National Weather Service
The National Weather Service North Central River Forecast Center publishes maps for soil temperatures and frost depths around the Upper Midwest.
Here is the latest frost-depth map from the River Forecast Center:
UW Soil Science
The UW Soil Science Department also maintains up-to-date archives of climate and soil data from around the state. Two reports in particular may be of use to nutrient managers.
Automated Weather Observation Stations
UW Soil Science has maintained a pair of automated weather stations in Dane County (Arlington) and Waushara County (Hancock) since the mid-eighties. In addition to climate data, these stations record the temperature of the soil at various depths every half-hour throughout the day. Here is a convenient summary of the last seven daily average soil temperatures. For more information about the stations and the available data (including a query engine to retrieve historical data), see their home page.
Textual HYD Reports from the Co-Op Observer Network
The maps from the River Forecast Center use data from the National Weather Service's cooperative weather observation network with 700-plus observers around the state. These data are reported daily, and the NWS cautions that they have not yet been quality-controlled when first published. In the winter months, soil temperature and frost depth reports are included for a few locations around Wisconsin (scroll to the bottom of the report). You can access the latest report from the Soil Science archive, which is in a scrolling calendar format; click on the appropriate day in the calendar to see the report.