Recent rains are welcome in parts of middle and east TN where moisture was running short, however heavy and frequent rains continue to visit west TN, making field operations very difficult. At-planting nitrogen and recently applied sidedress N have gone through numerous heavy rains– particularly in northwest TN, leading to calls about N loss.
Rainfall amount and intensity, soil temperature and texture, fertilizer source and application date can have an impact on potential N loss. In rare instances, surface-applied granular N can be lost as runoff when heavy rains pound the ground shortly after application, without enough time to dissolve granules and incorporate the N. Wet weather nitrogen loss is usually more due to sustained flooding or ponding where soils stay saturated for several days. Nitrogen loss as denitrification may occur when nitrogen in the nitrate (NO3) form converts to nitrogen gas by a bacterial process in the soil. The potential for denitrification increases where fields are saturated for more than 2 days. Urea and nitrate can also move through the soil profile, especially in coarse textured soils, during leaching and possibly be unavailable early in the season.
It is difficult to accurately estimate N loss, because there are a number of variables involved such as the 1) nitrogen source used, 2) soil temperature and how long the nitrogen has been on the field (percent converted into nitrate form), and 3) duration of flooding or ponding. Assume that 3 to 4% of soil nitrate may be lost to denitrification per day of saturation, after the first two days. Urea applied several weeks ago is mostly (90 to 100%) in the nitrate form. At 3 weeks after application (warm soil temperatures), about 80% of urea or UAN might be in the nitrate form. Using urea as an example: Urea applied at 150 lbs N/ac X 80% = 120 lbs N expected to be in nitrate form at 3 weeks. At 3 weeks, if soil is saturated for a total of 5 days (minus 2 days) = 3 days for potential loss through denitrification. At 4% loss/day X 3 days = 12% loss of nitrate (120 lbs N/ac X 12% loss= 14.4 lbs N/acre potentially lost). In fields where there is concern that N loss might impact yield, consider applying an additional amount (~30 lbs N/ac) using ground equipment while corn is small.
For N applications still to be made, apply layby nitrogen in the most efficient way to minimize loss. Injecting UAN behind a coulter and down into the soil can’t be beat. There is no leaf burn and recent UT research indicates a stabilizer is not necessary using this method. Use a stabilizer (urease inhibitor) with surface-applied urea or UAN unless application is timed the same day as a rain or an irrigation event. A topdress of granular urea with a good urease inhibitor such as Anvol has been comparable to SuperU or ESN in recent UT research, and will burn corn less than liquid UAN. Avoid spreading at night or early in the morning when there is dew on plants causing fines and particles to stick easier (if it’s stuck on the leaf it isn’t on the ground where it needs to be) and resulting in more burn.
In our research, broadcast spraying UAN over the top of corn has yielded either numerically (with stabilizer) or statistically (without stabilizer) less than other UAN application methods and causes varying degrees of leaf burn. Corn grows out of the burn, but if the N is on the leaf, it isn’t on the soil where it needs to be. Last year we had very little leaf burn in our research plots. This year at Jackson, we sprayed UAN over the top of corn that had been growing under cloudy conditions for 2 days prior to and 1 day after. Leaf burn looks to be considerable (can you say torched?) in the plots at 3 days. Watering immediately after broadcast spraying UAN might minimize leaf burn and incorporate N more quickly, possibly improving yield. Applying UAN in row middles with drop hoses or with splitters as a Y-drop directed to soil at the base of corn are effective ways to apply UAN directly to the soil with little crop injury. Just remember to include a urease inhibitor.