Spring soil nitrogen following the drought of 2012
How much soil N has been lost?
Dan Schaefer of C-BMP took both fall 2012 and spring (March) 2013 samples in a few fields in east central Illinois that can give us some estimates of N loss. In one field where N was applied in the early spring in 2012, October samples had about 16 and 13 ppm of nitrate-N in the top and second foot of depth. In mid-March 2013, those numbers had fallen to 6 and 10 ppm, respectively. Assuming no net conversion of N from ammonium to nitrate, this reflects a loss of about 50 lb of nitrate from the top two feet. Much of this probably remains in the soil below the top 2 feet, but some may have entered the drainage tile by now.
In a second field in which N was applied as spring sidedressed ammonia, nitrate-N went from 24 and 8 ppm in the first and second foot of soil last October, to 9 and 18 ppm, respectively, by mid-March of this year. That represents a net loss of only about 20 lb of N per acre, but a considerable amount of movement of nitrate from the top to the second foot. Nitrate losses have possibly been limited because it took so long (and so much precipitation) for dry soils to rewet to the point that water started to move through the profile.
Ammonium-N is immobile in soil, but in warm soils, microbes generally convert it rather quickly to nitrate-N. Because of this we normally see only 2 or 3 ppm of ammonium-N in either the fall or the spring. We found an average of about 5 ppm of ammonium-N in both the top and second foot of soil in the 2012 fall samples. In the second field described above, the fall sample had about 11 ppm of ammonium-N in the top foot. This could have come from spring-applied ammonia or from mineralization of soil organic matter after rain in September. The surprise is that the March sample still had 9 ppm of ammonium-N in the top foot. It is clear that there was limited net conversion of ammonium to nitrate in the top foot of soil between mid-October and mid-March.
Does this mean that ammonia applied in the fall of 2012 is still present? Most of the ammonia was applied after soil temperatures had dropped to below 50. Soils also stayed relatively cool, with average soil temperatures at the 4-inch depth of 49, 45, 34, and 34 degrees in November, December, January, and February, respectively. In contrast, 4-inch average soil temperatures in January and February were 39 and 41.5 degrees, respectively, in 2012. There were only 3 days in January and February 2013 with average 4-inch soil temperature of 40 degrees or more. So we think that soil temperatures have stayed low enough this winter to minimize the conversion of ammonium to nitrate.