Friday, June 20, 2014

Our Thoughts on Nitrogen Loss this Year


What Kind of Spring-Applied Nitrogen Loss Will I See Following Rain?
                Last year Burrus provided an educated guess in response to this question.  The question is this – “I had rain – how much Nitrogen did I lose?”  Burrus provided an answer to that question during the unique wet spring that followed warm winter weather in 2013.  Our answer to that question in 2013 can be reviewed at our YouTube Channel – Burrus Agronomy U in a session entitled “Nitrogen Loss." Some of the information provided during that session can help develop assumptions needed to provide our educated guess in 2014:
            Assumption 1:  It will take a few days for nitrogen fertilizer to convert into ammonium.  The first step on the road to nitrogen loss usually involves nitrogen being converted into ammonium.   Needless to say – most nitrogen sources are converted (at least briefly) into a form of nitrogen that can be stored by the soil (ammonium).  While the transformation into this form is not a really long delay in the nitrogen loss process, it is still a delay in the process of nitrogen loss.  We need to factor some kind of delay into any educated nitrogen loss guess.
                Assumption 2:  Ammonium will convert over to nitrate at a rate of about 1 percent per day in the spring.  The second step on the road to nitrogen loss involves nitrogen being converted from ammonium into nitrates.  Nitrate should be thought of as a form of nitrogen that is not held well by the soil and that is prone to being “chewed on” by microbes.  The actual rate of conversion into nitrate may be more like 0.8 to 1 percent per day depending upon location, environment, etc.  However, we need to establish some numbers to work with, so we will run with the higher end of that range.  Once again, our goal is to guess at nitrogen loss, and this at least helps us start to get to an educated guess.  
                Assumption 3: Denitrification of nitrate occurs at a rate of about 5% per day of saturated conditions.  For now, we will assume that no nitrification inhibitors were used, which would slow that conversion down.  Think of this process as simply nitrate being changed into a lost gas by “little critters in the soil.”  A few days of super saturated conditions are probably needed back to back before this process begins in earnest.  In other words, anything less than a few days of saturation/semi-ponding will result in nitrogen losses that are extremely low.  Go beyond those few days and our best guess (we stress guess) would be 5% loss per each subsequent day of saturation.
 So Where Does That Leave Us?  How much Nitrogen might we have lost following recent rains? 
Let’s consider nitrogen losses associated with three different periods of time this spring (just to keep things simple).  Let’s think of nitrogen being applied one month ago, two months ago, and about two and a half months ago.  Remember it takes a few days for us to reach that first stage in the nitrogen loss process (conversion into ammonium).  After that, conversion into a vulnerable form occurs at a rate of about 1% per day (nitrogen switches over from ammonium to nitrate).  That probably means that a quarter of the nitrogen applied a month ago is available for potential loss, more than half of the nitrogen applied two months ago is at risk, and about 70% of the nitrogen applied two and a half months ago could be lost (if some other “bad things” happen).  Let’s say this is the upper end of where we might be – this might be the most nitrogen converted over to a potentially lost form.
Now we need to consider what loss might have actually looked like following rain/ following soil saturation.  Let’s think about two different saturation scenarios.  Let’s consider a situation where the soil has been saturated for a few days (let’s say five) and a situation in which the soil has been saturated for several days (let’s say ten).  Five days probably represents the more common scenario if you encountered ponding, and ten days of saturation represents what happened to a few unlucky growers.  Remember, it takes a few days for saturation-related nitrogen losses to kick into gear.  So five days of saturation probably acts more like an actual day of nitrogen loss, and ten days of saturation probably acts like a few days (let’s say five to make it convenient).  Remember, we also said that losses of nitrogen as a gas occur at about the rate of 5 percent per day.  That’s 5 percent of the “at risk form” of nitrogen, not 5 percent of the total nitrogen applied.  That is an important difference.  The math can get a little tricky to type out, so let’s use a chart to describe what these scenarios might look like.
Period Between N App and Soil Saturation
Percent Nitrate/ Percent of Nitrogen
"At Risk"
5 Days of Saturation Percent of Total N Loss (Assuming ~5% Lost Nitrate)
10 Days of Saturation Percent of Total N Loss (Assuming ~25% Lost Nitrate)
1 Month
25
1 to 2%
6 to 7%
2 Moths
55
2 to 3%
13 to 15%
2.5 Months
70
3 to 4%
16 to 18%

            We have, in many ways, set up the worst case scenario in the above chart.  We have assumed the upper end of the range for nitrate formation, we have assumed 5 percent lost nitrate nitrogen per day of saturation (following a few days of saturation), and we have assumed that the 5 and 10 days of saturation occurred at one time rather than being spread out over a period of time.  We also considered nitrogen application to occur in one shot.  We did not open the door for a split application of nitrogen.  Back off on any of those and the actual amount of nitrogen loss is probably less than what we would guesstimate here.  For instance, split nitrogen applications would result in less loss than what we see here.  As you can see, many growers likely encountered nitrogen losses that ranged from a percent to a few percent at the max (considering the 5 days of saturation scenario).   A few unfortunate growers may have encountered some double digit percentage loss at the max.
            One final disclaimer on Nitrogen losses from recent rain – accurately estimating denitrification (nitrogen losses due to saturated soil conditions) is difficult (some would say it is nearly impossible).  We have taken a stab at providing a guess.  We have attempted to generate numbers that might be used by a grower to make more informed decisions.  However, we cannot and will not guarantee that these numbers are an “exact” representation of reality.  This is a guess, but it is our best one for now.  We hope it helps. (Dr. Matt Montgomery)

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