GLOBE III - Bulk Density

Purpose

To determine the bulk density of each soil horizon at the GLOBE soil study sites.

Overview

Bulk density is defined as the ratio of the mass of dry solids to the bulk volume of the soil. Students will take 3 bulk density samples from each horizon of the soil profile they sample at each of the study sites. A soil sampling can of known volume will be used. An auger may also be used, and the diameter and length of the hole must be recorded for each sample to determine its volume. The sample will be taken back to the classroom, weighed, oven dried, and reweighed. Rocks need to be separated from the dried sample, either by seiving, or by picking out the rocks carefully and their density of the rocks determined. Data reported on the GLOBE Data Entry Sheet include: volume of can (or diameter and depth of auger hole for each sample), weight of can, moist weight of soil sample plus can, dry weight of soil sample plus can, weight of rocks, volume of rocks for triplicate samples in each horizon. The final bulk density at the field moisture content for each soil horizon (in triplicate) can then be calculated.

Time Required

One class period is required in the field for sampling and preparation, given an existing pit or auger hole.
Half a class period is required in the classroom, to weigh the sample and prepare it for drying.
One class period is necessary, after the samples dry, to do additional weighing, separating the rocks, determining their volume, and recording the measurements.

Skill Level

Measurement Frequency

This measurement is reported one time for each soil profile. Three subsamples from each soil horizon are measured and reported once for each soil profile characterized.

Key Concepts and Skills

Concepts: density, volume, units of measurement, replication
Skills: sampling, drying, weighing, calculation of density, converting units from weight to volume

Materials and Tools

balance
drying oven
100 mL graduated cylinder
container for weighing rocks on balance
thermometer
optional: seive (no.10 with 2mm openings)

For Pit of surface method:

trowel or shovel
soil cans (12 - 15)
small nail

For Auger method:

Dutch auger
sample bags and marker
container to dry samples in oven

Background

Bulk Density is the weight of a dry soil in a specific volume. How dense the soil sample is depends on how many spaces (pores) are in the sample, hoe tightly they are packed, and also what the solid material is composed of. Soils made of minerals will have a different bulk density than soils made of organic material, even if they have the same amount of pore space. The amount of sand, silt, and clay in every sample effects the amount of pore space in the soil as well. Sandy soils have large pore spaces, but because sand grains are large, there are not as many pore spaces as in a soil with smaller particles. Silty soils have smaller particles with smaller pore spaces, but there is more total empty space in a silty soil than in a sandy soil. In general, clayey soils, which have good soil structure, have a great amount of pore space because the size of the particles are very small, and many small pore spaces fit between them. They will most likely have a lower bulk density than sandy or silty soils. Organic soils usually have the most pore space, and the dry weight of the organic material is less than the weight of mineral particles, so they will have the lowest bulk density. Usually, a soil sample will have a mixture of sand, silt, clay, and organic material, so its bulk density will depend on how much of each of these components are mixed together in the sample, and how tightly they are packed.

Knowing the bulk density of a soil is important for many reasons. Bulk density can give us information about how much porosity (number of pore spaces) are in a sample. This helps to determine how much air or water can be stored and moved through the soil. It also gives an indication of how tightly soil particles are packed together, and if it will be difficult or easy for roots to grow or shovels to penetrate through a soil horizon. Another important function of bulk density is using it to convert between weight and volume for a soil sample. If we know the weight of a soil sample, we can calculate what its volume is by dividing it by its bulk density. If we know the volume of a soil, we can determine its weight by multiplying it by its bulk density.

What to do and How to do it

There are 3 options for determining bulk density depending on how you perform your soil characterization protocol.

OPTION 1: (If you dig a soil pit)

