Three different kinds of evidence all indicate the same thing: the nutrient value of UK and US fruits and vegetables has declined over the last 50-100 years. This is particularly worrying:
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.
With fertilizers, clever pesticides, and genetic modification, farmers can grow more crops per acre of land but it's more difficult for people to eat twice as much food to get the same amount of nutrients. (via the meaningfulness of little things)