Introduction to Soil Chemistry

horizonation 7
or basic.Thus, the type and amount of salts present will affect a soil’s pH, which
will in turn affect the solubility and availability of both natural and contaminating
inorganic and organic compounds.
The species of components present will also be affected by
oxidation–reduction and pH. For example, iron is primarily in the Fe 3+ or the
Fe 2+ state depending on the oxidation–reduction potential of the soil. Speciation,
which depends in part on the oxygen status of soil, is of environmental
concern because some species are more soluble and thus more biologically
available than others. The occurrence of a specific species is related to the
chemistry occurring in a soil, which is related to its features. Thus, large
features must be considered when developing analytical and instrumental
methods.
1.1. HORIZONATION
The most striking large feature of most soils is the occurrence of distinct horizons.
For the analytical chemist, three soil horizonation conditions exist. Highrainfall
areas typically have tree or tall grass vegetation and extensive horizon
development. Low-rainfall and desert areas with sparse and desert vegetation
with little horizon development. Areas with rainfall between these extremes
will have variable vegetation and horizonation. It is not possible to draw sharp
boundaries between these areas because local conditions such as frequency,
time of year, and intensity of the rainfall will dramatically affect the climate
in transition areas. Each of these situations presents unique challenges to the
analytical chemist.
1.1.1. Horizon Development under Moist Conditions
When water falls on soil, parent material salts are dissolved. These salts are
leached into and eventually out of the soil. Plants grow, produce biomass,
exude organic matter (including acids), die, and thus add organic matter to
both the soil surface and subsurface. Silica and alumina, which are relatively
immobile, slowly dissolve and are eluted into the lower areas of a developing
soil. Organic matter is decomposed, mixed by macro- and microorganisms with
and leached into the soil. As this process continues, horizons develop in the
soil parent material and eventually a recognizable soil profile is produced (see
Figures 1.1–1.5). The depth of the soil and the number and types of horizons
that develop depend on the soil forming factors, the most active of which are
climate and biota, although parent material, as mentioned above, is also very
important [1,2].
Soil parent material is not always derived from the underlying rock. In some
cases rock is covered by other geologic materials deposited by ice (glacial till),
water (alluvium), wind (loess), gravity (colluvium), or a combination of these
transporting agents. A more complete list of transporting agents and the geo