This happened due to extermination of soil life through modern farming practices of heavy tilling, fertilizing and monoculture crops. Dead soil is more susceptible to erosion and requires ever increasing amounts of synthetic fertilizer to grow anything. More soil has been lost from the US than the amount of food that has been produced. Soil fertility can be returned to historical levels by changing farming practices…
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6104417/
Essentially there's around the same mineral content it's just the vegetables are also on average larger in size and mass - yet yield the same mineral content.