With RS-485 you can have up to 32 nodes on a single cable. I am looking at using Arduino micro controllers at the nodes. you could have say, 4 sensors at each node at different soil depths. That would give you up to 128 individually addressable sensors per cable.
I purchased some MAX487CPA transceiver chips from Mouser for ~$2.00 ea. There is already software available on the internet to make it work. All I need is some spare time…
Dan