Is It Too Early To Apply Fall Nitrogen?

Jim Camberato Agronomy Department, Purdue University,West Lafayette,IN

Rain has been plentiful across most of northern Indiana this entire growing season. Soils will be wetter than usual after harvest. Fall and winter rainfall will saturate soils earlier; potentially increasing the loss of fall-applied nitrogen (N) via tile drainage and denitrification to the air.

The rule of thumb for fall anhydrous ammonia (AA) is to delay application until average soil temperatures at the 4-inch depth fall below 50 °F and soils are becoming consistently colder. Following this advice may be more important this year given the high initial soil moisture levels. This same guideline applies to swine manure which is primarily ammonium nitrogen (NH4+).

Low soil temperature reduces the bacterial conversion of NH4+ to nitrate nitrogen (NO3- ). Slowing this reaction is critical to the efficient use of fall-applied N because NH4+ is retained in the soil whereas NO3- is easily lost from the soil by leaching and denitrification. The longer N remains in the NH4+ form in the fall, the lower the potential for N loss in the early spring when warmer soil temperatures and excess soil moisture invariably occur.

The first week of October is too early to apply N!

Soil temperatures typically do not cool to a consistent 50 °F or less until late October or early November, and sometimes even later. For example in Tippecanoe County from 2000 through 2010, the earliest consecutive 7 days of average soil temperature at or below 50 °F began Oct. 25 (see table). In 6 of 11 years, soil temperatures dropped consistently below 50 °F during the first 15 days of November. In 3 of 11 years, suitable temperatures for fall N application did not occur until Nov. 22 and in 1 year soils were above 50 °F until the 7 days beginning Dec. 5.

Year                   1st day of 1st 7 days with avg.       

                           4”soil T at or below 50°F

2000 Nov.            6

2001 Dec.             5

2002 Oct.             25

2003 Nov.            22

2004 Nov.            23

2005 Nov.            15

2006 Nov.            10

2007 Nov.            12

2008 Nov.            6

2009 Nov.            23

2010 Nov.            13

Thanks  to Ken Scheeringa for   

Compiling the weather data.

If AA or swine manure will be fall-applied, monitor soil temperature and the weather forecast to determine when soil temperatures at 4 inches are likely to fall below 50 °F. You should also consider adding a nitrification inhibitor to these N sources as an additional precaution to delay the conversion of NH4+ to NO3- .

Source : purdue.edu