An integrated value of soil moisture can be determine dby measuring the attenuation of vertically-polarized surface radio waves taht are propagated over the ground between a transmitting and receiving atenna. Soil moisture values in the root-zone region were measured over longitudinal distances typically ranging from 50 feet to 600 feet with good results. Integrated soil moisture measurements over greater distances are also possible. Received field strength of propaged radio surface waves closely matches theoretical calculations. The measurement is easily made and does not disturb the soil. Dense, green vegetation, such as alfalfa or corn, causes errors in measurement accuracy. Less dense vegetation, such as range land, does not seriously affect measurement accuracy. The described equipment is portable and can be used by an unskilled operator.
Chadwick, Duane G., "Integrated Measurement of Soil Moisture by Use of Radio Waves" (1973). Reports. Paper 571.