The following information and quotes (from Dr William Albrecht, PhD), come from Back to the Basics of Human Health by Mary Frost:

Early nutritionists and Dr Albrecht were adamant that mineral deficient soil is one of the original sources of disease in the world today.  “Simply stated, food crops grown on depleted soil produce malnourished bodies, and disease preys on malnourished bodies.”
“When we see a symptom in the plant, it will always correlate to a poison or deficiency in the soil: when we see a disease in the human, it will relate to a poison or deficiency in the food.” 
“Soil is the basis of all life.  Dr. William Albrecht, PhD conducted studies in the 1950s that proved beyond a doubt that plants can appear healthy but have low quantities of nutrients.  He also proved that the health of a plant is it’s own protection against insects.  When a plant is healthy it has no need for pesticides whatsoever.  In the plant’s root system there are little off-shoot rootlets that have hair-like fungi (called mycorrhizae) growing on them.  Good soil is composed of 45% minerals and is full of microbial life, which contains millions of bacteria.  The bacteria’s primary job is to decompose anything that falls on the land and to break down mineral deposits into plant food.  The plant isn’t devoured by the bacteria because the mycorrhizae secrete antibiotics to protect the plant.  Nature gave fungi and bacteria an interesting relationship.  They are natural antagonists.  They keep each other in check through their competition…The plant, thus protected, is free to absorb the minerals that soil microbial life has released without fear of infection from soil-borne bacteria.  If we see a fungus growing on a plant, it is a self-produced fungus because there was something inferior about the quality of the plant.  Nature grows a fungus on an inferior plant, which then dies, decomposes, and begins again – until it gets it right.”