> do not think that the soils are necessarily as mineral depleted as some may think
I agree, the soils are not depleted. In fact minerals and other nutrients are constantly being replaced through flooding, irrigation, dust, micrometeorites, animal wastes, plowing under of crops, nitrogen fixing cover crops, etc.
Any study can be made to find anything that someone wants. If I wanted to prove soil is nutrient deficient all I would have to do is take a sand sample from the Sahara desert and test it. The fact is though that soil nutrients are going to vary from place to place, even is a close proximity. For example, where I live our soil is highly alkaline and full of caliche (calcium/magnesium carbonate). If I go about 20 miles from here the area is loaded with gypsum (calcium sulfate), which we use to break up caliche due to the acidity of the gypsum.
Studies from back in the "dust bowl" days would also show poor soils because of the loss of nutrient top soils. Again though this was not the entire country, and the top soils have been replaced.
As for the lower nutritional values of the produce we get today, this has nothing to do with the soil. The plants are being picked unripe to cut production costs, and to make shipping and shelf life longer. Because they are being picked so early the plants are not being given the time to develop all the phytonutrients that they would have if left to ripen.
If the soils were so mineral deficient the plants would be so unhealthy that they would never live long enough to get a crop.