[iancmceachern]

One of these might work for you:

https://m.fr.aliexpress.com/item/32689223035.html?html=stati...

https://www.amazon.fr/Corrosion-Resistant-Capacitive-Moistur...

If not I run www.openagriculturesupply.com amd would love to help find something that does.

[greggsy]

I have a couple of these hooked into an rpi zero to monitor soil moisture under my house. They work, but wireless would be much better.

[iancmceachern]

I should have linked these for you in the first place. They have a whole ecosystem of sensors that are all wireless.

https://www.seeedstudio.com/SenseCAP-S2105-LoRaWAN-Soil-Temp...