This paper presents a system to estimate soil moisture through the reading of standard Ultra High Frequency (UHF) passive RFID tags that can be buried in the soil, allowing wireless moisture measurement without the need of using batteries in the field for long periods. In the proposed system, one or more passive EPC/GEN2 tags acts as sensors buried in the soil. The system dispenses external cables and antennas and may be composed of a single RFID tag buried in a specific soil depth or by several RFID tags buried at different depths. An antenna coupled to a RFID reader can be pointed to the place of installation of these tags, and by measuring the received signal strength indicator (RSSI) and other variables, direct estimation of the water content can be done. In addition to its simplified installation procedure, the system allows manual and automatic robotic reading through irrigation systems or other systems for irrigation scheduling.