Frost tubes filled with methylene blue solution are commonly used tools for measuring soil frost and thaw depths in the field. To quantify the accuracy of this method, soil frost depths were measured in agricultural fields in the Tokachi district of the northernmost of Japan's main islands. The frost depths measured using frost tubes coincided with those determined from direct observations of the soil profile. The root mean square error (RMSE) between these two methods (i.e. frost tubes and direct observations) was +/-0.035 m, which is expected for frost tubes in this region. The soil temperature profile was characterized over four winters using thermocouples to estimate soil frost depths in an experimental field in central Tokachi. Frost depths were also measured using frost tubes during this period. A scatter diagram of the frost depths obtained using these two methods showed a concentration of points along a 1:1 line. The RMSE between these two methods was +/-0.028 m, suggesting that frost depth can be accurately estimated from the soil temperature profile. Unlike soil frost depth, soil thaw depths measured using frost tubes were consistently deeper than those determined through direct observation of the profile of soil hardness. This might be true because the frozen soil around the frost tube thawed earlier because of the greater insolation and heat conduction from atmosphere resulting from the decrease in snow cover as a result of snowmelt around the frost tube. However, because a scatter diagram of the thaw depths obtained using these two methods showed an approximately linear relation, the thaw depths measured using frost tubes can be calibrated using this relation. (C) 2011 Elsevier B.V. All rights reserved.