In the cycling of carbon and nitrogen, one step in the cycle is converting the atmospheric form to a usable form. But not all elements are present in the atmosphere at a significant level. The third element we'll look at is phosphorus, which has no significant atmospheric form. This makes the phosphorus cycle simpler than the carbon and nitrogen cycles.

Phosphorus is an important building block of DNA, as well as a component of phospholipids in cell membranes, energy molecules like ATP, and even our bones. There is only one important inorganic form of phosphorus, and that's phosphate.

Plants absorb phosphate from the soil and water, and use it to synthesize organic molecules, which are passed on to consumers. Phosphate is added back to the soil by the decomposition of dead material and feces. Phosphates are also added to soil and water through the weathering of rocks. Phosphates generally bind to soil particles and cycle locally.

But some phosphates are carried away by groundwater and runoff, and are eventually deposited in lakes, rivers, and oceans. Phosphates in the ocean settle to the ocean floor and become sediment. For the phosphates to return to the cycle, a geological upheaval must bring the sediment to the surface.

In the same way that fertilizer adds excess nitrogen to lakes and rivers, it also adds phosphates to the water. Phosphates are often the limiting nutrient in an ecosystem. Even if excess nitrates are present, algae usually won't grow unless phosphates are added. Therefore, it's extremely important that we make sure phosphates aren't released into the water. In order to keep our ecosystems clean and safe, many states have banned detergents containing phosphates.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education