Considerations for Topdress Nitrogen

Topdress timing is rapidly approaching and there are a few considerations we need to think about before making fertility applications.  Recent rains and soils that are relatively saturated make possible a couple of potential nitrogen loss pathways.  Denitrificaiton and leaching. 

Denitrification is possible when soils are saturated for extended periods and depending on soil temperature can be quite severe (warmer soils will denitrify faster than cold soils) resulting in your soil nitrate (NO3) being microbially reduced to nitrogen (N2) gas that excapes into the atmosphere.

Leaching is possible because nitrate nitrogen (NO3) is mobile in the soil and can move down with soil water.  Leaching can result in NO3 moving off site below ground or below the plants root zone depending on soil type and soil depth.  Either way, your plant cannot access its food source in this scenario.  As soils begin to dry, mobile nitrients can move to more shallow depths in the soil provided they did not leach off site or denitrify.

Recently, I did some soil samples on ground that we applied 20 gallons (~60 units of N) of 28% UAN at planting in October.  At 6 inch depth sampled in Janurary, there was only 4 pounds of nitrate N. so, where did it go?

Normally, when sampling for a mobile nutrient like Nitrate, Sulfur, Boron or Chloride, we would go to a depth of 16-24 inches.  However, the morjority of the wheat's fiberous roots only proliferate the top 6-8 inches of soil, especially early in the plants life.  

So, the moral of the story is that we are likely short on available nitrogen and will have to add additional units of N to reach our desired yield goals.  In my opinion, based on current conditions, we need to sample at 6 inch and 16 inch depths for Nitrate N to make an educated quess on how much nitrogen will be availabe and when. Additionally, we need to reduce environmental loss by adding a nitrogen stabilizer that has both NBPT and DCD to all urea and UAN application including topdress wheat.

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