Alpine Archaeology: Soil Chemistry Theory and pH Testing
Fig. 1 Soil Range of typical pH values with local Alpine juniper and mountain azalea
preferences in local geosols (plant habitat can be a good indicator of soil pH)
Fig. 2 Juniperus communis alpina Fig. 3 Rhododendron ferrugineum ssp.
Fig. 4 Patrick Hunt at Grand-St-Bernard Pass in alpine Roman road
excavation during 1997, initial phase around 2400 meters elevation.
Note soil color contrasts from topsoil with dark vegetative root staining
to more ochrous colored soil at 5 cm depth. )
For the last ten years our Stanford Alpine Archaeology Project has been conducting soil chemistry research as another method of characterizing archaeology microcontexts horizontally and microstratigraphy vertically. As director of the project, my premise is that most of the existing methodologies for distinguishing microfeatures in archaeology are visually dependent on physical characteristics. Some of these distinguishing factors include soil color (Munsell Charts), soil granularity (Wentworth-Udden grain size indices), and other somewhat quantifiable parameters such as soil texture as a function of particle symmetry or asymmetry along with homogeneity of grain size or lack thereof, soil compression as a function of density in cubic centimeters and soil moisture as measurable water content. Pedology or soil science as a separate discipline is well-researched with many directions in agriculture, forestry management and similar fields of study, and pH analyses is one of many such pedological evaluative instruments. This brief report explores this field application of pH field testing in experimental archaeology, especially in the Alps.
But utilizing soil chemistry as a further method to distinguish microcontexts and microstratigraphy in archaeology is, while uncommon, potentially beneficial for adding an invisible characterization to visible characterization. Taken all together, when reinforcing other characterizing methods, pH soil chemistry as one of many field tests has an immediate theoretical advantage in that it provides another method to complement, refine or even negate prior analyses in ways that merely visually-dependent field methods cannot.
Flourishing plant habitats can often be good indicators of immediate local pH, as both alpine juniper (alkali-loving) and alpine azalea (acid-loving) are found in abundance about 300 m below our excavations, whereas the immediate excavation site soil context is very close to a neutral 7.00 pH mostly derivable from the local schist geosol pH combined with organic matter. Plants have long been recognized as clues to surface geology as well since calcium carbonate (e.g., limestone) will yield an alkali pH whereas granitic rock will often yield an acid pH. Both rock types are present in the alpine region of the Grand-St-Bernard pass but not in our immediate excavation vicinity.
Recent history of pH testing in Archaeology
I first began suggesting and researching pH testing while at the Institute of Archaeology, London, in the 1980’s and began experimentation with pH testing in 1993 in the Junipero Serra Park Project for the County of San Mateo. As field use was streamlined and incorporated into analyses, it was also found useful for short-term cultural resource management projects for the Stanford Management campus property consultancies between 1994-96 and subsequently elsewhere in the world including the Stanford Alpine Archaeology Project between 1994 to the present. Still theoretical, it has nonetheless yielded important new data in the field with portable pH testing equipment and is now a component of all my archaeological field research where applicable for soil chemistry.
Use of pH testing in archaeology is not limited to my own field experiments in the Alps and elsewhere. It has apparently been used, for example, in Britain, Central America and other fieldwork for some time and soil chemistry is but one of many analytical tools in archaeology: “Activities performed over long periods of time tend to leave soil chemical residues as evidence of those activities. Some of the questions studied in this paper deal with the interpretive capabilities provided by chemical patterns.” (1) Use of pH testing in archaeological field conditions has even been attempted for a possible explanation of why bones might survive better under certain soil conditions than in others. (2) In a superb text dealing with a broad spectrum of field analyses Shackley posits that a high pH was destructive in diatom sampling of a Saxon-related context in Romney Marsh (but without providing the exact pH testing methods other than in indirect citation of field reports). (3) In an edited text on house activity areas, pH is one variable out of many used for “untangling complex formation” and “recognition of composite signatures”. (4) Application in archaeology can be useful “when a genuine measured value is called for [where] it must be carried out using calibrated meter” as “one of the many stratigraphic characteristics that may be needed as part of the unraveling of site formation processes.” (5) Nonetheless, few excavation projects take soil chemistry seriously or use pH analyses for macro or especially microstratigraphic differentiation where it could be useful.
Fig. 5 Scale of pH in natural contexts
Definitions of pH, geosols and archaeosols
Some technical definition is necessary. What “pH” actually measures is “p” as a coefficient of the “H” (Hydrogen) on a scale generally from 0-14 where pH is the negative logarithm of hydrogen ion concentration (although pH can actually be negative and thus below 0; this would be extremely unusual in archaeological soils). A reading of around 7 is neutral, 2-6 is acidic and above 7 (normally to 12 although the scale goes up to 14) is alkali (or basic). To be more precise, pH is expressed as the logarithm of the reciprocal of hydrogen-ion concentration in gram atoms per liter. (6) ) Each numeric value change, for example, from 5 to 6 pH means that a 5.0 ph is ten times more acid than a 6.0 ph; a reading of 9.0 pH is ten times more alkali than a pH of 8.0. Therefore, incremental change between soil pH readings of 6.8 to 7.2 in microcontexts is meaningful. This must be emphasized in archaeology where human-influenced change must usually be assumed.
Values of pH below 2 and above 11 are not very supportive of life and are in fact toxic in high or low concentration at either end of the scale, therefore rarely found in nature. I have found most soils that support living organisms or are mostly derived from once-living organisms to be in the mid-range from 5-8. Soils found in typical archaeological contexts (archaeosols) are most often mixtures of inorganic geological erosionally-derived elements (sand, gravel, etc.) and organic decomposition of dead matter, whether from plant and tree detritus, leaves, humus and other natural composting organic material. Natural geosols are non-archaeological. Because our alpine contexts are at elevations (2200-2500 m) considerably above the treeline or timber biosomes (1900 m), our local soils (mostly shallow around a 1 meter depth over schist bedrock here), we do not encounter slightly acidic podzols as expected from the dominant larch conifer forest below where soil depth is greater (up to 2 m) above often similar neutral pH schist bedrock.
Fig. 6 Stanford Alpine Archaeology team members with portable pH meter
soil testing (l-r Katia and Larry Reeves, 1997 with Corning Check-Mite pH-10
meter being calibrated in buffer)
Fig. 5 Portable field pH meter
The hypothesis of the Alpine Archaeology Project was that application of pH testing validity in an archaeological context can be potentially used to differentiate microstratigraphy where pH is complementary with color and other characterizations or where it is difficult to differentiate microstratigraphy by color (where it is the same) and other characterizations. Our sampling and pH testing over several years suggests this hypothesis is valid.
Thus, on the one hand, validity of pH testing in archaeology may be optimal when used to complement heterogenous microstratigraphic and microcontextual elements. On the other hand, validity of pH testing in archaeology may also be optimal when used to differentiate homogenous microstratigraphic and microcontextual elements whose characterization may be better served by invisible soil chemistry than by mere visual characterization. Because application of soil chemistry pH in archaeology is still theoretical, it is expected that corrections of field methods and calibrations for pH testing will be forthcoming as assumptions are modified and pH testing processes are further refined.
Figs 1 & 3 images from Rose Magazine and University of Florida (MG092) respectively. Fig. 2 from Moosfluh, Valais, Courtesy of Villardebelle Arboretum, France.
TO READ MORE, SEE PATRICK HUNT'S NEW BOOK ALPINE ARCHAEOLOGY (2007).
Copyright © 2006
Dr. Patrick Hunt