Passive RFID tags have diverse low-cost sensing applications, with sensors either connected to the tag or the tag antenna acting as a sensor, allowing close-range meas-urements. However, an emerging approach utilizes wave propagation for remote sensing without physical contact. This study investigates how wave propagation across dense grass alters the phase difference of arrival, enabling the estimation of vegetation volume ratio. A tag positioned 19 meters from the reader is moved vertically into a dense grass medium, then without grass. By analyzing the change in phase measurements caused by the wave slowness in the medium, the grass volume ratio can be estimated. The RFID-based estimate of 0.75% aligns with the 0.53% obtained from weight measurements. Looking forward, this method might help to monitor crops in agriculture, but also to sense other complex medium.