What if ices and salts and such checked if they were walled by firing 100 invisible arrows in all directions from it's epicenter for 200 tiles in all directions, and if 69%+ of the arrows interact with a player object, then all the walls those arrows hit will then take a decay 50% decay tick? This could happen once every 8 real life hours.
Your counter argument might be, it would be abuseable because people would use this as a siege mechanic, and I would disagree, people aren't totally stupid and will just not build near those special resources for their permanent bases. Sure this might inconvenience mega villages, since it would be more difficult to find spots without salts etc, but in my opinion fuck them. Mega villages will end up ruling the game one way or another and don't need special consideration.
The exploit is if people build a wall that is 201 tiles around a resource to avoid the arrows then at least they have to vclaim all the walls and deal with increased upkeep, otherwise they will be repairing those walls all day everyday.
I wouldnt increase the arrows range more than 200 tiles, because it's still good to leave the walling option technically on the table for the peoples.