Environmental Engineering Reference
In-Depth Information
45-65 % sand, and 0-20 % silt. Textural classes can be identified in the field using a
flow chart (Fig.
4.9
). The process is difficult with very dry samples so it is helpful to
moisten a dry sample with a spray bottle.
This flow chart is intended only for soil materials that are predominately
mineral. For organic soil materials, see
Soil Organic Matter
later in this chapter.
A difficult call, even for an experienced soil scientist, is for mineral soil materials
enriched with organic material to the extent that it displays characteristics of
organic material. In that case, the mineral texture is assigned a mucky modifier,
for example, mucky-modified silt loam. This issue is also addressed in
Soil Organic
Matter
.
The most important mineral soil texture separation is between loamy fine sand
and loamy very fine sand as this break determines which field indicators can be used
for identifying the soil as hydric. Based on the diagram, a soil that does not ribbon is
generally sandy (loamy fine sand or coarser) and those that do form a ribbon are
loamy or clayey (finer than loamy fine sand).
4.7 Soil Structure and Bulk Density
In general, mineral soil particles do not occur as independent units. Instead, multiple
particles are grouped into secondary units called peds or aggregates. This aggregation
is promoted by oxidized Fe, organic matter, and physical forces associated with
wetting and drying cycles, freezing and thawing cycles, or vehicular traffic. Soil
structure refers to the shape and distribution of peds and the resistance of peds to
physical change. Examples of structural shape classes are presented in Fig.
4.10
.
Structural units are also rated for strength and are reported as weak, moderate,
or strong.
Bulk density is defined as soil dry weight per unit volume. Sand has a higher bulk
density than clay. Organic soil materials have a lower bulk density than mineral soil
materials. O horizons have lower bulk densities than mineral horizons, and an A
horizon generally has a lower bulk density than the underlying B horizon. Bulk
density decreases as porosity (% pore space by volume) increases. Compaction is an
increase in bulk density; it can be caused by vehicular traffic or by long-term
inundation. Structural classes are associated with general ranges in bulk density.
For example, granular structure is prevalent in A horizons and is promoted by
organic matter; it is associated with low bulk densities. The single grain class is
associated with sandy materials, C horizons, and high bulk densities. Blocky
structure is associated with B horizons enriched with clay and intermediate bulk
densities.
Use of the Field Indicators does not require familiarity with structure or bulk
density. However, as addressed later, knowledge of these soil characteristics are
critical to the installation of monitoring wells and the assessment of wetland
hydrology as they can significantly impact the flow path of water in soil. Figure
4.11
shows the impact of structure on percolation of water through soil.
