Unit D3 - Nutrient Cycles


Unit D3


All things are made up of tiny atoms (too small to see and hard even to imagine) which mix and join together to make the different substances like air, water, rock or the materials of which our body is made. The study of the many kinds of atoms, the ways they combine together and the substances they make is called chemistry, and is too complicated to explain here. What is important to understand is that there are different kinds of atoms, and that each substance or material is made up of specific combinations of these atoms. It usually takes energy to put atoms together in bigger and bigger combinations, like those we find in living things. Energy may be given up in the form of heat (as in a fire) when the atoms are separated again. However the atoms themselves are not changed in these normal chemical reactions; they just change their form, and they go through these combinations and changes over and over again indefinitely.


For example, a tree may take in air through its leaves and water from the ground through its roots. Using energy from the sunlight, it breaks the water and air down to get carbon, oxygen and hydrogen atoms, and then puts these together first to make kinds of sugar and then to make wood. If the wood is burned, the atoms go back to the air as smoke, and the energy from the sun that held them together becomes the heat of the fire. The atoms that went back to the air may be taken up by another tree and used again. If some of the sugar made by the tree goes into the fruit, you could eat the fruit and your body would break down the sugar to give it energy (that is where your body heat comes from). The atoms from the sugar would be breathed out of your lungs as air. These are examples of cycles (meaning to go around in a circle) because something takes place or is used over and over again.

Cycles are an essential part of all natural systems and all living things. Growth and change and life itself involve taking in materials, using them in some way, and discarding them as other materials. Eating and breathing are obvious examples.


Many atoms are so common that their cycles do not matter much in managing the environment. However, some other atoms or chemicals are essential for life but are much harder to get in a form that can be used. These things that are needed as food are called nutrients, and if they are in short supply, plants and animals cannot grow the way they should.

The three nutrients that living things most often need but cannot always find enough of contain the atoms nitrogen (N), phosphorus (P) and potassium (K). Other things may also be needed, but less frequently or in smaller quantities. Only the most important nutrients are discussed here, but similar principles apply to the others.

Nitrogen occurs in air, but it cannot be used by most living things until it is fixed or attached to oxygen or hydrogen atoms. Phosphorus and potassium are found in some soil materials. Plants need these nutrients in order to grow, and if they cannot get them in the soil they will be stunted or even die. A plant suffering from the lack of a nutrient may have leaves that turn yellow or die at the edges, and may be shorter or weaker than healthy plants. If a soil is not too rich in nutrients, they will quickly be taken up and used by the plants, leaving the soil exhausted. However, when the plants die and decay, the nutrients go back into the soil where they can be used again, making a simple nutrient cycle.

Animals make the nutrient cycle more complicated. They must get their nutrients and energy from the plants they eat (or from eating animals that have eaten plants). When animals deposit their wastes or die, the nutrients in their bodies go back to the soil, but not necessarily in the same place where the plants grew. Thus nutrient cycles may involve the movement or transport of nutrients from one place to another.

It is important to understand the principles of these nutrient cycles, since respecting or managing them is an important part of keeping environmental resources productive, and ignoring them is a good way to fail in development efforts.

Nutrients in the environment

In most stable natural environments, the total quantity of nutrients stays the same as the nutrients cycle within the ecosystem. A productive ecosystem like a tropical forest or a coral reef is often able to build up quite high levels of nutrients over time. The long roots of forest trees reach down deep in the soil for nutrients which they bring up and add to the system. A large stock of nutrients is held in the vegetation and is quickly recycled if released. It is important to note that in such systems the reserve of nutrients is held in the plants and animals that make up the ecosystem, and not in the surrounding environment which may be quite poor. If the system of living things is damaged or destroyed, the nutrients that maintain its productivity may be lost very quickly.

Nitrogen is a special case, because it is made available as a nutrient by the biological activity of nitrogen fixation carried out by certain microbes and algae, sometimes in association with the roots of leguminous plants. It can also be lost as a nutrient when microbes break down nitrogen compounds. The fixing of nitrogen is a normal process in most natural systems, but when man disturbs or develops land, he usually destroys this capacity.

While the above description of nutrient cycles has focused on the land, the same principles apply in the sea. Nutrients occur in seawater, although the amounts in tropical seas are very low except where they are added to the sea by runoff from the land. Nutrients are stocked in sea plants and animals, and are accumulated and recycled in productive systems like coral reefs.

Man's impact on nutrient cycles

It should now be clear that man's activities can easily create environmental problems by interfering with the nutrient cycles. Cutting, clearing or burning the vegetation releases the stock of nutrients, which may be washed away quickly by rain if there are not other plants to take them up. Polluting coastal waters, or dynamiting or poisoning the reef have the same effect. The mechanisms by which new nutrients are accumulated and recycled may also be destroyed.

In order to keep up the productivity of man-made systems like farming, it is necessary to add nutrients to the system, either by applying natural materials like compost or manure, or by adding chemical fertilizers of which nitrogen, phosphorus and potassium compounds are the most common. The careful management of nutrients is one of the most important aspects of agricultural development.

Most environmental problems concerning nutrients result from the lack of nutrients in agriculture or forestry production or when soils become degraded, but there can occasionally be problems of too much of some nutrients, particularly in rivers, lakes and lagoons. The excess nutrients may come from fertilizer washed off the land, or from human and animal wastes (high in nitrogen) or detergents (high in phosphorus). The runoff from fields, livestock areas, villages and towns may collect in the water, causing the rapid growth of algae or other aquatic plants which may smother more useful resources like the corals on a reef. The accumulated algae may eventually rot, using up the oxygen in the water, killing the fish and producing bad smells. Nitrogen compounds can also get into human water supplies, particularly ground water, where they can present a risk to human health.


What is a cycle?

Why are nutrient cycles important?

Where do nutrients come from?

How do animals participate in nutrient cycling?

Where are the nutrients in a tropical forest? In a coral reef?

What makes nitrogen compounds different from other nutrients?

What does man often do to nutrient cycles?

What can be done to maintain nutrients in agriculture?

Is it bad to have too much of some nutrients? Why?

Instructions for trainers in the use of this unit

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Last updated 27 December 2007