This easy-to-read stainless steel Soil Thermometer has a 15cm (6") probe suitable for seed starting and transplanting. It is scaled from 30 to 90 degrees Fahrenheit and features a corrosion resistant aluminum body. Guidelines for germination and transplant temperature are included. The top dial has a diameter of 6cm (2.5"). For some seeds (e.g. mustard seeds), soil temperature does not have much impact one way or another on the speed of germination. But for many other kinds of seeds, germination only occurs at a certain soil temperature, and germination will be faster and more uniform if you use a Seedling Warmer. For untreated seeds like corn, soil temperature is absolutely critical - if it is too cold, untreated corn seeds may simply rot. The Soil Thermometer is a very useful tool when it comes to planting seeds, applying nematodes, and transplanting seedlings.