Rapitest Soil Thermometer Gives You A Better Rate Of Success With Your Planting. Fahrenheit and Celsius readings. Durable aluminum casing. Most seeds require a warm soil to a depth of at least 3" to encourage germination. Transplanted seedlings can go into shock if planted in soil that is too cold. Don't trust the calendar to know when it is safe to plant. How warm the soil is as important as how warm the air.