Some of the design challenges of a soil sensor include how to power it over long time, and how to transmit the data out. For good and reliable sensing, it’s often necessary to bury the sensor underground, but that makes it harder to power over a long time and have a reliable wireless transmission method. Many years ago I took a look at some of the available options:
https://rayshobby.net/wordpress/reverse-engineer-a-cheap-wireless-soil-moisture-sensor/
but none of them seems a great option. I am not sure what’s the state of the art. I see Vegetronix mentioned quite often — the VH400 sensor is a wired sensor, to go wireless you need a WiFi hub, powered by 9V battery. I don’t know if this is a long-term solution or not — it seems cumbersome to have to replace the battery once every few months. So honestly I don’t know which way to proceed.