Analytical Methods:Bulk Density

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Bulk Density
Bulk density is the mass/volume in soil science it refers to the mass (weight) of a standard volume of soil.

Questions
Because of differences in the porosity of soils and sediments and in the mass (weight)between organic and mineral components the density of different soils can vary widely. Most laboratory analyses are carried out on dried soils that have been broken down into their constituent parts and then sieved to remove all particles larger than 2 mm. Concentrations expressed relative to soil weight cannot be directly related back to the amount of that component in the soil layer. For example knowing the a soil contains 400 mg kg-1 of Ca doesn't allow you to calculate how much calcium is actually in a single deposit, and thereby how much shell or bone has been added. If the depth and extent of the deposit, and it's bulk density are known, this sort of calculation can be very easily made.

Bulk density is affected by compaction, either through trampling or burial. Measurements of bulk denisty, therefore, are important for monitoring the impact of visitors and developments on archaeological deposits. As bulk density increases porosity decreases, hence the amount of rainfall that the soil can absorb also decreases. Following heavy rain, theredore, more water runs-off the surface of the soil and this can result in accelerated soil erosion.

Case studies where bulk density has been used in geoarchaeological studies include:

Sampling
Bulk density can be measured in the field using a cone penetrometer or gamma radiation meters. However, if such equipment is not available it is a very simple procedure to measure bulk denisty in the laboratory. All that is needed is a sample of soil of known volume. The easiest way to take a sample is using a square tin such as a kubiena tin. The sample should be carefully cut from the soil gradually working the tin over the block, if the tin is hammered into the sample substantial compression can result. Porosity in soil is variable and compaction can be highly localised, replicate samples should be considered to account for this where necessary.

Analysis
In the laboratory the mass (W) of the soil block should be measured in grams. Density is the mass divided by the volume (cm3) of the sample (length x width x height); the units are g cm-3. This will give the field moist bulk density. The sample can also be oven dried (105oC) and weighed to give the air-dry bulk density, to correct for changes in soil moisture over time or across a site.

Data and interpretation
Sampling is key to estimating bulk density. Unfilled tins, large voids, and large stones can bias measurements. The reliability of individual measurements of bulk density should be considered when interpreting the data. When attempting to reconstruct loadings to soils the importance of post-depositional processes such as leaching should also be considered as these have the potential to significantly alter chemical concentrations once in the soil.

Related Techniques

 * Soil Moisture

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