That article body doesn't support soil depletion as the cause. It asserts it and then cites a study that doesn't support it. Pretty similar to what you just did actually.
>Davis and his colleagues chalk up this declining nutritional content to the preponderance of agricultural practices designed to improve traits (size, growth rate, pest resistance) other than nutrition.
The article only cites a few studies that show the nutrient content of crops is decreasing. For example, the study you mentioned [1] was "Changes in USDA Food Composition Data for 43 Garden Crops, 1950 to 1999" and showed "apparent, statistically reliable declines" in 6 nutrients.
The part of the article that you've quoted is simply Davis' opinion, not the subject of that study. The author has a different opinion. I don't know who is right, which is why I said it's "not an uncommon view".
I don't know if that debate has been settled. The article doesn't cite any studies examining the cause of this nutrient depletion, and I didn't find any studies of the cause with a few Google Scholar searches for soil depletion and related terms. If you know of any research that answers the question I'd love to read it.
>Davis and his colleagues chalk up this declining nutritional content to the preponderance of agricultural practices designed to improve traits (size, growth rate, pest resistance) other than nutrition.