Along with the synthetic fertilizer comes other interests in our industrialized food system: Plant varieties are bred and grown for their shelf life, not their nutritional density or taste. Here in the US our food comes half way across the country (or all the way if it was grown in California and you live on the east coast) to get to a supermarket so it needs to stay good for a long time. And that's why we end up with cheap, flavorless iceberg lettuce that has no real nutrition in it. The greens worth eating are expensive.