The warm, spring-like temperatures that we have been experiencing lately have been making it hard for people to remember it is still February! The warm sunshine and higher than normal temperatures feel like spring is already here and it’s time to start planting your vegetable garden. One thing that is commonly overlooked when it comes to spring planning is soil temperature. Since the air temperature is so warm, it is easy to forget that the soil underneath our feet is still in the 30’s right now. I found a short column about soil temperatures and how they can affect the crops you plant from K-State Research and Extension’s Horticulture department to share about this often overlooked item.
One of the most neglected tools for vegetable gardeners is a soil thermometer. Soil temperature is a much better measure of when to plant than air temperature or the calendar. Planting when soil is too cool can cause seeds to rot and transplants to refuse to grow.
A number of vegetables can germinate and grow at cool temperatures. For example, peas will germinate and grow well at a soil temperature of 40 F. Though lettuce, parsnips, and spinach can sprout at a soil temperature of 35 F, they prefer at least 45 F for best germination and growth. Radishes also do well at a soil temperature of 45 F. Warm-season crops such as tomatoes, sweet corn and beans prefer at least 55 F for germination (or transplanting), but others such as peppers, cucumbers, melons and sweet potatoes need it even warmer, about 60 F.
Taking soil temperature accurately is a bit of a science. First, use a metal soil thermometer, which is sold in many garden and hardware stores. Take temperature 2.5 inches deep at about 10 to 11 a.m. Temperature variations throughout the day and night affect soil temperature, with lowest readings after dawn and warmest around mid-afternoon. The late-morning reading gives a good average temperature. If taking the soil temperature at this time is not practical, take a reading before you leave for work and a second when you return home and use the average. Also be sure to get a consistent reading for four to five days in a row before planting, and make sure a cold snap is not predicted.
An excellent guide sheet on this subject is published by the Alabama Cooperative Extension System and is titled “Soil Temperature
Conditions for Vegetable Seed Germination.” It can be found at http://www.aces.edu/pubs/docs/A/ANR-1061/ANR-1061.pdf.  
Alicia Boor is the Agriculture and Natural Resources agent for Barton County K-state Research and Extension. One can contact her by email at aboor@ksu.edu or calling 620-793-1910