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The phosphorus cycle

The phosphorus cycle

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Where do animals get phosphorus from?

The phosphorus cycle

Life on Earth is dependent upon certain ingredients. One of them is phosphorus. Phosphorus moves between animals and earth, between plants and water. Plants absorb phosphorus from the soil, then animals eat plants, or each other. When plants wither, when animals pee or poo, or when things die, the phosphorus is released and the cycle starts again.

A phosphorus atom circulates in this way for a while on land, but will eventually get washed out to sea by the water runoff. Here it gets picked up by some organism, and can circulate in the ocean for a hundred thousand years, until - trapped in some shell - it sinks down to the ocean floor. Small shell fragments - together with clay and other tiny materials - slowly form thin layers in the deep: sediment. The many thin layers of sediment are gradually pressed together, so tightly that they transform into rock! The earth's crust changes with time; and after millions of years what was once ocean floor has become land.

Now the phosphorus is bound in rocks in the land again. But it doesn't stay there! With time the rocks are weathered, and broken down into tiny grains. These are geological processes, and they are very, very, slow: They take many millions of years. The weathering releases phosphorus in a form that can be absorbed by plants.

The plants build the phosphorus into their tissues, and the whole cycle continues! Phosphorus exists then, in three different places on the planet. In land ecosystems - that is in every living thing and in the ground. In water ecosystems - here too, in every living thing, and in the sediment layers on the ocean floor. And phosphorus is also in the bedrock.

Many of nature's other cycles - like the carbon or nitrogen cycles - have phases in the atmosphere. But phosphorus wants nothing to do with air! Phosphorus goes via the bedrock instead. This makes the phosphorus cycle more sensitive than the others. Which means the phosphorus cycle can more easily become unbalanced.

We humans have accomplished that with our modern agriculture. We mine phosphorus from the bedrock for use in fertilizers on our fields. The phosphorus is absorbed by crops. And when we harvest, the phosphorus level decreases faster in arable soil than in the soil under natural conditions - where plants wither down, and the phosphorus returns to the soil. In most modern agricultural methods, we add a lot of extra phosphorus, and we also completely remove the plants when we harvest.

So firstly, the phosphorous we add does not stay in the fields: and secondly, unnatural amounts of phosphorus are washed out into lakes and oceans. And phosphorus does the same thing in the water as it did in the fields: it functions as a plant nutrient: fertilizer! Too many nutrients in the in the water is not good at all. It leads to eutrophication. Eutrophication can cause lakes and other bodies of water to overgrow.

It can also cause explosive growth of algae - called algal bloom. Some algae are toxic, so we can't swim! When the balance of ecosystems becomes disrupted like this, there are not only consequences for nature, but also for us humans! We are dependent upon nature and what it provides - in the form of food and other goods. If we change ecosystems, we will no longer be able to enjoy the same benefits.

If we keep doing what we're doing now, quickly freeing phosphorous from the bedrock, millions of times faster than natural weathering, and causing it to gather in the water systems, we are disrupting the phosphorus cycle even more! Our arable soils will weaken, and harvests decline. At the same time lakes and oceans will become overgrown, and ecosystems devastated. With some changes in agricultural practices, and in the management of sewage and agricultural waste, much of the phosphorous that gets washed out could be recycled as fertilizer, and the balance of the phosphorous cycle could be reestablished, to some extent.