To answer the original question...

Most environments have one 'rate limiting' nutrient- that is, the lack
of one nutrient constrains growth much more so than others. In marine
environments, it is often iron, in freshwater lakes, it is often phosphorus.

For example, in the case of your body, it's iron, which your body goes
to great lengths to tie up and make unavailable and pathogens go to
great lengths to get at. (Often by blowing up red blood cells and
grabbing the iron in the hemoglobin)