Grade Level: 5-10
"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
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.
- 3 100ml graduated cylinders per group (or a measuring cup and two clear
- Fine, playground-style sand and coarse, aquarium-style gravel
- Blank piece of paper and something to write on
- Pencil or pen
- Metal spoon or gardening spade
- 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)
- With each group taking 3 graduated cylinders, fill one cylinder with 100ml
of sand, one with 100ml of gravel, and one 100ml of water.
- Pour the water slowly into the gravel and stop when the water just covers
the top of the gravel. Record the amount of water used in each data table.
Refill the cylinder of water to 100ml.
- Repeat step 4 with the sand and record the amount of water used in the data
- Discuss the experiment: Which substance has more pore space: gravel or sand?
How did you make this decision?
- 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.
||Volume of Water used (ml)
- Find a place outside where it is permissible to collect small soil samples
and have each group choose a survey area.
- 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.
- 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.
- 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
- 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.
- 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?
For more, visit www.nature.nps.gov/views.