From the source:
"The high rate of magnesium deficiency now postulated [5,6,7,8] can be attributed in part to a steady decline in general magnesium content in cultivated fruits and vegetables, a reflection of the observed depletion of magnesium in soil over the past 100 years [11,12,13]. A report to Congress was already sounding the alarm as far back as the 1930s, pointing out the paucity of magnesium, and other minerals, in certain produce [14]."
"Three kinds of evidence point toward declines of some nutrients in fruits and vegetables available in the United States and the United Kingdom: 1) early studies of fertilization found inverse relationships between crop yield and mineral concentrations—the widely cited “dilution effect”; 2) three recent studies of historical food composition data found apparent median declines of 5% to 40% or more in some minerals in groups of vegetables and perhaps fruits; one study also evaluated vitamins and protein with similar results; and 3) recent side-by-side plantings of low- and high-yield cultivars of broccoli and grains found consistently negative correlations between yield and concentrations of minerals and protein, a newly recognized genetic dilution effect. "
The easiest of those references to link to is https://journals.ashs.org/hortsci/view/journals/hortsci/44/1... :
"Three kinds of evidence point toward declines of some nutrients in fruits and vegetables available in the United States and the United Kingdom: 1) early studies of fertilization found inverse relationships between crop yield and mineral concentrations—the widely cited “dilution effect”; 2) three recent studies of historical food composition data found apparent median declines of 5% to 40% or more in some minerals in groups of vegetables and perhaps fruits; one study also evaluated vitamins and protein with similar results; and 3) recent side-by-side plantings of low- and high-yield cultivars of broccoli and grains found consistently negative correlations between yield and concentrations of minerals and protein, a newly recognized genetic dilution effect. "