This paper focuses on the study of a mathematical model to measure the moisture content of agricultural soils. After the proposal and analysis of a measurement method based on the propagation of elastic waves in a granular unconsolidated medium, a mathematical relationship between the propagation velocity of an acoustic wave and the moisture content of the soil is obtained. The authors carefully study and verify the conditions that guarantee the applicability of the model; the analysis shows that a right choice of the frequency of the transmitted signal allows the applicability of the model to a wide range of agricultural soils.