I would not assume the land owner knew the stump was there just because it had disk marks on it.   Cabs are quiet these days, equipment is large, and not everyone running said equipment is always aware of what they are doing.  I've seen tillage equipment pull up huge rocks, take out fences, and even an electric line pole all without the operater having a clue about it.  It's the sort of thing I just have a hard time believing even when I see it with my own two eyes, but it does happen.

I'd tend to consider the person operating the equipment to be responsible for avioding whatever there is to hit.  I never work a field and with the assumption that even a part of the field is free of obstructions.  We have rocks here, so staying alert becomes second nature to anyone who has spent much time in the fields around here.  Perhaps it's different where you are.  I can't imagine working a field and not feeilng responsible for watching for things like that.  It sure would be nice to sit back in the seat and relax once in a while.  I'd probably fall asleep if given the chance.:)