It can not be done using ground textures.
Well, perhaps I should take that back a bit. Provided that your field edge runs either north-south or east-west, you could take the two ground textures in question and create a third ground texture in photoshop with as sharp an edge as you need between the two textures, and then use that to create the field edge line in surveyor. A lot of trouble to go to though. You might just be better to disguise the fuzzy line with shrubbery or stone walls.