When it comes to applying fall anhydrous ammonia or manure with a high ammonium N content (like liquid swine manure), we recommend that soil temperatures at a 4-inch depth are below 50° F and continue to trend cooler to help minimize nitrogen loss prior to the next growing season. While soil temperatures at the 4-inch depth dropped down into the low 40s and even upper 30s across the state of Iowa by November 2, warmer air temperatures this week will raise soil temperatures, possibly above the 50° F threshold.
You can track soil temperatures at the 4-inch depth using the Nitrogen and Phosphorus Knowledge web page to view daily, previous day, and 3-day history of average soil temperatures in every Iowa county.
Historically, the soil temperatures at the 4-inch depth cool below 50 degrees in the northern third of the state the first week of November. In central and southern Iowa, soil temperatures cool below 50 degrees during the second week and third weeks of November.