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].<p>[0]: <a href="https://www.ecowitt.com/shop/goodsDetail/19" rel="nofollow">https://www.ecowitt.com/shop/goodsDetail/19</a><p>[1]: <a href="https://github.com/garbled1/homeassistant_ecowitt" rel="nofollow">https://github.com/garbled1/homeassistant_ecowitt</a>