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Soil Porosity: Sand, Silt, and Clay

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Author: Mike Calhoun

Soil texture is determined by the relative proportion of sand, silt, clay and small rocks (pebbles) found in a given sample. Sand is gritty to the touch and the individual grains or particles can be seen with the naked eye. It is the largest of the three size classes of soil particles. Silt is smooth and slippery to the touch when wet and the individual particles are much smaller than those of sand. These individual particles can only be seen with the aid of a microscope. Clay is sticky and plastic-like to handle when wet. The individual particles are extremely small and can only be seen with the aid of an electron microscope.

Soils are made of particles of different types and sizes. The space between particles is called pore space. Pore space determines the amount of water that a given volume of soil can hold. Porosity refers to how many pores, or holes, a soil has. The porosity of a soil is expressed as a percentage of the total volume of the soil material. Porosity is an important measurement in areas where drinking water is provided by groundwater reserves.

Research Questions

  • What is soil porosity and why is it important?
  • How can porosity be measured?
  • Which of the soil samples tested (sand, clay, and pebbles) has the greatest porosity?
  • Which of the soil samples tested (sand, clay, and pebbles) has is the least pareses?
  • Which particle size had the most amount of empty space? The least amount of space?
  • Is there any relationship between particles size and pore space?  

Materials

  • 3 metric measuring cups
  • 100 ml graduated cylinder
  • Water
  • Permanent marker
  • Soil samples: sand, clay and small pebbles (can be obtained from various field locations such as a rock quarry, road cuts, stream beds, etc.)

Procedure

  1. Fill one measuring cup to 200 ml with sand, the second cup with 200 ml of clay and the third with 200 ml with small pebbles.
  2. Fill a graduated cylinder to 100 ml with water.
  3. Slowly and carefully pour the water into the first cup until the water just reaches the top of the sand.
  4. Pour slowly so no water spills out of the measuring cup. Record exactly how much water was used.
  5. Use the formula below to calculate the percent porosity for the sand:

Porosity = (Amount of water added to sample ÷ Total sample volume) x 100

  1. Repeat the same procedure with the clay and the pebbles.
  2. Record the results in a table similar to the one shown
 
Soil Type
     Total sample volume
    Amount of water added to sample
 Porosity
Sand
         200 ml
 

             %

Clay
          200 ml
 

             %

Pebbles
          200 ml
 

             %

Digging Deeper

After measuring the porosity of sand and clay, make a mixture of these two samples by adding them together. Repeat the water addition procedure and calculate the % porosity then compare the results to that of the two individual soil samples. 

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