The chemical
composition of soils can be stated to consist of just 8 major
elements: O, Si, Al, Fe, Ca, Mg, Na, and K. Yet, given the abundance of
elements in the Universe, this is an unusual composition for something that
seems so common place.
In this lecture, we examine the processes that formed the mix of elements
in the universe, the processes that differentiated this mix to form the earth
and the solar system, and the subsequent re-organization of elements into
the core, mantle, and crust. Finally, from a pedological perspective, the
processes of chemical weathering and organic matter additions by plants generally
deplete soils in Si, Ca, Mg, Na, and K and enrich it in C, N, S, ....We will
review figures from our reader on how the earths water, plants, and soil chemistry
relate to that of the crust to illustrate the importance of these processes.
One concept we introduce in this discussion is the importance of "normalizing"
chemical data to an "immobile reference element" in order to gain a quantitative
measure of gain or loss during a chemical transformation.
We discuss that the earth
is greatly depleted in C, N, S, etc - the elements of life. In fact, recent
research suggests that these elements would not be present in appreciable
quantities at all without cometary impacts late in the accretion of earth.
A very approachable, and colorfully illustrated, paper that discusses this
is found in a recent article in the
American Scientist (2001, 89:432-442).
A web
connection to a an interactive periodic table
is on our web site. This provides a wealth of interesting concepts about the
elements. Most importantly, the periodic table (and the abundance of elements
on the table) represents a history of the Universe and in a general way,
some insights into how old it is.
The earth (and soils) , because of its overwhelming
dominance of O, Si, and Al, must be ultimately understood by considering the
way these elements combine into minerals
(orderly and repeatable assemblages of elements), and ultimately into
rocks (assemblages of minerals). Soils are dominated by
silicate minerals , and we will spend future lectures discussing the
classification of both primary
(formed by geological processes) and
secondary (formed by weathering) silicates - and their properties (especially
the secondary minerals).
