Soil temperatures are updated daily and posted on the Iowa State University Extension Soil Temperatures for Agriculture Web site. This is valuable information for farmers and other applicators who want to apply nitrogen in the fall.
Waiting until soil temperatures are cool enough to apply N can help reduce the risk of nitrogen loss. The following guidelines are from ISU Extension climatologist Elwynn Taylor and ISU soil fertility specialist John Sawyer.
"Soil temperature is the best indicator we have to reduce the risk of nitrogen loss when fall application of anhydrous ammonia, or manure with high ammonia content, is deemed expedient," says Sawyer. "Experience has taught us that the conversion to nitrate is greatly reduced at soil temperatures below 50 F, although the conversion is not totally avoided."
Keep tabs on soil temperatures
Fertility in the right place and at the right time is highly important, he points out. When the risk of nitrogen loss is high both the "E" (ecosystem) and the "$" (pocketbook) should be considered.
For example, soil temperature in Iowa's counties was in the mid- to upper-50s on October 9, 2008. Although soils have occasionally cooled to below 50 F by the third week of October, as in 2002, it is usually the latter half of November (such as it was in 2003, 2004 and 2006). "Do not anticipate that soil temperature will remain below 50 F until after mid-November," says Taylor.
Soil temperature can vary several degrees depending on the slope of a field and soil conditions. The general temperature for each county in Iowa is mapped and linked to the Extension Soil Temperatures for Agriculture Web site.
The county temperature maps are updated daily. Visitors to this site can get an idea of temperature direction trends from maps of the previous two days.