It started almost 10 years ago in Kentucky with a tractor equipped with GPS, soil sensors and a radio connection to the University of Kentucky. There a computer (and an application specifically developed) took care of directing the tractor over the field to spread the right amount of fertiliser in the appropriate area in the field.
The robot, see it at work in the photo, uses a laser system to find its way among the stalks, avoiding them and spraying fertiliser right at the point the stalk emerges from the soil. By using GPS the robot knows where the cornfield ends and moves on to the next row of stalks.
There are alternative ways of spraying fertiliser, from the old fashion one, done by a farmer walking through the rows of stalk to irrigation (but you are wasting quite a bit of fertiliser and it gets unevenly spread) to using drones (but you are getting fertiliser on the leaves rather than delivering it to the roots).
Using a tractor in corn field during the stalks growth is out of the question since the rows area packed.
Hence the interest for this robot. I am not an expert but a business model based on pay per use seems, at least from the point of view of the farmer, a very good deal.
It is interesting to see robots entering in many areas and I bet will see them much more in the future, to the point that we will no longer notice them!