Explainer: The fertilizing power of N and P | Science News for Students

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
Jan 4, 2018 — 6:35 am EST
algae bubbles
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

Power Words

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adenosine triphosphate      (ATP) This is a molecule that cells make to power almost all of their activities. Cells use oxygen and simple sugars to create this molecule, the main source of their energy. The small structures in cells that carry out this energy-storing process are known as mitochondria. Like a battery, ATP stores a bit of usable energy. Once the cell uses it up, mitochondria must recharge the cell by making more ATP using energy harvested from the cell’s nutrients.

algae     Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.

atmosphere     The envelope of gases surrounding Earth or another planet.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

bacteria     ( singular: bacterium ) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).

bloom     (in microbiology) The rapid and largely uncontrolled growth of a species, such as algae in waterways enriched with nutrients.

carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chlorophyll     Any of several green pigments found in plants that perform photosynthesis — creating sugars (foods) from carbon dioxide and water.

copper     A metallic chemical element in the same family as silver and gold. Because it is a good conductor of electricity, it is widely used in electronic devices.

DNA     (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

ecosystem     A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.

element     (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

eutrophication     The process by which a body of water becomes full of nutrients, which then stimulates the excessive growth of plants and algae. When these organisms die, bacteria break them down. But this bacterial activity can temporarily use up much of the water’s dissolved oxygen. With too little oxygen, animals can suffer — even suffocate. In short order, a eutrophic ecosystem can collapse.

fertilize     (in biology) The merging of a male and a female reproductive cell (egg and sperm) to set in create a new, independent organism. (in agriculture and horticulture) To provide basic chemical nutrients for growth.

fertilizer     Nitrogen, phosphorus and other plant nutrients added to soil, water or foliage to boost crop growth or to replenish nutrients that were lost earlier as they were used by plant roots or leaves.

forest     An area of land covered mostly with trees and other woody plants.

hydrogen     The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.

leach     (in geology and chemistry) The process by which water (often in the form of rain) removes soluble minerals or other chemicals from a solid, such as rock, or from sand, soil, bones, trash or ash.

microbe     Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

mineral     Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nitrogen     A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn.

nutrient     A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

organism     Any living thing, from elephants and plants to bacteria and other types of single-celled life.

oxygen     A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

phosphate     A chemical containing one atom of phosphorus and four atoms of oxygen. It is a component of bones, hard white tooth enamel, and some minerals such as apatite.

phosphorus     A highly reactive, nonmetallic element occurring naturally in phosphates. Its scientific symbol is P. It is an important part of many chemicals and structures that are found in cells, such as membranes, and DNA.

potassium     A chemical element that occurs as a soft, silver-colored metal. Highly reactive, it burns on contact with air or water with a violet flame. It is found not only in ocean water (including as part of sea salt) but also in many minerals.

protein     A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

runoff     The rainwater that runs off of land into rivers, lakes and the seas. As that water travels through soils, it picks up bits of dirt and chemicals that it will later deposit as pollutants in streams, lakes and seas.

science fiction     A field of literary or filmed stories that take place against a backdrop of fantasy, usually based on speculations about how science and engineering will direct developments in the distant future. The plots in many of these stories focus on space travel, exaggerated changes attributed to evolution or life in (or on) alien worlds.

suffocate     To be unable to breathe, or to cause a person or other organism to be unable to breathe.

tundra     A cold, usually lowland area in far northern regions. The subsoil is permanently frozen. In summer, a tundra's top layer of soil thaws and can support low-growing mosses, lichens, grasses, shrubs and trees (some only a few centimeters high).

wetland     As the name implies, this is a low-lying area of land either soaked or covered with water much of the year. It hosts plants and animals adapted to live in, on or near water.


Website: A. Bernhard. The nitrogen cycle: processes, players, and human impact. Nature Education Knowledge. Vol. 3, 2010.

Website: C. Hyland et al. Phosphorus basics – the phosphorus cycle. Department of Crop and Soil Sciences, College of Agriculture and Life Sciences, Cornell University, 2005.