Introduction to Soil Chemistry

problems 89
blocks, and thermocouple psychrometers are usually installed in the field and
measurements are taken on a regular basis. Neutron probe and time-domain
reflectrometry (TDR) equipment are usually carried to the field each time a
measurement is made. In addition, the neutron probe requires an access hole
into which it is lowered to determine the of water content. These methods
provide data that are seldom generally used in soil analysis and will not be
discussed further [11–16]. Additional information about these methods can be
found in the text by Brady and Weil [17].
4.14. CONCLUSION
The compounds contained in soil air are basically the same as those in atmospheric
air, but are more variable.Also, the volume of soil occupied by air varies
greatly. Water is a unique molecule in both its physical and chemical characteristics.
It has higher than expected boiling and melting points and can dissolve
a great variety of compounds. In the soil it is even more unique in
that it occurs in the liquid, gaseous, and solid (frozen) states.The water content
of soil is highly variable, ranging from air dry, with as little as 1% water to
saturated where all void spaces are filled with water. The soil solution
contains many inorganic and organic compounds ions and gases, the concentration
of which changes dramatically when soil water content increases or
decreases.
In the laboratory soil water content is measured by drying in the oven and
with pressure plate apparatus. A number of different field measuring methods
are used mostly to determine the amount of water available for plant use.
Drying soil can change the form and species of components present, and for
this reason, most soils are dried carefully at or only slightly above room temperature
before analysis.
PROBLEMS
4.1. Describe the differences between atmospheric air and soil air.
4.2. Soil water can be thought of as existing as layers around soil particles.
Explain how these layers are differentiated.
4.3. Explain how pores can affect the composition of both the soil atmosphere
and the soil solution.
4.4. Using equations, illustrate how soil carbon dioxide affects both the pH
of soil and the ions present in the soil solution.
4.5. Give both general and specific examples of how organic compounds can
lead to the formation of cation exchange sites in soil.