Soils & Soil Physical Properties
Part 2 – 10 | Unit 2.1
E. Soil Classification: 12 Orders
- Soil scientists have come up with systems for classifying soils, in much the way plants
and animals are classified. There are currently 4 main classification schemes: Russian,
FAO, Canadian, and Soil Taxonomy (Euro-American in origin, but used worldwide). Soil
taxonomy is similar to plant and animal classification in that the system is based on
genesis—how it is thought the soil developed, similar to the evolutionary classification of
plants and animals. Also, like plant and animal classification systems, soil taxonomy is not
static. As more is learned, the system changes. - The highest category of this system is the Orders. There are 12 soil orders (see u Table 2.4,
12 Orders in Soil Taxonomy).
Lecture 1: Soils—An Introduction
u TABLE 2.3 | COMMON POPULATIONS OF SOME SOIL MICROORGANISMS
ORGANISM NUMBER PER GRAM OF SOIL
Bacteria 108 –10^9
Actinomycetes 107 –10^8
Fungi 105 –10^6
Algae 104 –10^5
Protozoa 104 –10^5
Nematoda 10 –10^2
u TABLE 2.4 | 12 ORDERS IN SOIL TAXONOMY
Alfisols form in areas with low rainfall, but wetter than deserts
Andisols form in volcanic ash
Aridisols form in deserts
Entisols young soils (form in recently active areas, such as floodplains and mountains)
Gelisols form in very cold climates, with permafrost near the surface
Histosols soils very high in organic matter, common in wetlands
Inceptisols fairly young soils, but with more soil development than Entisols
Mollisols form in grasslands (such as the Midwestern prairies), have thick, dark, fertile soil
Oxisols old soils formed in the tropics, have very low fertility
Spodosols generally form in temperate coniferous forests, have very low fertility
Ultisols form in humid temperate and tropical regions in older landscapes, are highly acidic
with low fertility
Vertisols soils rich in clay, which causes them to swell when wet and shrink (causing large
cracks) when dry
Animals are classified first by kingdom, then phylum, then class, and so on down to species.
Similarly, soils are classified first by order, then suborder, great group, and on down to
series, the soil equivalent of species. Soils in a series have horizons that are similar in their
key characteristics. Series names are usually taken from local geographic features or place
names. There are over 20,000 recognized soil series in the U.S.