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Soil bulk density (BD)
Soil BD is an important indicator of soil Structure. It affects infiltration, rooting depth, available water capacity, soil porosity, and aeration, availability of nutrients for plant use, and activity of soil micro-organisms, all of which influence key soil processes and productivity.
Soil BD is calculated as the dry weight of soil divided by its volume. This volume includes the volume of soil particles and the volume of pores among soil particles. Bulk density is typically expressed in g/cm3.
Modified slide (from Camp and Lund).
Assessment of Soil Bulk Density (BD) by the Common Method:
In this chapter, I will present the common method used for assessing the soil BD when researchers are sampling so many cores without using up valuable oven drying space to dry the complete BD soil cores. Also, this method helps researchers to archive soil from the sampling location that was not oven dried to preserve chemical and biological characteristics.
The standard technique that the researcher can find the oven-dry weight of our BD sample, as follows:
Weigh the air-dry sample (a),
Take a subsample and weigh that subsample and record its mass.
Archive the main air-dried BD sample,
Dry the subsample until it no longer changes weight and records this mass,
Determine the gravimetric water content which is equal to ((initial subsample mass - dried subsample mass) / dried subsample mass) this is the mass of water/mass of dry soil (b),
Use this information to find the dry weight of the BD sample (x) using the following equation
x = a/(1-b), where
x is the dry weight needed for determining BD,
a is the wet weight of the bd sample (i.e., the mass of the air-dry bd sample from step 1), and
b is the gravimetric water content which is the mass of water in the subsample divided by the mass of dry soil in the subsample).
Now we have the dry weight needed to calculate the BD using the field volume of the sample.
This equation is derived from a = x+xb where a = wet mass, x = dry mass, b= gravimetric water content (i.e. ratio of water mass to dry soil mass), and x times b gives the mass of water).
Solving for x gives the equation x = a/(1-b))
The above equation solves the problem of preserving the bulk of the sample for archiving. This also will allow us to process our samples in the convection oven more rapidly as we will be able to put more samples in the oven and it will take much less time to dry the samples.
Therefore, we can test to see how much time is needed for samples to dry by weighing periodically until the sample no longer changes and then we can use this as a standard drying time (plus a little buffer time to be conservative).
However, another approach is to use a microwave for drying the subsamples. One can search on youtube for a video describing the microwave technique.
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