http://www.microsoil.com/liebigs_law_of_the_minimum.htm Justus Liebig 1803-1873

The total yield or biomass of any organism will be determined by the nutrient present in the lowest (minimum) concentration in relation to the requirements of that organism.

"The limiting nutrient is the one in least abundance."

    In terrestrial ecosystems, Nitrogen is usually limiting (least abundant relative to requirements).
    In aquataic ecosystems, Phosphorous is usually limiting.

If the limiting nutrient is supplied in abundance, then Liebig's law states that the next least-abundant nutrient will become limiting, and if the needs for that nutrient are then met, then the next least-abundant nutrient will become limiting.
For example, if Phosphorous is added to a lake, then the rate of growth will increase until it is limited by Nitrogen. Then, if sufficient Nitrogen is added, another nutrient will become limiting.


In the deep ocean, Iron may be the limiting nutrient. [Nature]

Links
http://www.microsoil.com/liebigs_law_of_the_minimum.htm
http://www.yorku.ca/faculty/academic/cblanche/nats1760/n17liebig.html
http://www.aglukon.com/News_-_Info/Liebig_s_law/liebig_s_law.html
http://www.ravensdown.co.nz/SPS/html/law_of_minimum_k1.html