What Is Present - Field Sampling: Principles and Practices in Environmental Analysis - page 220

This in turn leads to a complex solution of these components in water and the occurrence
of many unexpected components in the environment.
The inorganic phase is mostly silicon, aluminum, and oxygen. The organic fraction is
made up mostly of carbon, hydrogen, nitrogen, and oxygen. Neither of these
characterizations provides any useful information about the composition and activity of
soil, however. Even small amounts of clay particles, particularly the very active clay
fractions, can make a large difference in a soil's characteristics. This includes how easily
a soil is sampled. A sandy soil with small amounts of clay can be sampled using a core
sampler. Sandy soils without any clay must be sampled using a bucket-type sampler. The
same thing can be said for the other components in soil, including organic matter.
The soil solution is made up of water. In most cases it contains a complex mixture of
inorganic and organic ions and molecules and dissolved gases. Relatively small changes
in any of these constituents can have a pronounced effect on the soil solution's
characteristics. At pH 7 the soil solution contains 0.0000001 moles of H + per liter. * This
is not very much! A slight increase in H + to 0.000001 moles/liter makes the solution
acidic, while a change to 0.00000001 H + moles/liter makes the soil solution basic.
In like fashion, small changes in the gaseous content of soil can dramatically affect its
characteristics. An increase in the carbon dioxide concentration in soil air will change the
soil solution pH. It can also cause the precipitation of various ions as carbonates.
All of this leads to three important considerations in sampling. First, changing soil
conditions will change the amount of a component that is found. In some cases it may
even determine if a component is found or not. Because such a variety of components is
present, sampling must always be accompanied by controls. It is not good enough to
simply say that lead, chromium, arsenic, or any one of innumerable other inorganic and
organic compounds is found in a soil sample. The concentration at which it is found is
critical because it must also be determined if this level is natural for the soil or not. Is the
component significantly above background levels or not? Another important question
concerns the biological availability of the component. Is the component in a form that is
biologically available or unavailable? Only when these questions have been answered can
one begin to assess the need for further sampling and, potentially, remediation.
9.1. IN TOTAL
The three spheres of the environment have very different compositions when looked at
from the perspective of gas, liquid, and solid phases. The atmosphere is predominantly a
gaseous phase. Relatively small amounts of liquids and solids are suspended in it. The
hydrosphere is predominantly a liquid phase. It has variable amounts of salt, relatively
small amounts of gas dissolved in it, and small amounts of solid suspended in it. On the
other hand, soil has relatively large amounts of gaseous, liquid, and solid phases in
intimate contact with each other.
* It is more accurate to call this the hydrogen ion activity in soil rather than concentration;
however, for our purposes here concentration illustrates the point better.