Article recevied from Dale Cown, Senior Agronomist SCS / Sales Manager AGRIS – June 14, 2013
Once again farmers are concerned with N loss. The form that is lost from the soil is the nitrate form. Any preplant N sources are likely all converted to nitrate by now. So length of saturation of the soil is a concern due to leaching potential on sands and denitrification on other soil textures. Because it is much warmer now than in March and April the losses will be much higher/ day approaching 10% per day or more.
I will try to make it easy to talk with growers . The assumptions are preplant N applied at 180 lbs N as SMART N ( includes starter N) the majority of the days have been over 65 F. For every day of saturated soil ( where you cannot walk in the field) I estimate 8 lbs of N per acre/ day of loss therefore 7 days of saturated soil – 2 days to get denitrification started, the loss is estimated at 40 lbs of N /acre after 7 days of waterlogged soil. IF no Smart N was used then the loss could be as high as to 100 lbs/ acre after a week of saturated soil.
A daily loss with preplant SMART N of 8 lbs / acre/ day of saturated soil
A daily loss with preplant unprotected N sources all converted to nitrate 20 lbs / acre/ day of saturated soil
UAN sidedress with Agrotain PLUS – 2 lbs of N/ acre/ day of saturated soil
Saturated soil where you cannot walk on the field and water is lying on the surface. This is different than being fit for tillage that would take longer but may not be saturated. This may only be for 2 days. In which case it takes 2 days to start denitrification. If only 1 day of saturated soil after the 2 days to start it at then losses are only 8 to 20 lbs / acre. And 2 lbs maybe with Agrotain plus sidedressed UAN.
Side dressed UAN with Agrotain plus: only protects the urea or ammonium form. Therefore I estimate the remaining loss to be 2 lbs of N/ acre/ day of saturated soil . Under same scenario of 1 week of saturated soil I only expect a loss of possibly 8- 10 lbs of N/ acre. Without Agrotain losses could easily exceed 4 to 5 lbs per day. No question on low CEC sands there will be a physical movement downward of the entire application to lower depth due to low CEC and no place to hold the ammonium N. That is more difficult to determine. Under preplant scenario N loss may be double the estimated amount above. With sidedress applications in sand we may want to consider applying another 30 lbs- 50 lbs with UAN and Agrotain on those fields with 3 inches of rain. We can expect anywhere from 21 to 28 days of protection from denitrification. Likely count on 3 weeks.
Recent sidedress with UAN will still have a majority of the N in ammonium form. Remember UAN is already 25% nitrate. UAN treated with Agrotain PLUS with the nitrification inhibitor will offer 21 to 28 days protection from application timing.
The trick is to determine how much of the unprotected UAN has converted completely to nitrate. See chart , it is driven by soil temperatures. And secondly denitrification losses are dependent on length of time soils are saturated. Taking a lesson from wheat experiences with topdressing and considering the recent PSNT survey of lower than expected soil nitrate levels to start with we may need to consider reapplying some N on sandier/ sandy loam and clay soil types on corn that received preplant N applications. The poorer the drainage the greater the need to consider reapplying some N .
Fields that received less than an inch of rain and do not have ponding are likely ok . Fields with more than an inch and ponding may need to be touched up again. I would suggest side dressing another 30 – 50 lbs of actual N with Agrotain plus on fields that received preplant N and heavy rains.