What Is Present - Field Sampling: Principles and Practices in Environmental Analysis - page 226

pH. Normally soil pHs do not go above 9, thus higher pHs may indicate that there are
high levels of the alkali metals and that something unusual is happening and needs to be
investigated further.
Aluminum is associated with acid soil conditions. Soils with low pHs will be found to
have high levels of available Al 3+ . This is of concern because this cation is toxic to most
plants. The exceptions are the so-called acid-loving plants, such as rhododendron and
blueberries [13].
Elements found in multiple oxidation states will generally be found in the state
appropriate for the conditions existing at that time. Under reducing conditions iron will
be found as Fe 2+ , and under oxidizing conditions it will be in the Fe 3+ oxidation state.
This is generally true for all the cations, which exist in several oxidation states (i.e., lower
oxidation state under reducing conditions and higher oxidation state under oxidizing
conditions). Manganese is found as 2 + , 3 + , and 4 + in soil and acts as if it is between

oxidation states because of this mixture [14].
Another exception is molybdenum (Mo). It is an exception not because of multiple
oxidation states, but because it is found as an oxyanion, molybdate (MoO 4 2− ), rather than
a cation in soil [15].
When sampling and analyzing soil it is important to keep the oxidation states of the
metals of interest in mind. Samples from reducing conditions will typically show higher
levels of soluble transition metal cations than those taken from oxidizing conditions. A
second component of this is that during the sampling, samples are usually exposed to
oxygen. Such exposure can lead to changes in the oxidation states of the metals the soils
contain. This would also happen if these samples were exposed to air during storage or
transport. Oxidation at any step can have a pronounced effect on the analysis and the
apparent biological availability of a metal.

9.2.4. Nonmetal Cations
There are two common nonmetal cations found in soil. The first is the H + , a proton,
which does not really occur as H + but as the hydronium ion H 3 O + . The second is
ammonium Both of these can be important indicators of unusual soil conditions.
The hydronium ion is associated with acidity in all media and is only present in
appreciable amounts when the pH is acidic; that is, below 7. Because of its size and
hydration it does not fit easily into any scheme designed to describe cations in soil.
Soil pH is rarely below 4.0, thus when pHs occur appreciably below 4 they indicate an
unusual situation. Usually such low pHs are associated with some cold region forest soils
or with sulfur and its oxidation to sulfuric acid. This is particularly likely in and around
mining operations, in which sulfur is brought into oxidizing conditions, especially when
it is mixed with soil or water. Whenever very low pHs are found in soil, the source
should be found, because low environmental pHs indicate a serious problem that needs to
be found and corrected. Very low pHs can solubilize otherwise inert toxic minerals and
can ultimately lead to the total destruction of the soil itself.
The second common nonmetal cation is ammonium Ammonium is important
because plants use it as a source of nitrogen for plant growth and development. Ammonia
is also important because it can be oxidized to both nitrite and nitrate.