Before making your bulk density measurement, obtain enough soil cans to take 3 bulk density measurements for each horizon in your soil pit and clearly label them. Determine the volume of each can and record the result on the GLOBE data entry sheet. The volume of the can is determined by filling it to the top with water and pouring the water into a graduated cylinder to measure the number of mL (equal to cubic centimeters) of water that fills the can. Once the volume has been measured, poke a small hole at the bottom of the can with a nail, to allow air in the can to escape when soil is being pressed into the can. Weigh each can and record its weight and volume on the GLOBE Data Entry Sheet.
Proceed to the field and perform the following measurement on the same soil profile that you use for your soil characterization protocol. For each horizon in your soil profile, push a can with a known volume into the side of the horizon. (NOTE: The soil in the profile should be moist, so that it will stick together, and be easy to press the can into. If necessary, wait to do this procedure after a rain, or wet the soil before doing this measurement.), Be sure that the soil has filled the can by observing when you can see some of the soil poking throught the small hole in the bottom of the can. If it is difficult to push the can into the soil, you may need to use a hammer or other object to force it in.
Using a trowel or shovel, remove the can and the soil surrounding it. Trim the soil from around the can until it is flat against the edges of the can so that the volume of the soil is the same as the volume of the can. Once it has been trimmed, cover the can with the lid and return it to the classroom. Repeat this procedure 3 times for each horizon so that you have 3 bulk density samples for each horizon. Be sure to label the cans in the field with the horizon name, depth, and number of replicate (1, 2 or 3 for each horizon).
As soon as possible after taking the sample, bring it back to the classroom and weigh it. Record this field moist weight (including the can) on the GLOBE Data Entry Sheet. If a drying oven is available, remove the lids and place the cans in the oven at 105o until it is completely dry (this can be done by removing the sample from the oven and reweighing each day. It is dry when the weight remains stable and no longer changes). If no oven is available, carefully remove the soil from each can and spread each sample on a separate piece of paper to air dry. Drying can go faster if the soils are left in a sunny window. Be sure to keep each sample labeled and separate from the other.
Once the soil is dried, obtain a weight of the dry soil and can, and record this weight on the GLOBE Data Entry Sheet.
Once it has been dried and weighed, rocks need to be separated from each sample if they are present. Pour the sample into a #10 sieve (2mm openings) and carefully push the dried soil material through the mesh into the sieve bottom or piece of paper. Rocks will not pass through the mesh and will stay on top of the sieve. Remove the rocks from the sieve and their density calculated. If no sieve is available, carefully remove the rocks by hand.
Once the rocks have been separated, weigh them, and record this weight on the GLOBE Data Entry Sheet. Next, place 30 mL of water into a 100mL graduated cylinder, and one by one, add the rocks to the water. Read the level of the water after all the rocks have been added and record this value (and the original volume of water) on the GLOBE Data Entry Sheet.

OPTION 2: (If you use an auger to do your soil characterization measurement)

If you choose to use an auger to do your soil characterization protocol, and do not dig a soil pit, the auger can also be used to take a bulk density sample.

Before going into the field, locate 3 sampling sites close together to make auger holes. Be sure to have identified the depth of the horizons in you profile with your soil characterization protocol first, so that you know how deep to go to take a sample of that horizon.
In the field, auger to the specified depth, and fill the auger head full of a sample for each horizon. Only turn the auger once and remove it from the hole so that the soil does not bevome compacted. As each horizon sample is removed, remove the soil from the auger and put into a sample bag. Try to place as much of the soil as possible from the auger head into the bag, without losing any on the ground. If the horizon is deep, you may need to go into the hole more than once with the auger, and add the soil each time to the same bag for the same horizon. Once the entire horizon has been sampled, measure the diameter of the hole that the auger made, and the depth of the hole. Label the outside of the bag with the horizon name, diameter of the hole, and depth of the top and bottom of the horizon (the same way that the horizons are designated in the soil characterization protocol; that is, record where the top of the horizon starts, and where it ends. The top horizon should start at 0 cm). Repeat this procedure in 3 different holes, next to each other, so that you obtain 3 sample of the same horizon.
As soon as possible after taking the sample, bring it back to the classroom and weigh it in the bag. Record this field moist weight on the GLOBE Data Entry Sheet. If a drying oven is available, place the soil in a can or on a piece of paper and put in the oven at 105o until it is completely dry (this can be done by removing the sample from the oven and reweighing each day. It is dry when the weight remains stable and no longer changes). If no oven is available, carefully remove the soil from each bag and spread each sample on a separate piece of paper to air dry. Drying can go faster if the soils are left in a sunny window. Be sure to keep each sample labeled and separate from the other. Once the soil sample has been transferred to the drying location, weigh the empty sampling bag and record this value on the GLOBE Data Entry Sheet.
Once the soil is dried, obtain a weight of the dry soil and can or paper. Record this weight on the GLOBE Data Entry Sheet.
Once it has been dried and weighed, rocks need to be separated from each sample if they are present. Pour the sample into a #10 sieve (2mm openings). Before proceeding, weigh the empty can or paper which held the soil for drying, and record the weight on the GLOBE Data Entry Sheet. Next,carefully push the dried soil material through the mesh into the sieve bottom or piece of paper. Rocks will not pass through the mesh and will stay on top of the sieve. Remove the rocks from the sieve and their density calculated. If no sieve is available, carefully remove the rocks by hand.
Once the rocks have been separated, weigh them, and record this weight on the GLOBE Data Entry Sheet. Next, place 30 mL of water into a 100mL graduated cylinder, and one by one, add the rocks to the water. Read the level of the water after all the rocks have been added and record this value (and the original volume of water) on the GLOBE Data Entry Sheet.

