Thanks. The schematic is simple and good but I wanted a rugged waterproof solution so would be choosing EC Sensors from below
https://www.ebay.com/itm/254277104876
I want it to last a couple of years so above two seem perfect for me.
There is a whole KIT based EC Sensor as well ((Sensor & Kit) Soil Temp + Humidity + Electrical Conductivity EC Sensor Arduino | eBay) but I guess would be opting for above 2 to keep things small.