Nitrogen Cycle II: Fertilised Algae

The nitrogen cycle is an example of a global cycle, representing how nitrogen moves through the spheres of the Earth – the biosphere, lithosphere, hydrosphere and atmosphere.

Nitrogen (N) is an essential element for plant growth, forming the structure of DNA and amino acids. Review the nitrogen cycle in the AusEarthEd video Nitrogen Cycle I: Legumes.

It is common practice to add nitrogen to the soil in the form of fertilisers so as to increase plant growth. However, sometimes more fertiliser is added than is required by the plants, and the surplus nitrogen can leach through the soil into local water bodies. This leads to excessive growth of aquatic plants, such as algae and duckweed, and can cause algal blooms. In extreme situations, this can cause the death of the other organisms living in the water body, through a process known as eutrophication.

Algal bloom

By growing duckweed or algae over a period of 3-4 weeks, you can see how the application of different levels of nitrogen fertiliser can increase aquatic plant growth. This is demonstrated in the Nitrogen Cycle II: Fertilised Algae video. Remember to use different levels of liquid fertiliser to measure the response of the aquatic plants and include a control, with no fertiliser added. Carefully calculate the amount of fertiliser to add, as excessive fertiliser can be toxic to plants and in your sealed container it is unable to leach away.

The growth of your plants can be measured in a variety of ways. In our video we count the increased number of duckweed leaves over the period of the investigation. Other methods include, estimating the percentage of water surface covered by the duckweed, counting the number of algal cells that can be viewed through a microscope field of view and even harvesting, drying and weighing the plants.

Leaf counts

Note any changes in leaf colour. This can indicate that the plant is not able to source the nutrients it requires and indicates the importance of nitrogen for plant health. This was noted in the control plants (no fertiliser) which became pale and almost colourless. Extended root growth also indicates that the plants are seeking nutrients in the water.


Now that you have investigated how fertiliser increases the number of aquatic plants, relate this to the impact of excessive fertiliser use on healthy water bodies. How important is it to correctly calculate the amount of fertiliser required by our plants, to prevent run-off occurring? Research ways to manage the run-off.

Where does the excess nitrogen from your garden end up?

Find out more about fertiliser run-off:

  • Research further information about the nitrogen cycle and investigate more activities at our WASP online resources.
  • Learn about environmental monitoring in your local environment and how to sample safely in the video Field Water Sampling.
  • Find out more about the impacts of fertiliser run-off on our lakes and rivers at the National Oceanic and Atmospheric Administration (NOAA).
  • What else ends up in our water bodies? Stormwater Treatment Blogspot.