Geoscience Reference
In-Depth Information
basics. The subsequent chapters trace soils as a material entity and soils in the mind
through historically and geographically distinct cases. The
nal paragraphs discuss
implications for sustainability.
3.2 A Soil Science Primer
Soils are varied and manifold. According to the most widely accepted attempts at
classifying them, 12 soil orders and a multitude of sub-orders and classes can be
discerned.
8
While the air can be viewed as largely homogeneous, and its origin is of
little concern for historians, one must understand the formation of soils, because
their resulting qualities are so different and because cultivation is a major factor
interacting with soil development.
The natural history of soil is called pedogenesis, an evolutionary development of
soils over time which was
rst described fully by the Russian soil scientist Do-
kuchaev in the late 19th century.
9
The human history of soils is the history of their
cultivation. Taken together, natural and human histories create the history of human
interaction with soils, their environmental history.
Hans Jenny
rst detailed the factors of soil formation: climate, parent rock
material, topography and organisms interact to form soils.
10
Soils form the surface
layer of the earth in a range from several meters thickness to a few centimeters.
11
There is no single de
nition of soil that all soil scientists accept, but most would
agree that soils are three-dimensional entities composed of mineral and organic
matter, with their own architecture comprising micro- and macropores through
which water and air circulate, and particles of different sizes and surface textures,
which form a multitude of quite different habitats for microbial and macroscopic
soil organisms. Particle size is an important soil characteristic, with sand, silt and
clay being the three categories most often discerned in order of decreasing particle
size. A typical soil (if such a thing exists) consists of roughly 25 % each of air and
water, 45 % mineral particles, and 5 % soil organic matter (SOM), most of which is
comprised of large organic compounds called humus. The rest of SOM is roots and
soil organisms. Processes in soils can be physical (such as aggregate formation),
chemical (such as nutrient dissolution and leaching) or biological (such as earth-
worm digestive action). Taken together they control a major part of global bio-
geochemical cycles, in particular the cycling of reactive nitrogen, and of carbon and
its compounds.
Soil processes (in all three senses) depend very much on surfaces, and many
involve exchanges at active surfaces such as clay minerals offer. The origin of life
8
Blaser (
2004
).
9
Evtuhov (
2006
) for a longer history of pedology see Feller et al. (
2006
).
10
Jenny (
1941
) Compare the overview about soils in: Muoghalu (
2003
).
11
For example, Pidwirny and Heimsath (
2008
).
