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Spring Precipitation Reduces N: ISU Soil Test Results

May 8, 2013
By: Davis Michaelsen, Pro Farmer Inputs Monitor Editor

Precipitation is a double edged sword this spring. The moisture is welcome after the parching summer of 2012. But a study from Iowa State University shows that as soil moisture levels increase, N levels are decreasing. When soil samples taken last fall were compared to recent soil tests on the same ground, the results showed significant N loss.

According to ISU agronomist, John Sawyer, "...the amount of nitrate-N that might remain for a 2013 corn crop depends on springtime rainfall. Tile lines are flowing again, and nitrate in the profile will move with percolating water." Sawyer draws three basic conclusions based on the soil test data:

  • One, samples collected before the large spring rains still had high nitrate-N amounts.
  • Two, samples collected after the large spring rains show nitrate movement deeper into the profile.
  • Three, samples collected after the large spring rains generally have less total profile nitrate-N than last fall.


Results vary by soil type, but the table below from ISU affirms the findings and while all of the samples were collected in Iowa, the implications are the same for the entire Corn Belt.


Add N loss to a narrow application window, and we have a recipe for sidedress applications later on. UAN solutions posted the only gains in the Monitor's Index this week, and while anhydrous and dry urea continue downward movement, UAN may be responding to expected demand post-emerge.

This is not the spring to skimp on N applications. Recent rains and snow have gone a long way toward recharging soil moisture, but as that water has moved through the soil profile, it has taken N with it, either deep into the subsoil, or down tile lines to the creek.

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