By Elwynn Taylor and John Sawyer, Department of Agronomy
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. Experience has taught us that the conversion to nitrate is greatly reduced at soil temperatures below 50 F, though not totally avoided.
Fertility in the right place and at the right time is highly important. When the risk of loss is high both the “E” (ecosystem) and the "$" (pocketbook) should be considered.
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 (2003, 2004, 2006). Do not anticipate that soil temperature will remain below 50 F until after mid-November.
Soil temperature can vary several degrees depending on the slope of a field and soil conditions. The general temperature for each Iowa county 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.
Elwynn Taylor is a professor of agronomy with research and extension responsibilities in climatology. John Sawyer is a professor of agronomy with research and extension responsibilities in soil fertility and nutrient management.