Soil and water are two fundamental resources of agriculture. The whole system is seldom in a state of equilibrium. In nature, the properties of soils undergo ceaseless changes due to the processes such as temperature fluctuation, rainfall precipitation, infiltration, drainage, water redistribution, evaporation, creep, and compaction. No any laboratory test, no matter how well designed, can simulate such complicated real world conditions. It is valuable for soil scientists to monitor the evolution of soil properties in natural conditions. To do so, an acoustic technique is invoked for a long-term field soil survey by sound speed measurement. In this study, an acoustic probe with five acoustic transducers is inserted into the ground with minimum disturbance of soils. At different depths, sound speed, temperature, moisture, soil suction, and rain precipitation are recorded continuously. It is found that temperature has a negligible effect on the sound speed, moisture plays a minor role, and soil suction is the predominant factor governing the sound speed of the shallow field soil. Terzaghi's principle of effective stress is brought to explain the observations.