An earth heat pump was set in operation to abstract heat from soil assumed to be in thermal equilibrium. Temperature‐time graphs of the antifreeze and of the soil at distances of six, twelve, and eighteen inches below the center of the antifreeze line were plotted from experimental data. From these graphs data are obtained from which the thermal diffusivity of the soil is calculated. The soil was analyzed and found to fall in the general Casagrande classification of lean clay (CL) with low plasticity.