Characteristics of the Environment - Field Sampling: Principles and Practices in Environmental Analysis - page 35

commonly called pine) forests. These soils have a subsurface horizon called a spodic
horizon in which aluminum oxides and organic matter have accumulated. Iron oxides
may or may not be included in this area. The occurrence of iron and aluminum oxides
and organic matter can affect analytical procedures applied to samples from this horizon.
This is thus an important horizon that requires special consideration during sampling and
analysis.
2.3.7. Other Soil Orders
Each of the other soil orders has its own unique characteristics, but these do not influence
sampling procedures as directly as the orders mentioned above. Their various unique
horizons, such as the Bt, however, require special attention. Persons carrying out the
sampling need to know what soil orders or soil types are present. Local soil scientists can
provide needed information about the local soils and their characteristics, which will
affect the sampling plan and analytical methods [16].
2.4. SOIL TEXTURE, STRUCTURE, AND BULK DENSITY
Knowledge of soil texture and density is essential in field sampling. Texture can be
described using either soil science or engineering descriptions. Density is described as the
mass per unit of dry soil, Mg/m 3 or g/cc, and is universally the same.
2.4.1. Texture
The two different methods of describing soil texture depend on whether the soil is to be
used for agriculture or engineering. In agriculture it is used for growing plants, and in
engineering as a medium to support structures. Soil scientists are most interested in the
relative amounts of sand, silt, and clay, as well as the biological characteristics of soil.
Engineers are interested in the characteristics of larger stone and gravel, sand, and fine
particles. Each of these components is called a soil separate and is defined as having a
particular size. (See Table 2.1.) The soil in a field to be sampled may be described by
either of these approaches, so it is important to be familiar with both.
2.4.1.1. Texture—Soil Scientist Definition
Soil scientists name soil textures using the percentage of sand, silt, and clay present. Sand
is between 2.00 and 0.02 mm in diameter, silt between 0.02 and 0.0002 mm in diameter,
and clay less than 0.002 mm in diameter. Using the percentages of sand, silt, and clay and
a textural triangle (see Figure 2.5), a