How to Find the Porosity of Soil

Fill the measuring container with water. Measure the radius of the container -- half the center diameter -- square it and multiply by pi (3.14) to determine the area of the container. Multiple this number by the height to which you fill the water to determine the volume of water. For example, if you have a container with a radius of 10 centimeters and water filled to 10 centimeters, the volume of the water is 3,141.6 cubic centimeters.

Collect a sample of the soil you want to test and measure its volume. For this example, assume the volume of the sample is 250 cubic centimeters.

Heat your oven to 225 degrees F. Place the soil in the oven for 30 minutes to evaporate any water. Remove the soil sample.

Immerse the soil sample in the water container and leave it for approximately one hour to allow water to saturate the soil.

Remove the sample and measure the volume of the remaining water using the formula hπr², or the height of the water in the container times pi times the square of the container's radius. Subtract this volume from the original water volume. Assume the remaining water in this example has a volume of 3,050 cubic centimeters; 3,141.5 minus 3,050 is 90.5. The soil therefore absorbed 90.5 cubic centimeters of water.

Divide this result by the original soil sample size. Using the figures you've obtained, you can see that 90.5 divided by 250 equals 0.362. Multiply this result by 100 to convert to a percentage. This soil sample has 36.2 percent porosity.