A novel method and system is disclosed for using a commodity RFID system for automatically measuring levels of soil moisture in planting containers. A large number of planting containers are used to grow pots in soil in a greenhouse. An RFID reader interrogates passive RFID tags affixed to the planting containers. The RFID reader can be attached to a robotic arm configured to move above multiple rows and columns of containers. Signal features of specific passive RFID tags affixed to specific ones of the containers are automatically monitored, including a minimum response threshold of RFID reader transmission power to activate the passive RFID tag (“MRT”), based on the wireless interrogation of specific tags by the reader. Soil moisture levels of specific containers are then automatically determined based on the signal features of the attached tags, and effects of soilure moisture on electromagnetic fields of antennas of tags.