Recent storms covered much of the region with 4-6” of precipitation. With large rain events like this our staff gets questions about potential nitrogen losses due to denitrification and leaching. These big rains came in May with very cool temperatures and even snow in some areas (brrrrrrrrrr). Soil temperatures during and after the rain actually dropped into the low 40 degree F range or lower. This low temperature should have prevented the conversion of ammonium to nitrate in the soil. To get an idea of the magnitude of N losses, we worked with farmers near Northwood ND and tested the soil nitrate and ammonium levels in a few of their corn fields after the rain. These fields were soybeans in 2014 and had urea applied and incorporated with tillage before the big rain.
As you can see from the data in the table, urea was applied to Field 1 on April 27, which allowed more time for the urea to convert to ammonium and nitrate. If you add the soil nitrate and the ammonium levels together, you can see that most of the nitrogen fertilizer is still in the 0-24” soil profile for Field 1. This is a heavy loam soil and nitrate nitrogen will not likely be leached. Field 2 and 3 had urea applied later, on May 4 & 5, a few days before the big rains. You can see that these two fields have much more N in the ammonium form remaining in the topsoil (0-6”). This means the temperatures in the soil has been too cold to convert the ammonium to nitrate. This is very important for these two fields because they are well drained sandy loam soils which are subject to leaching losses. Since much of the N fertilizer was still in the ammonium form (NH4+) and is help on the negatively charged soil, it could not be leached from the soil during the big rainfall events.
These test result are only from three fields, but they do confirm that urea N that was applied and incorporated two weeks before the 4-6” rainfall event, is still in the 0-24” soil profile and in position for crop uptake. This is very good news for growers!