Much like us, plants are what they eat. If you feed a plant poor nutrition, it produces poor nutritional products. And obviously, the better nutrition we feed the plant, the healthier and more nutritional the products (fruits and vegetables) will be for us.
Today our farmlands are so over-farmed with synthetic fertilizers and pesticides that our soil is stripped of almost all of its microlife and trace elements. Without these trace elements and microbes our farmlands are producing crops with little or no nutritional value at all.