How closely can I plant my vegetables?

I have raised beds filled with 12 - 18" of 50% leaf compost and 50% good screened topsoil. Every year I add composted manure or Bumper Crop. The Ph, nitrogen, etc are all excellent. Everything seems to grow well over the years. The typical recommendations involve rows with 18 - 36" separations.That doesn't seem to make sense in a raised bed with quality soil.

I'm specifically looking to hear experiences with corn, onions, eggplant, and cucumbers.

I'd like to try corn spaced 1 plant per square foot. And onions 4" apart in rows 4" apart.

Any thoughts? Thanks

Scott