Explainer: The fertilizing power of N and P

How two elements get out of the air and soil, and into all of life on Earth

These green bubbles are an algal bloom caused by too much fertilizer in the water. When the algae die, the organisms that break them down can suck up much of the oxygen in the water. This can potentially harm other aquatic life.

F. lamiot/Wikimedia Commons (CC BY-SA 2.5)

Science fiction often describes Earth as being home to carbon-based life forms. That may be true, but calling them nitrogen- or phosphorus-based life forms might be just as accurate. Like carbon, both nitrogen and phosphorus are necessary to all living things. Too little of either, for instance, and plants or animals won’t grow. Get too much and ecosystems can suffer — big time.

Nitrogen atoms are required ingredients for the amino-acid molecules that make up the proteins in every cell on Earth — plants, animals and microbes alike. Nitrogen atoms are building blocks of our DNA. They even turn up in the chlorophyll that allows plants to turn the sun’s energy into food.

Phosphorus atoms wear several cellular hats as well. They are in our DNA. And they make up the critical part of the molecule adenosine triphosphate (Ah-DEN-oh-seen Try-FOS-fate) — or ATP. This tiny but mighty chemical has three groups of atoms called phosphate groups. Each phosphate group is made of five atoms: one phosphorus and four oxygens. ATP serves as a cell’s rechargeable battery. Hacking one phosphate group off releases the energy in its chemical bonds. Picking up a new phosphate group allows it to regain (and store) the lost energy again.

Without nitrogen and phosphorus, there would be no DNA, no energy to power our cells, no proteins, no plants and no people. Indeed, there would be no life on Earth.

Fortunately, Earth’s inhabitants are surrounded by both elements. Nitrogen makes up 80 percent of the gas in of our atmosphere. Phosphorus is part of many of the minerals in soil. But most atoms of N and P are not present in forms that plants (or animals) can use.

When it comes to nitrogen, luckily, there are microbes to come to the rescue. They perform a service called nitrogen fixation. In this process, they convert nitrogen in our atmosphere (two nitrogen atoms bound together) into a form that other organisms can use (one nitrogen with three hydrogen atoms bound to it). Plants also can get nitrogen from other organisms that have died and decomposed.

Phosphorus tends to be in a similar bind, tied up in minerals and away from plants. Over time, however, as soil breaks down, that phosphorus will slowly become available to bacteria. The microbial middlemen then convert it into plant-friendly forms.

Microbes are constantly fixing nitrogen. Soil is always releasing some phosphorus. Still, most ecosystems remain hungry for more. Scientists often describe areas such as forests, wetlands and tundra as being nitrogen- or phosphorus-limited. By that they mean that if they had more nitrogen and/or phosphorus available to them, the plants in these areas would grow bigger or faster.

For example, some areas may have soil that has just become exposed to the air. This new soil has few microbes producing nitrogen. It will usually be nitrogen-limited. Areas with soil that has been used by plants for a very long time, on the other hand, may have lost much of their phosphorus, making them phosphorus-limited.

Farmers often add both nutrients to their fields as fertilizer. Essentially, fertilizers are plant foods. You might also fertilize a houseplant or garden. Fertilizers usually contain a mix of nitrogen and phosphorus. (Most contain other elements as well, such as potassium and maybe a dash of copper or zinc.) Adding fertilizer is a quick way to restock the soil pantry so the plants can eat well and grow.

But too much food is not necessarily a good thing. When soil receives too much nitrogen or phosphorus — more than its plants need — these nutrients can leach away into rainwater. It now can be carried into lakes and streams. At first, the extra plant food will make algae in the water bloom and proliferate explosively.

But sooner or later, those masses of algae will die and sink to the bottom. There, microbes will break them down. Now their populations will proliferate. To power their activities, these organisms will suck up much of the oxygen in the water. With little to no available oxygen, other organisms in the water can suffocate. This process is called eutrophication

Bethany is the staff writer at Science News for Students. She has a Ph.D. in physiology and pharmacology from Wake Forest University School of Medicine.

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