The nitrogen and phosphorus cycles

Nitrogen gas (N2) makes up about 78% of the atmosphere, but most organisms cannot use it directly in that form. Nitrogen fixation (done by certain soil bacteria, some bacteria living in the root nodules of legume plants, lightning, and the industrial Haber-Bosch process) converts N2 into forms plants can use. From there: nitrification converts that nitrogen into nitrate, plants take it up through assimilation, decomposers release it back into the soil through ammonification when organisms die, and denitrifying bacteria eventually convert some of it back into N2 gas, completing the cycle.

The phosphorus cycle works differently in one crucial way: phosphorus has no significant gas phase, so it does not cycle through the atmosphere at all. Instead it moves slowly through rock weathering, soil, living organisms, water, and sediment -- a much slower, more geology-driven cycle than nitrogen's. Both nitrogen and phosphorus are essential plant nutrients and the main ingredients in crop fertilizer, which is exactly why both show up again later this year as the two nutrients most responsible for water pollution when they run off farmland in excess.