Laboratory Studies
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 this photo 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.
Profile 3
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.