> "If all the earth's crust was converted into bio-mass, what would be the limiting element?"
I think selenium would run out long before phosphorus.
The crust contains selenium at 0.05-0.09 ppm, but most plants require around 0.1-1 ppm. It's also one of the hardest nutrients to remediate because you can't just dump a bunch of selenium on the surface when 5-10 ppm starts to become toxic to lots of other organisms.
I was supplementing with selenium for various reasons, and it seemed clear that it's one of the minerals that used to be supplied via our diet, but is quite deficient now, due to exhaustion in the soil.
Selenium, along with taurine, is one of the supplements which genuinely drove my physicians vehemently crazy, so I doubled up, and ensured that they were the focal point of my daily regimen.
Remember all those post WWII photos showing how fertilizing with phosphorous resulted in incredible crop yields?
Guess what element China (which has some of the world's largest reserves) has quit exporting?