Introduction to Field Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 7

TABLE 1.1 The Twelve Soil Orders and Some Important Characteristics

Soil order Characteristic
Entisol No horizon development
Inceptisol Minimal horizon development
Andisol Developing from volcanic ash
Histosol Organic soil
Gelisol Frozen soil
Aridisol Desert soils
Vertisol Soils developing large cracks when dry
Alfisol Horizon development under trees
Mollisol Horizon development under grass
Ultisol Horizon development under tropical and subtropical trees
Spodosol Horizon development in sandy soil under coniferous trees
Oxisol Most highly weathered

They are given generally in order from least to most developed—that is, from those
orders showing the least to the most horizon development [3].

1.2.2. Engineering Description
The engineering description of soils is very different from that of soil science given
above. For engineering purposes the carrying capacity, shrink-swell, and plasticity
characteristics of soil are more important than its chemical or biological characteristics.
The engineering classification gives more emphasis to larger components such as gravel
and its characteristics, and less to erosion and crop production characteristics. Engineers
describe soils using a two-letter designation that provides an indication of its
composition. Thus a soil composed of well-graded gravel would have the designation
WG, while a soil composed of clays with high plasticity would be designated as CH.
A soil's plasticity and liquid limit play a large role in its engineering characteristics.
The plastic limit is the moisture level when the soil just becomes plastic; that is, it can be
molded into a shape and retain it. The liquid limit is the moisture level when the soil
becomes liquid, has a loose consistency, and will run.
A more detailed description of soil horizons, underlying materials, and engineering
characterizations will be presented in Chapter 2. The importance of these horizons and
ways of describing soils in developing a field sampling plan will also be discussed.