Laboratory soil characterization studies for archaeology are
used to 1) verify and supplement the field morphology of soil
profiles and 2) develop specific interpretation based on various
chemical, physical, and mineralogical properties of soils.
Particle Size Analysis
Particle size distribution is one of the key laboratory
analyses for soil interpretation of archaeological sites. In
addition to being one of more stable properties of soils,
particle size is used to correlate archaeological levels,
evaluate soil formation intensity, and detect
discontinuities in soil profiles. Particle size analysis
includes determining the coarse fragments (>2 mm), sand
(2.0-0.05 mm), silt (0.05-0.002 mm), and clay (<0.002 mm).
Methods to determine the particle size distribution in soils
include sieving for the coarser separates (>0.05 mm) and
hydrometer and pipette for the finer fractions (<0.05 mm).
The soils at al-Mudaybi’ were nearly all
very silty, with average sand, silt, and clay percentages at
15%, 60%, and 25%, respectively. The coarse fragments shown
in the profile in
Figure 1 
have resulted from the collapse
of walls; this was a major component in many profiles at al-Mudaybi’.
The high content of silt is very characteristic of soils
developed on loess or wind-blown silts.
Organic matter
Organic matter and iron compounds are the most important
coloring materials or paints for soils and sediments
throughout the earth. Organic matter accumulation in surface
soils results from the decomposition of vegetation and other
organic remains. In desert or near-desert areas, the lack of
vegetative cover results in low content of organic matter
and subsequently light-colored soils. Refer back to Figure 9
to observe the differences in soil color on Karak Plateau
resulting from rainfall variations. The organic matter of
surface soils at al-Mudaybi’ is usually < 0.5%; however, the
buried former surface soil horizon shown in Figure 10 had
>1% organic matter. Organic matter is determined by
analyzing for organic carbon content. The common methods for
organic carbon are (a) wet combustion by an acid dichromate
solution and back titration with ferrous ammonium sulfate or
(2) dry oxidation of carbon in a furnace and analyzing for
evolved CO2.
Soil pH
Soils that are acid will have pH values < 7.0 and those
soils that are alkaline have a pH > 7.0. Because of the
limited rainfall to leach bases out of the soil (e.g. Ca,
Mg, Na) on the Karak Plateau, most of the soils will have pH
values > 7.0. The profile shown in Figure 10, for example
had pH values >7.5 on the surface. The soils also have free
carbonates throughout the profile which is typical of most
arid soils. Carbonates in the dust also recharge the soils
with bases with time in many desert regions.
Elemental analysis
As a result of the recent method of rapidly evaluating the
elemental composition of soil with inductively coupled
plasma atomic emission spectrometer (ICP), the content of
elements such as P, Ca, Mg, Na, Pb, Cu, Mn, Ba, and others
are commonly determined on soils at archaeological sites.
Elemental composition of soils has been used for decades in
soil archaeological studies; P has been especially useful in
evaluating the intensity of occupations of many
archaeological sites. Phosphorus is rather immobile in many
soils and is an important constituent of waste products for
habitation; thus this elemental provides a good elemental
index for site evaluation.
The phosphorus content in the profile shown
in Figure 10 was 109 mg/kg in the surface and dropped to 5 mg/kg
at 50-66 cm. The buried surface horizon that represents the
living surface about 700 years B.C., however, had 307 mg/kg of
extractable P. This increased P is the result of waste products
produced by the people living in this area during the Iron Age.
