How to Calculate Bulk Density

By Bryan Perkins
Arid soil image by Igor Baryshev from Fotolia.com

Bulk density is a measure of the mass of soil per unit volume. Soils are composed of pores that hold air and water, minerals and other organic matter. According to the University of Missouri Cooperative Soil Survey, "an ideal soil can be described as being 50% solids and 50% pore space, with half the pore space filled with air and half with water." Low bulk density soils reduce stormwater flow, minimize runoff and improve the overall quality of water available, making lower soil bulk density desirable for plant growth.

In the Field

Drive the 3-inch diameter ring into the soil you are sampling, beveled edge down. Place the block of wood on top of the ring and hit it with the hand sledge. The ring does not need to be completely covered in soil. Your measurements will be more accurate if the ring is driven into the ground evenly on all sides.

Measure the height from the soil surface to the top of the ring. To get a more accurate reading of the depth of the ring take four evenly spaced measurements from around the ring and record the average of the measurements.

Dig around the ring with the garden trowel. Place the trowel under the ring to prevent the loss of soil and carefully lift it out of the ground.

Remove any excess soil from the bottom of the sample with a flat-bladed knife. Ensure the soil is flat and even with the edges of the ring.

Push the soil sample into a plastic bag using the flat-bladed knife. Seal and label the bag, touching and disturbing the sample as little as possible.

In the Lab

Weigh the soil sample on the scale while it is still in the bag. Record the data. Weigh an empty plastic bag to account for the weight of the bag and record this data as well.

Knead the bag with your fingers to thoroughly mix the soil sample.

Scoop 1/8 cup of loose soil from the plastic bag and place it in the paper cup. Weigh the soil sample in the paper cup and record the data. Weigh an empty paper cup and record the data as well.

Place the cup containing the soil sample in a microwave and dry it on full power for four minutes. Open the microwave door for one minute to allow venting, then dry the soil sample on full power for four more minutes.

Weigh the dry soil sample in its paper cup and record the data.

Calculations

Subtract the mass of the paper cup found in step 3 of the lab work section from the mass of the dry soil in the paper cup found in step 5. Record this data as the dry weight of the soil sample.

Convert the volume of the soil sample, 1/8 cup, to centimeters cubed. One cup is about 236.6 centimeters cubed, so 1/8 cup is about 29.6 centimeters cubed. This is the volume of the soil sample.

Divide the dry weight in grams of the soil sample by the volume in centimeters cubed of the soil sample. The resulting value is the bulk density of the soil.

About the Author

Bryan Perkins is a writer from Baton Rouge, La. His work appears in various online publications, covering topics related to science. Perkins holds a Bachelor of Science in biological sciences from Louisiana State University.