OPTION 3: (If you can only take a sample from the soil surface)

Before making your bulk density measurement, obtain 3 soil cans to take 3 bulk density measurements for the surface horizon at each soil sampling site and clearly label them. Determine the volume of each can and record the result on the GLOBE data entry sheet. The volume of the can is determined by filling it to the top with water and pouring the water into a graduated cylinder to measure the number of mL (equal to cubic centimeters) of water that fills the can. Once the volume has been measured, poke a small hole at the bottom of the can with a nail, to allow air in the can to escape when soil is being pressed into the can. Weigh each can and record its weight and volume on the GLOBE Data Entry Sheet.
Proceed to the soil sampling site, and choose 3 locations close together to take surface soil samples. Be sure that these are close to the location where you performed your soil characterization protocol. Remove vegetation and other material from the soil surface. For each of the 3 locations, push a can with a known volume into the surface of the soil. (NOTE: The soil in the profile should be moist, so that the soil will stick together, and be easy to press the can into. If necessary, wait to do this procedure after a rain, or wet the soil before doing this measurement.), Be sure that the soil has filled the can by observing when you can see some of the soil poking throught the small hole in the bottom of the can. If it is difficult to push the can into the soil, you may need to use a hammer or other object to force it in.
Using a trowel or shovel, remove the can and the soil surrounding it. Trim the soil from around the can until it is flat against the edges of the can so that the volume of the soil is the same as the volume of the can. Once it has been trimmed, cover the can with the lid and return it to the classroom. Repeat this procedure at each of the 3 locations so that you have a total of 3 bulk density samples for the soil surface. Be sure to label the cans in the field with the horizon name, depth, and number of replicate (1, 2 or 3).
As soon as possible after taking the sample, bring it back to the classroom and weigh it. Record this field moist weight (including the can) on the GLOBE Data Entry Sheet. If a drying oven is available, remove the lids and place the cans in the oven at 105o until it is completely dry (this can be done by removing the sample from the oven and reweighing each day. It is dry when the weight remains stable and no longer changes). If no oven is available, carefully remove the soil from each can and spread each sample on a separate piece of paper to air dry. Drying can go faster if the soils are left in a sunny window. Be sure to keep each sample labeled and separate from the other.
Once the soil is dried, obtain a weight of the dry soil and can, and record this weight on the GLOBE Data Entry Sheet.
Once it has been dried and weighed, rocks need to be separated from each sample if they are present. Pour the sample into a #10 sieve (2mm openings) and carefully push the dried soil material through the mesh into the sieve bottom or piece of paper. Rocks will not pass through the mesh and will stay on top of the sieve. Remove the rocks from the sieve and their density calculated. If no sieve is available, carefully remove the rocks by hand.
Once the rocks have been separated, weigh them, and record this weight on the GLOBE Data Entry Sheet. Next, place 30 mL of water into a 100mL graduated cylinder, and one by one, add the rocks to the water. Read the level of the water after all the rocks have been added and record this value (and the original volume of water) on the GLOBE Data Entry Sheet.

Data Entry

When you are done, the following should have been recorded on your GLOBE Data Entry Sheet:

Depth and horizon information
Sample volume (mL)
Can/bag weight (g)
Wet weight (of the moist soil and can/bag) (g)
Dry weight (of the dry soil and can/bag) (g)
Rock weight (g)
Prerock volume (of the water in the graduated cylinder before rocks are added) (mL)
Postrock volume (of the water in the graduated cylinder after rocks are added) (mL)

Calculations

The sample volume for the can is just the can volume (found by pouring water into graduated cylinder); for the auger, the sample volume is:

         Sample volume =  pi x (inside dia./2)^2 x sample thickness (cm3)

The volume of rock, in terms of displaced water is:

           Rock volume = Postrock volume - Prerock volume (mL = cm3)

The bulk density (in units of g/cm3) can now be calculated for each sample by:

                         (Dry weight - Can weight) - Rock weight
       Bulk Density  =   ---------------------------------------
                                Sample volume - Rock volume

The soil water content (SWC) of the soil at the time of sampling is equal to:

                            Wet weight - Dry weight
     Soil Water Content  =  ------------------------
                            Dry weight - Can weight

For Advanced Students

Using the bulk density measurements, you may wish to also calculate the porosity (or percent pore space) in each sample. The porosity tells you the maximum amount of air or water that can be held in the soil sample. To calculate porosity, divide the bulk density by a value called the "particle density". Particle density is the density of the individual sand, silt, clay, or organic particles in the soil. For mineral particles (sand, silt, or clay) use a value of 2.65 g/cm3 For organic particles , use a value of 1.4 g/cm3

         Porosity (%) = bulk density / particle density

Design and Learning Approach

Hands-on protocol. Will help to integrate soils with other aspects of the ecosystem by better describing soil properties related to rooting depths, porosity, hydraulic conductivity, and others. Also allows conversion of units for modeling purposes, and scaling of soil properties to larger areas.

Cost Impact

may need to buy new soil cans in addition to those for soil mositure since a small hole will be placed at the bottom of these.
Sieve set (Forestry suppliers) $46.00; for this procedure, only need #10 (2mm mesh) sieve

Training Impact

Simple method. Needs to be added to the soil charcterization protocol for sampling and for lab measurements. Should only take a few extra minutes to add to the training schedule for soils.

Systems Impact

Adds one field to soils profile characterization data

Assessment Requirements

Students need to be evaluated as to whether or not they are doing the measurements properly, and whether they understand weight, volume, and density, and the relationship between them.

Last updated: 1/24/97 Comments? globe@hwr.arizona.edu