Introduction to Soil Chemistry

soil chemistry, analysis, and instrumentation 19
A landscape will contain many different soils, which can be expected to
change with position in the landscape, vegetation, slope, climate, and parent
material. It is also to be expected that the soil will change only over longer
periods of time, namely, decades and centuries. However, groupings of soils
can be and are made for all soils on this planet, and all soil types can be found
on all continents except Antartica. Ultisols, Mollisols, Alfisols, Spodosols, and
other soil types occur in North and South America, Asia, Europe, and Africa,
Australia, and New Zealand. Thus researchers and analysts all over the world
need to know about soils, soil types, and their nomenclature.
1.6. RELATIONSHIP OF LARGE FEATURES TO SOIL CHEMISTRY,
ANALYSIS, AND INSTRUMENTATION
Each topic discussed above provides a great deal of information about the soil
and its chemistry. This information is invaluable to the soil chemist, the person
developing a soil analytical procedure, or a person wishing to make an instrumental
analysis of some soil characteristic or component.The discussion below
should not be taken as exhaustive, but only as providing examples of the types
of information available and why knowledge of the large features of soil are
important in understanding its chemistry and analysis. The following chapters
will refer back to these characteristics where appropriate.
1.6.1. Soil Types
What do the names Alfisol and Mollisol tell us about the chemistry of these
soils? Alfisols develop under humid climates and thus are acidic and have a
medium base saturation. All the easily leached salts have been removed.
However, basic parent materials often underlie them. Mollisols develop under
lower rainfall conditions than do Alfisols and so have higher pH levels and
higher concentrations of more easily leached salts, and—although they contain
significant levels of calcium—they are still often slightly acidic. Mollisols have
higher organic matter throughout the upper portions of the soil profile than
do Alfisols. Both soils have well-developed B and Bt horizons.
Knowing which of these types of soils is present tells us much about its
chemistry, the likely pH or pH range of the soil, and its salt content The occurrence
of clay, especially in the Bt horizon, will affect the availability and solubility
of both inorganic and organic components. For instance plant available
phosphorus is decreased by low (acid; pH < 7) and high (basic or alkaline;
pH > 7) soil pH. Likewise, the type of clay present will dramatically affect the
extractability of a soil component or contaminant. Clay type and its chemistry
will be discussed in Chapter 2.
Ultisols and Spodosols develop under different conditions but have some
important similarities. Ultisols are the ultimate in soil development. Salts have
been leached out; however, they are generally considered soils having