Soil thermal properties are critical to many processes. For example, germination, crop growth, and nitrification depend on soil temperature, which is controled by thermal properties. Thermal properties are usually measured in situ using specially designed probes that enclose a heat source and a thermometer. Here we proposed and validated an alternative procedure to estimate soil thermal diffusivity. We measured changes in temperature at the center of a soil core subjected to a constant thermal boundary condition in a constant temperature water bath. This procedure formerly required a numerical analysis because no analytical solution was known. We introduced an analytical solution derived by Carslaw and Jaeger (1959). Estimates of thermal diffusivity of an Andisol at various moisture contents, obtained using the new procedure, compared favorably to results derived with the conventional probe methods. The new procedure constituted a viable alternative.