How to Determine the Porosity of a Soil Sample

Collect a soil sample by pounding a metal tube 6 inches into the ground. Make a mark on the outside of the tube at the soil surface so that you know exactly how deep of a soil sample you have taken.

Determine the volume of your soil sample by measuring its diameter in centimeters. Divide the diameter by two and multiply it by pi (3.14). Multiply this number by the height of your soil sample in centimeters to find its volume. You do not need to remove the soil from the metal tube to perform these calculations.

Place the soil sample in a pan and dry it in an oven set on low. Twelve hours of drying time should be sufficient.

Weigh the dry soil with a metric scale.

Find the bulk density of the soil by dividing the dry soil weight by the volume of the soil sample. The units for your answer will be grams per centimeter squared or g/cc. For example, if your dry soil weight is 63 grams and the volume of your sample is 35 centimeters squared, your bulk density will be 1.8g/cc.

Calculate the porosity of your soil sample by dividing the bulk density by a standard particle density value of 2.65. Subtract this number from 1 and multiply the result by 100. For the above bulk density you would divide 1.8 by 2.65. This will give you 0.68. Subtract this from 1, which will give you 0.32. Now multiply this by 100 to get your soil's porosity, which would be 32.