I think those values are calculated automatically based on the surrounding terrain. The problem is the editor doesn't update the display promptly (or ever, sometimes), so it's not terribly obvious.
From what I've been able to determine, if you place flat land with either the kingdom or fallen environment (so it's either green grass or that purple stuff), and it's a big enough area, you get 4 wheat/2 prod. Add a forest, and the edges get 3/3 and a little further into the forest gets 2/4.
Crystal deposits and shards give essence in the 8 tiles around them (which stacks, if they're close enough). I've never seen this show up in the editor though, only by placing them and starting a game.
The production resources like clay pits seem to give a +1 prod bonus to the surrounding 8 tiles.
I don't think there's any way to set this manually, even if you edit the map by hand (didn't see anything in the XML that looked promising).
