Planting by the calendar is too risky when there are simple tools to help make more accurate decisions about soil temperature. Checking regional soil temperatures can be as simple as checking a website.
- Current Soil Temperatures for Illinois: http://www.sws.uiuc.edu/warm/soiltemp.asp
- Past week soil temperatures: http://www.isws.illinois.edu/warm/soiltemp/archive.asp
- Compare these with Long-term Soil Temperature Averages:
http://www.isws.illinois.edu/atmos/statecli/Soil-Temperature/soil_temperature.htm
- Regional Soil Temperatures and Frost Depth: http://www.crh.noaa.gov/ncrfc/content/soilTemp/soilTemp_gd.php?pe=TV&depth=02
These websites give you the big picture of how the spring is warming up. To be more accurate, you should take soil temperatures of your individual fields. Soil temperatures depend on numerous factors including soil type, soil moisture, soil color and tillage practices. Soil temperatures will also change over the depth of the soil profile. The surface soil fluctuates the most, with daily fluctuations of 10-20 F, while temperature deeper in the soil remains more steady and take longer to change.
Knowing the soil temperature can be as simple as purchasing a refrigerator thermometer that has a probe. This low cost purchase can be a valuable tool in your toolbox. There are more sophisticated thermometers available as well that can download to your computer.
To determine the soil temperature, simply push the thermometer into the soil to the depth of planting. For transplants it is best to determine the soil temperature at 4 inches. If the soil is very dense, you can use a screwdriver to make a initial hole to the right depth so that the thermometer doesn't get bent when pushing it into the soil. It is best to do this in several locations throughout the field. Because soil temperatures are influenced by air temperatures and sunshine, take soil temperatures for several days and average the temperatures to determine the average soil temperature for the field. Soil temperatures tend to be coolest between 6 and 8 am in the morning and should be used as a guide as to when to plant or when to look for germinating weeds. In the heat of summer you can check for the maximum soil temperatures between 3 and 5 pm.
Knowing the soil temperature can help guide planting to guarantee good germination. Planting into cold soil can encourage diseases and even stunt later crop growth. For recommendations on when to plant crops in Illinois, check The Midwest Vegetable Production Guide. http://www.btny.purdue.edu/Pubs/ID/ID-56/
Soil temperature can also be an indicator of when to begin scouting for spring weeds. The Weed Emergence poster can help guide you on what to scout for based on the weed germination time.http://weeds.cropsci.illinois.edu/extension/Other/WeedEmergePoster.pdf