Soil sensors that output digital signals are already supported and the statuses are pushed to MQTT in firmware 2.1.9(7). If you are referring to analog soil sensor — the firmware does NOT support analog sensor because the GPIO pins used to read sensor status is not an analog pin. You would need to use either an i2c type of soil sensor, or have an i2c type of analog to digital converter in order to interface it with opensprinkler.