Please read this article from AgriGold Agronomist Bob Berkevich regarding our nitrogen situation this year. If you have questions, call me and we can discuss.
Thanks,
Tim
From: "Tim Harbaugh" <Tim.Harbaugh@agreliantgenetics.com>
Nitrogen Loss in 2013
June 07, 2013 by Bob Berkevich
While wet weather has many of us thinking about getting the rest of the crops planted, it's also important not to forget about the ones that have been planted. Warm soils and continuous soil saturation lead to losses of nitrogen that may have already been applied. The nitrogen cycle is complex and there is no way to know exactly how much nitrogen may have been lost, but understanding how nitrogen acts in the soil can help growers decide if additional nitrogen should be applied to corn in extremely wet areas to avoid the N deficiency and yield loss shown in Figure 1.
Forms of Nitrogen
Generally, fertilizer nitrogen is composed of ammonia, ammonium, urea, nitrate, or some combination of these sources (Table 1). Once applied, the fertilizer initially becomes either ammonium (NH4+) or nitrate (NO3-).
In the soil profile, ammonium cannot be lost, because its positive charge is held tightly by the negative charge of soil particles. Over time, all sources of fertilizer transform into nitrate (Figure 2) in a process called nitrification. As soil temperatures rise above 50°F, the nitrification rate increases.
Leaching
Since nitrate has a negative charge, it is not held by soil particles and flows wherever soil water flows. Nitrate leaching occurs when soil water is drained through natural outlets or in tile lines. The amount of nitrate lost from leaching depends on how much of the original fertilizer was in the nitrate form at the time of water drainage out of the field. Dr. Peter Scharf at the University of Missouri tracks rainfall amounts and identifies areas where nitrate leaching could be severe and where growers should be aware of the potential for nitrogen loss and deficiency. Fields in the cross-hatched areas of the "N Watch" in Figure 3 have significant leaching potential, especially if they are lighter soils or are tile drained.
Denitrification
Nitrogen can also be lost in a gaseous form. Aerobic bacteria in the soil need oxygen to survive. In saturated soil conditions where oxygen has been depleted from the soil profile, the bacteria break the oxygen off of the nitrate (NO3-) molecule. With the loss of the oxygen (the O's in the NO3-), all that remains is nitrogen, or N2 gas. Nitrogen is lost to the atmosphere in this gaseous form. Once again, the amount of nitrogen lost depends on the amount of the original fertilizer nitrogen that was in the nitrate form at the time of soil saturation. Areas in the cross-hatched areas of the "N Watch" in Figure 4 and localized fields in other areas that have been saturated for many days have significant denitrification potential.
How much nitrate has been lost?
There are several factors that influence how much of the originally applied nitrogen fertilizer might be present in the nitrate form at the time of heavy rainfall, and therefore available for loss.
•UAN solutions initially have some nitrogen in the nitrate form that could be lost immediately. Anhydrous ammonia, urea, and ammonium found in DAP/MAP/AMS nitrogen is initially in the ammonia form and is not immediately available for loss. See Table 1.
•Earlier applications give more time for ammonium to convert to nitrate and lead to higher loss potential.
•Nitrification inhibitors like N-Serve and Instinct delay nitrification, keeping N in the ammonium form, which is not at risk for loss. Urease inhibitors like Agrotain protect urea from volatilizing from the soil surface but do not protect from nitrate loss via leaching or denitrification.
•Soil temperatures in the 70-80°F range have occurred during parts of this spring, leading to 4-5% loss of nitrate-nitrogen per day, compared to 2-3% loss per day when soil temps are in the 55-65°F range.
•Soils don't need to have a foot of water on them to be considered saturated. If you kneel down in the field and your knee feels wet, there is enough moisture there for denitrification to occur. Longer periods of saturation lead to more nitrate loss.
The worst case scenario of fall-applied N, no nitrification inhibitor, warm soil temperatures, and continuously saturated soils could equate to N losses greater than 50% at this time. Growers who utilize spring-applied N, nitrification inhibitors, split applications, and in-season applications will have less N loss, and therefore less uncertainty about how much additional N (compared to the planned program) the crop might need.
Sources:
Fernandez, Fabian. 2013. Determining How Much Nitrogen is Present. The Bulletin, University of Illinois Extension. 3 June 2013. <http://bulletin.ipm.illinois.edu/?p=523>
Lamb and Kaiser. 2012. University of Minnesota Extension.
Scharf, Peter. Nitrogen Watch 2013. University of Missouri Extension. 3 June 2013. <http://plantsci.missouri.edu/nutrientmanagement/Nitrogen/Nitrogen%20watch%202013/nitrogen%20watch%202013.htm>
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