How big do you mean when you say 'huge fields'? Attending to single plants individually, this would be far to slow to work over a huge area.

'In contrast to modern agriculture, where things are done with huge monster tractors' - You might be interested in what http://www.bluerivert.com/ are doing. They are essentially attending to every plant individually in smaller cropping areas.

Once you actually get to a large area, I.e 1000ha fields, it doesn't make sense to attend to every plant individually throughout the season. We do however have technology to plant each seed down to mm precision, and apply nutrients based on zones created within fields.

Also, does it make sense to duplicate all the infrastructure required for one of these, over every single paddock on a farm, when we could have a single, essentially autonomous, machine crop and manage the entire area itself?