http://www.umm.edu/altmed/articles/phosphorus-000319.htm
Humans don't usually suffer from phosphorus deficiencies and too much phosphorus is not good for you. However, all plants need phosphorus to grow. Without adequate supply of phosphorus growth of plants is very small.
soils.usda.gov/sqi/publications/files/prole.pdf
http://www.energybulletin.net/node/33164
Phosphorus is a limited resource like oil. As we have peak oil, we also have peak phosphorus. Peak phosphorus is estimated to happen soon.
http://www.energybulletin.net/node/33164
There are no substitutes for phosphorus in agriculture.
http://www.naturalnews.com/031516_agriculture_phosphorus.html
http://www.eurekalert.org/pub_releases/2011-02/uow-wpu021411.php
Availability of phosphorus defines the fundamental maximum life sustaining capacity of planet earth. In many ways phosphorus is more fundamental to human life than oil. Oil has substitutes, phosphorus does not have any.
http://phosphorusfutures.net/
http://www.greenpeace.org/international/en/news/Blogs/makingwaves/chemical-fertilisers-china/blog/44517/