In order to monitor and understand the temporal variations of sound speed due to the changes of soil physical properties under natural conditions, a long‐term field soil survey has been conducted. Measurements of sound speed, soil temperature, soil moisture content, and water potential were performed continuously, along with the measurements of surface temperature and precipitation over a period of 2 years. The effects of water potential, moisture content, soil temperature, and depth on sound speed were assessed. Analysis of the data shows that there is a power law relationship between the sound speed and water potential. It is also found that the water potential is the dominant influence on the sound speed, whereas the moisture content and temperature have relatively minor impacts. Brutsaert and Luthin’s theory was employed to calculate the sound speed as a function of the effective stress. The theoretical predictions were compared with the experimental data and they are in good agreement. The study suggests that sound speed measurement could be used as a new and effective tool for water potential measurement.