Agriculture Reference
In-Depth Information
CHAPTER
7
EXTRACTION
Soil analyses involving instrumentation generally have extraction procedures
associated with them. Thus, in addition to the references cited below and listed
at the end of this chapter, additional understanding of extraction procedures,
how they are designed and used, can be found along with the descriptions of
spectroscopic and chromatographic methods and the corresponding refer-
ences cited and listed in Chapters 8 and 9.
As has been pointed out in previous chapters, soil is a complex mixture of
inorganic and organic solids, aqueous and gaseous solution, and suspension of
inorganic and organic ions, molecules, and gases, which can be sorbed, dis-
solved, or free. In most cases analysis for a particular component first involves
isolation of that component from all the other myriad soil components, specif-
ically, the soil matrix. Isolation may involve physical separation such as pre-
cipitation or distillation or, more often, as a first step, after sieving is extraction
of soil using an appropriate extracting solvent or solution. Once isolated,
the component can be directly or indirectly measured. Direct methods
usually involve spectroscopy, while indirect methods involve production of
a colored product by reaction with an applicable reagent and carrying out a
colorimetric measurement.
Both direct and indirect measurement involves the preparation of a stan-
dard or calibration curve. With an accurate calibration curve, the concentra-
tion of the component can be determined taking into account interferences,
dilution, and extraction efficiencies.
7.1.
ISOLATION
Physical methods of separation and isolation of components from soil are com-
monly used in soil analysis, for instance they are particularly useful in nitro-
gen compound determination, as described in Chapter 6, and by head space
analysis, as described below.
