I've blogged about the great nutrient collapse previously, but that article focused on crop varieties and (mainly) CO2 levels. This article focuses on the soil:

Broccoli. One of the most nutritious vegetables on the planet.   But 70 years ago, it contained twice the calcium, on average, and more than five times the amount of Vitamin A. The same could be said for a lot of our fruits and vegetables.   Why? How?  The answers lie in the soil and how Americans farm it. Over the last two centuries, U.S. population growth and food production methods have stressed and degraded our dirt.   Our soil is not as alive as it once was, and experts say that's a problem.