lets say the base tile tield on fertile ground is 2/2. Then it checks the hidden numbers (which would be decimal points) to figure out if there is enough essence to add one point.
world resources add more to tiles: these are approximations off the top of my head.
shards bump essence up to 2.
rivers add 1 grain
clay adds 1 materials
iron adds 2
forest adds 1
grain, orchard, apiary add 2
not sure about dragon locations, i think they give 3 essence
crystal adds 1 essence
the location with the powerful ogre (i think ogre shaman - has about 40 attack) gives 3 essence when you kill him
so if you are on a spot next to a grain, iron mine, crystal, and a shard you will get +2 grain, +3 materials, +3 essence for a total of 4/5/3 tile yield.
That is pretty much the basics, i might be off a number or two on the grains.