The Phosphorus Cycle for Middle School Science

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The Phosphorus Cycle describes how phosphorus moves through the environment. Unlike carbon and nitrogen, phosphorus doesn't exist as a gas in the atmosphere. Instead, it is found in rocks and water.

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The Phosphorus Cycle

The phosphorus cycle describes how phosphorus moves through the environment. Unlike carbon and nitrogen, phosphorous does not appear as a gas on Earth because it does not combine with oxygen to form a gas like carbon dioxide, and its boiling point is over 500° Fahrenheit.

Instead of floating in the atmosphere, phosphorous spends most of its time on Earth trapped in sedimentary rocks. As the rocks are weathered by wind, water, plants, and animals, they release phosphorus into the soil and water. Plants absorb phosphorus through their roots. Phosphorus is a limiting factor for plants because they need it to convert sunlight into food. It is also in their DNA, cell membranes, and ATP.

When animals eat plants, they get the phosphorus to use in their own DNA, cell membranes, and ATP. Vertebrates also use phosphorus in their bones. Teeth and bones make up about 80% of the phosphorus in the human body.

Animals release phosphorus back into the environment when they excrete waste or die. Some of the phosphorus is taken up by detritivores, and the rest of it remains in the soil.

Surface runoff carries phosphorus from the soil to the water. Plants in the water absorb the phosphorus, and animals get phosphorus by eating the plants. Phosphorus not absorbed by plants falls to the ground and joins the layers of sediments. Over time, these layers become sedimentary rock.

Phosphorus can escape the sedimentary rock through weathering, but if the rock is pushed into the mantle at a subduction zone, it will melt. Phosphorous hangs out in the mantle until it is released as a part of the ash during a volcanic eruption. The phosphorus stays in the atmosphere as tiny dust particles until it either falls to the ground or combines with water to form acid rain.

Because phosphorus is a limiting factor for plants, farmers add it to their crops using fertilizer. When it rains, phosphorus in the soil from the fertilizer is carried into nearby rivers and streams. Extra phosphorus in the water leads to eutrophication, so animals can't survive in the area. When oxygen is depleted, dead zones can form in lakes and oceans. Runoff from farms in the Mississippi River Basin has created a dead zone in the Gulf of Mexico. There are over 400 dead zones worldwide.

Phosphorus cycles through the environment naturally. When we add phosphorus to the soil, we change the phosphorus cycle. Scientists and farmers are working to find ways to get nutrients to crops without damaging other parts of the environment.