How do scientists measure the porosity of soil?

1 Answer
Jan 18, 2018

You can find the porosity of soil by measuring the amount of water it takes to fill all the tiny pores in the soil.


The porosity of soil is the volume of space in between the mineral particles of soil. Porosity varies greatly from one kind of soil to another because the grains of soil are loosely or densely packed.

Here's an image of different kinds of soil with different porosities:

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To calculate the porosity of a soil sample, determine the volume of the empty spaces between particles by seeing how much water it takes to fill all those little pores.

Here is one method to find the porosity of soil.

Calculating Porosity Experimentally by #"Saturation"#

1) Gently dig up a sample of the soil you want to measure.
Don't be rough with the sample because handling it too much may disturb the structure of the soil.

2) Transfer the soil sample to a graduated beaker to measure the volume of the soil. Record this volume exactly.
This measurement is the #"Volume of the Sample"# #V_s#

3) Measure out a generous amount of water into a graduated cylinder. Start with more water than you will need. Record this volume exactly.

4) Saturate the soil sample with the water. Get the water as close as you can to the exact top of the soil in the beaker -- but don't go over the top

5) Subtract the volume of water that you used up from the starting volume in the graduated cylinder. That shows you how much water it took to saturate the soil.

6) The volume of the water used is equal to the Pore Volume #V_p#

#"Porosity"# = #("Pore Volume")/("Total Volume")# #xx##100%#

7) You find "Total Volume" by adding #V_s + V_p#

8) So you can use a scientific calculator to come up with this number by doing this math:

#"Porosity"# #=# #("Pore Volume") / ("Sample Volume + Pore Volume")# #xx# #100%#

9) This is the formula for this calculation

#P_t= (V_p )/ (V_s +V_p) xx 100%#

Here's a WikiHow article that describes other ways to calculate soil porosity: