A Bluetooth Low Energy  soil moisture sensor
Ecowitt make a range of low-cost sensors which all transmit to a central hub over 433/915MHz.
They have around 400 feet of range, and it is easy to get the base station to constantly ping a URL with current sensor values.
The soil moisture sensor is available on Amazon or directly from Ecowitt[0].
There is also a HomeAssistant integration[1].[0]: https://www.ecowitt.com/shop/goodsDetail/19[1]: https://github.com/garbled1/homeassistant_ecowitt