Soil Properties

National Park Service

Activity Source: 

Adapted with permission from Kristen Lucke, Views of the National Parks, National Park Service.

"Soil porosity" refers to the amount of pore, or open space between soil particles. Pore spaces may be formed due to the movement of roots, worms, and insects; expanding gases trapped within these spaces by groundwater; and/or the dissolution of the soil parent material. Soil texture can also affect soil porosity

There are three main soil textures: sand, silt, and clay. Sand particles have diameters between .05 and 2.0 mm (visible to the naked eye) and are gritty to the touch. Silt is smooth and slippery to the touch when wet, and individual particles are between .002 and .05 mm in size (much smaller than those of sand). Clay is less than .002 mm in size and is sticky when wet. The differences in the size and shape of sand, silt, and clay influence the way the soil particles fit together, and thus their porosity.

Soil porosity is important for many reasons. A primary reason is that soil pores contain the groundwater that many of us drink. Another important aspect of soil porosity concerns the oxygen found within these pore spaces. All plants need oxygen for respiration, so a well-aerated soil is important for growing crops. Compaction by construction equipment or our feet can decrease soil porosity and negatively impact the ability of soil to provide oxygen and water.

Materials

  • 3 100ml graduated cylinders per group (or a measuring cup and two clear plastic bottles)
  • Fine, playground-style sand and coarse, aquarium-style gravel
  • Blank piece of paper and something to write on
  • Pencil or pen
  • Ruler
  • Metal spoon or gardening spade

Procedure

    1. Divide into small groups. On a piece of paper, make a data table like the one below for each group.
      Soil particle type                    Volume of Water used (ml)
      gravel
      sand 
    2. With each group taking 3 graduated cylinders, fill one cylinder with 100ml of sand, one with 100ml of gravel, and one 100ml of water.
    3. Repeat step 4 with the sand and record the amount of water used in the data table.
    4. Discuss the experiment: Which substance has more pore space: gravel or sand? How did you make this decision?
    5. Before leaving the classroom, though, refill two of the graduated cylinders with 100ml of water. You will also need paper, pens, and pencils to record observations. Draw the data table below for each group.
                                 Survey area                                                       Volume of Water used (ml) 
      # 1  
      # 2

 

Find a place outside where it is permissible to collect small soil samples and have each group choose a survey area.

    1. Record observations of this survey area. Look at the types of plants growing in the soil, evidence of wildlife, etc. Is the soil in the shade or in direct sunlight? Sketch what you see.
    2. Once survey area observations have been made, obtain a small sample of soil to determine its texture. Is the soil wet or dry? If it's wet, does it feel gritty (sand), smooth and slippery (silt), or sticky (clay)? Can you see and measure individual particles? Record all of your texture observations.
    3. Now have each group fill its empty graduated cylinder with 50ml of soil. Pour water from one graduated cylinder into the soil until water just covers the top. Record the volume of water used in the data table next to Survey Area #1.
    4. Pick a new survey area (if possible, with different vegetation). Repeat steps 3 through 5, and record the volume of water used in the data table next to Survey Area #2.
    5. Return to the classroom and discuss your results: Was there a difference in soil porosity? Were there similarities? For the soil samples with similar porosities, did they have the same soil textures? Do you think these soils provide adequate water and air for plants? What types of plants live in these soils? Do factors such as sunlight or soil texture seem to affect the porosity of the soil?

 

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