Characteristics of the Environment - Field Sampling: Principles and Practices in Environmental Analysis - page 41

A and underlying B horizons. Good water-stable peds result in a soil having increased
percolation and aeration rates, making it easer to work with and easier to sample and
remediate [19].
The increased rates of percolation and aeration are a result of increased porosity in the
soil. Pores occur both within peds and within the lines of weakness between peds. Some
pores are large and drain readily, while other smaller pores retain water, which is used by
plants. Still smaller pores remain filled with water even under the driest soil conditions.
More information about pores and their effect on water movement is given in Chapter 7.
2.4.3. Bulk Density
Both soil texture and structure are related to a soil's bulk density. Because bulk soil is
composed of both solid and void space or pores it has a variable density, which is
specifically called its bulk density. Bulk density is the dry mass of oven-dried soil
divided by its volume. Typically the density is obtained by inserting a ring of known
volume into the soil using an instrument that does not cause compaction of the sample
being taken. The ring is removed and the soil in it leveled. The ring and the soil are
placed in a oven at 105° for 24 hr.
The ring plus soil is weighed again, and the weight of the ring is subtracted from the
total to give the soil weight. (Equations 2A and 2B in Figure 2.7 are used for these
calculations.) At this point one need only divide the mass by the volume of the ring to
obtain the bulk density. (Equation 2C is used for this calculation.) Bulk density is usually
obtained as grams per cubic centimeter. Soil bulk density is most often reported as
Mg/m 3 , however.
Typically soil bulk densities range from 1 to ~1.7 Mg/m 3 . Sandy soils generally have
higher and silty and clayey soils lower bulk densities. This is variable, however,
depending on the structure and compaction of the sand, silt, and clay. Subsoils have
higher bulk densities than surface soils, partially because of lower organic matter content
and pressure from overlying soil. Examples of common bulk densities, their associated
void space, and associated soil types and conditions are given in Table 2.4.
The void volume in soil is determined using the average particle density of individual
soil particles. On the basis of a great many measurements, soil scientists take the particle
density of soil to be 2.65 Mg/m 3 . The amount of solids in a soil sample is calculated by
dividing this into the bulk density. Subtracting this from 1 gives the amount of void
space. Multiplying these by 100 gives the percentage of each. Equations for these
calculations are given in Figure 2.7, Equations 2D and 2E.
Knowing the bulk density of soil in a field we can make many useful and important
calculations. Calculations of the kilograms or tons of soil that must be removed or
remediated can be determined, as can the volume of soil. Such calculations will also
allow mass balance calculations, which allow accounting for all the contaminant present.
In field sampling