Phosphorous

Phosphorous is one of the big 3 (4 if you count water) that the plant uses in fairly large amounts. Below article has a good explanation.

http://www.rainbowplantfood.com/agronomics/efu/phosphorus.pdf

Phosphorous will be one of the first nutrients to come under regulation. States that have a lot of chickens and hogs have excess phosphorous. I my opinion we need to learn how to concentrate these nutrients so they can be shipped to states that have short phosphorous. Phosphorous is a problem in fairly low amounts because it can cause algae blooms and contribute to eutrophic conditions.