com/2013/09/19/nitrogen-rich-strips/). The N-rich Strip will allow you to detect N stress early, which for grazers is important. Close attention needs to be paid on fields with wheat being grown for grazing, N deficiencies will reduce forage production and gain. If the N-Rich strip shows up or there are signs of N deficiencies (yellowing of older leaves from the tip toward the collar) its time to be looking at applying N. For grain only fields we have some time. It is important though that as we get closer to spring and hollow stem we are taking care of the crops N needs. Here is a link to a blog on reading the N-Rich Strips to get a N rate rec https://osunpk.com/2014/02/24/sensing-the-n-rich-strip-and-using-the-sbnrc/ and here is a link to one of my latest blogs on Timing of Nitrogen Application for Wheat https://osunpk.com/2018/10/01/how-long-can-wheat-wait-for-nitrogen/.

For more information please contact me at b.arnall@okstate.edu

Below is a Sunup TV video on the subject of Nitrogen Losses with the recent rains.