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Soils and water storage (under the microscope)

About this lesson

By looking at different soil types under the microscope, students will hypothesise which soil has the ability to store the most water.


Year level: 3, 4, 5, 6

Theme: Water conservation


Learning objectives

Students can:
  • understand the terms porosity and permeability
  • examine different types of soil through a microscope and determine which soil type will store the most water
  • explain the difference between well sorted, medium sorted and poorly sorted grains.

Curriculum links

Science

  • Living things can be grouped on the basis of observable features and can be distinguished from non-living things ACSSU044
  • Living things, including plants and animals, depend on each other and the environment to survive ACSSU073
  • Living things have structural features and adaptations that help them to survive in their environment ACSSU043
  • The growth and survival of living things are affected by the physical conditions of their environment ACSSU094

Humanities and social sciences (geography)

  • The importance of environments, including natural vegetation, to animals and people ACHASSK088

Things you will need

Lesson description

Discuss

  • Explain to students the terms porosity and permeability.
  • Using the soil types fact sheet discuss the differences between well sorted, medium sorted and poorly sorted soil grains.

Activity

  • Explain to students that they will look at different soil types under the microscope and observe the size of the grains to determine which soil is likely to hold the most water. 
  • Set the microscope to magnification 60X and hold it over a cup of one of the sand types. Estimate the amount, as a percentage, of air space between the grains as seen through the microscope.
  • Prepare a sticky tape slide using the microscopes fact sheet.
  • Sprinkle a few granules of the sand in the middle of the sticky part of the slide. Label the slide.
  • Place the slide on the microscope stage and view the composition of the sand using the microscope on magnification 60X.
  • Have students describe and draw what they see on the Microscope activity sheet. Things they should consider:
    • size of the grains (put a metric ruler under the microscope to get an idea of the size)
    • shape of the material (is it smooth or rough?)
    • type of material and colours that make up the sand (is it made from minerals like quartz, feldspar or mica, or from tiny bits of broken shells or plant material?)
  • Students repeat the above steps with the other sand types.
  • Upon completion students decide which soil sample they think could store the most water and provide an explanation for their choice.

Reflect & summarise

  • As a class discuss:
    • How is beach sand or river sand (which has been pushed around by moving water) different from garden sand?
    • Which soil sample has the most amount of space between grains?
  • Have students vote on which soil sample they think has the ability to store the most water. Get students to explain their rationale.
  • Ask student show they could test their predictions to determine which soil in fact does store the most water?

Extension activities

  • Have students investigate one of the following:
    • Which type of soil would allow rainfall to soak in most easily?
    • Which soil type would have the greatest runoff during heavy rain?
    • Why do people in Perth's coastal suburbs use a lot of water on their gardens during summer?
    • Which soil type would allow water to be drawn from it easily?
    • What effects do soil-wetting agents have?
    • The coastal soils near Perth are formed over limestone, while the oils in the Darling Range are formed over granite. Which of these rock types allows water to move through easily? Explain your answer.

Teacher background information

Soil type affects how well plants grow, and how effectively we can water them. Soil is a mixture of mineral particles, water, air, living organisms and decomposed organic matter.

The size of the mineral particles determines the spaces available for the other elements and, hence, the fertility, water-holding capacity and drainage capability of the soil.

What is porosity?

Porosity or pore space is the amount of air space or void space between soil particles. Porosity depends on both soil texture and structure. For example, a fine soil has smaller but more numerous pores than a coarse soil. A coarse soil has bigger particles than a fine soil, but it has less porosity, or overall pore space.

Water can be held tighter in small pores than in large ones, so fine soils like clay can hold more water than coarse soils, such as sand. The shape, arrangement and packing of soil particles also help determine porosity. Particles exist in many shapes and these shapes pack in a variety of ways that may increase or decrease porosity. Generally, a mixture of grain sizes and shapes, results in lower porosity.

What is permeability?

Permeability refers to the movement of air and water through the soil, which is important because it affects the supply of root-zone air, moisture, and nutrients available for plant uptake.

The size of pore space and interconnectivity of the spaces help determine permeability, so shape and arrangement of grains play a role. Water can permeate between granular void or pore spaces, and fractures between rocks. The larger the pore space, the more permeable the material. However, the more poorly sorted a sample (mixed grain sizes), the lower the permeability because the smaller grains fill the openings created by the larger grains.

Resources

Did you know?

One tablespoon of soil contains more organisms than the entire population of people on Earth.

Key vocabulary

  • Clay: A very fine-grained soil that turns sticky when water is added
  • Permeability: The ability of a fluid to move through soil or limestone rock
  • Porosity: The amount of space in a rock or soil that is able to store water
  • Sand: A loose material consisting of grains of rock or coral