Here's another helpful article on RS-485.Also, even if buried in a trench, you will need to protect against lightning by opto-isolating the nodes. See the ISOLATED LINK section of the Axelson article. You can use direct burial CAT5 cable to avoid the need for conduit.

If you can provide more detail about the number of nodes and the nature of the messages we might be able to suggest less costly methods. It might be less costly to use WiFi which can be done for under $60 per node. It's inherently isolated and you can use serial communications.