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Nitrogen and phosphorus cycles

Above Image: The nitrogen entry from the periodic table © 3dalia, iStockphoto

Air is made up of nitrogen (78%) and oxygen (21%) with small amounts of other gases such as argon and carbon dioxide. It probably then comes as no surprise that nitrogen is important for all organisms as it helps them to live and grow.

The Nitrogen cycle (Figure 1) describes the way that nitrogen moves through plants and animals, water, soil and the atmosphere.

Nitrogen can be introduced into the soil through natural process such as precipitation and lightning (1), as well as through human means such as fertilizers (2). Plants and animals cannot directly use atmospheric nitrogen (N2 gas) because it does not easily react with other biological molecules. Because of this, plants and animals need to get their nitrogen from more reactive nitrogen compounds.

In soil, specialized bacteria known as nitrogen-fixing bacteria (3), are able to covert (or fix) atmospheric nitrogen into more chemically-reactive nitrogen compounds such as ammonia (NH3) and ammonium (NH4+). Other bacteria, called nitrifying bacteria (4) can convert the ammonium in the soil into nitrite (NO2-) and then into nitrate (NO3-).

Nitrates can be taken up and used by plants (5) to make proteins. As animals eat plants, the plant proteins are taken up by the animals (6). When the animals make excrement (poop) (7) or when plants and animals die (8), this matter is broken down by decomposers (9) and ammonia is re-introduced into the soil. Denitrifying bacteria (10) convert nitrates back to unreactive nitrogen which can be re-introduced back into the atmosphere – and the cycle begins again.

Nitrogen fixation in plants

Certain plants also have the ability to fix nitrogen so that it can be taken up and used directly by the plants. These types of plants are called nitrogen-fixers. They are most typically plants from the legume family such peas, beans, and clover. The roots of these plants contain lumps or nodules (Figure 2) where nitrogen-fixing bacteria called Rhizobium are found and where nitrogen is stored.

The nodules with stored nitrogen are very beneficial to garden plants and agricultural crops. Although the nitrogen-fixing plant does not release extra nitrogen into the soil during its life, after it dies and during decomposition, the nitrogen that was stored in the nodules is released into the soil and becomes fertilizer for the next plants in the garden or the new crop.

Farmers intentionally plant field crops in rotations that include nitrogen-fixing crops to supplement the nitrogen requirement of the next crop that is planted. In Southwestern Ontario, corn is a commonly grown crop which requires a large amount of nitrogen for proper growth. Using too much nitrogen as part of added fertilizers can pollute surrounding waters sources and lead to the degradation of the field soil. In order to satisfy the need for large amounts of nitrogen in the soil for growing corn, farmers will rotate the field by planting a legume crop such as soybean in one year and then back to corn the next.

The phosphorus cycle

Phosphorus is an essential element for all life. The way that phosphorus is moved through plants and animals, the water, and soil is known as the phosphorus cycle (Figure 3). The phosphorus cycle is a very slow process.

Phosphorus is mainly stored as phosphate ions (PO3−4) in water, soil, rocks, and sediments. Most soils have very low amounts of phosphate. Phosphorus can occur both as inorganic and organic compounds.

Inorganic phosphate ions can be distributed throughout soil and water due to weathering (1) and erosion (2). Plants take up inorganic phosphate from soil (3), but since inorganic phosphorus is not very soluble in water, only a small amount of the total phosphorus is available to plants. When plants are eaten by animals (4), phosphate can be used to build organic molecules such as DNA and proteins. When a plant or animal dies (5) or an animal produces excrement (6), organic phosphate is returned to the soil. Mineralization occurs when bacteria break down organic matter into inorganic phosphorus. The reverse is called immobilization.

Addition of fertilizers and decomposition of plants and animals can increase the total phosphorus in the soil. The removal of phosphorus from soil occurs mainly through plant uptake (3) but can also occur through leaching (7). The leaching of excess phosphorus into waterbodies can cause excessive growth of plants, leading to eutrophication (8).

Fertilizing tomatoes

Nitrogen and phosphorus are important for growth in all plants, including tomatoes. When adding fertilizer to help tomato plants grow, be sure to not add too much nitrogen because although it helps with the growth of leaves, nitrogen can discourage development of flowers necessary for fruit formation. Phosphorus, on the other hand, helps with flowering and therefore is important to add to tomatoes to help with the development of the tomato fruits.



A compound of nitrogen and hydrogen (NH3).


An organism, such as bacteria, that breaks down dead plants and animals.


A process by which nitrates are converted into atmospheric nitrogen.

DNA (deoxyribonucleic acid):

A molecule which carries genetic information for living things.


Excess nutrients such as nitrogen and phosphorus in bodies of water. It can cause aquatic plants and algae to grow rapidly and use up the oxygen in the water.


A process in which wind or water moves rock, soil or other dissolved materials from one place to another.


A substance such as nitrogen, phosphorus or potassium that helps with plant growth. It can be natural such as a manure, or synthetic.


Is the reverse of mineralization. It occurs when inorganic phosphorus is converted to the organic form which can be incorporated into plants.

Inorganic compound:

A compound that is not organic and does not contain carbon atoms.


The loss of substances from the soil as a result of precipitation.

Legume plants:

Plants belonging to the family Fabaceae which includes crops such as peas, beans, and clover.


A process that occurs when bacteria break down organic matter into inorganic phosphorus.


An ion of nitrogen and oxygen with the molecular formula NO3-.


An ion of nitrogen and oxygen with the molecular formula NO2-.

Atmospheric Nitrogen:

Nitrogen in the atmosphere which occurs as nitrogen gas with the molecular formula N2. Due to the strong triple bond between the two nitrogen molecules, it does not easily react with other chemicals.


Plants with roots containing bacteria that fix nitrogen.


Lumps on the roots of legume plants where nitrogen is stored and nitrogen-fixing bacteria live.

Organic compound:

A compound that contains carbon atoms.


A species of bacteria that fixes nitrogen in the roots of plants in the legume family.


The breakdown of soil and rocks.