Having started this before getting a job I've had no time to actually do much with it, besides stare at the pile of parts on my bench before going to bed. I plan to try some testing this week, get the arduino to send measurements to a web page. Settle on a power storage medium etc.
A mesh sensor network relies on nodes that are always on to be able to receive the message from battery powered nodes and transmit it further till it reaches the central node. Here most of your nodes are battery powered, so how do you want to get the message across?
I thing this is a good application for LoRa radio transcieveres. Of course, it depends on the size of the field, but since you are talking about farmers...