Introduction

In the context of archaeological research soil/deposit chemical analysis should be viewed as an additional data set or tool for interpreting the archaeological record. Because chemical signatures are not exclusively anthropogenic (they are not uniquely of human construction like artifacts), there is always a non-anthropogenic component or effect. Human activity either indirectly modifies a soil’s chemical characteristic, as with pH, or it directly adds or subtracts material creating an anomaly by altering the amount of carbon, phosphorus, nitrogen, or carbonates in the deposits. Anomalies can only be detected if there is baseline data that characterizes the deposits prior to human intervention. This is accomplished by setting up control sampling locations or, if that is not possible, obtaining background data from preexisting sources (e.g., from sources like Shacklette and Boerngen, 1984). Interpretation of chemical data in archaeological contexts involves comparisons to control samples and an understanding of the evolution and maintenance of the anthropogenic soil anomaly (Carr, 1982). This is no small feat given the complexity of temporal and spatial occupation histories at many archaeological sites and the complex pedogenic response over time to anthropogenic activity.

Because this entry is about soil chemical analysis in archaeology, it seems appropriate to define soil from a soil chemist’s perspective:

Soils are multi-component, open, biogeochemical systems containing solids, liquids and gases. That they are open systems means they exchange both matter and energy with the surrounding atmosphere, biosphere, and hydrosphere. These flows of matter and energy to or from soil are highly variable in time and space but they are the essential fluxes that cause the development of soil profiles and govern patterns of soil fertility. (Sposito, 1989, 3)

The definition emphasizes that soils are open systems that adjust to variations in input. Knowing or hypothesizing about those adjustments after anthropogenic input over archaeological time scales is important for interpreting chemical data from archaeological contexts. The state factor’s model of soil formation first developed by Jenny (1941) and advanced in geoarchaeology by Holliday (1994, 2004a) is an excellent conceptual framework for interpreting soil chemical data in archaeological contexts. The model consists of five external factors that govern soil formation. They are (1) climate, (2) organisms (plants and animals), (3) relief (landscape position), (4) parent material (anthropogenic and non-anthropogenic deposits), and (5) time. Both these factors and soil-forming processes vary, resulting in changes in soil morphology, hydrology, and chemistry. The human animal can be considered with all the other organisms involved in soil formation or, perhaps, more appropriately as the sixth factor. Human populations, although they are just a player in the ecological drama, are the dominant one. They modify all of the factors of soil formation in major ways at scales from a single dwelling to the global climate (Hooke et al., 2012).

Control sampling

Chemical analysis in geoarchaeology is comparative so it demands two or more data sets to be of much analytical use. Control samples should be taken in the field and analyzed to determine the background or natural level of the chemical of interest. This is equivalent to analyzing blanks in the laboratory, a standard and necessary procedure. The point of control sampling is to determine the non-anthropogenic or natural background chemistry of the soil off-site, and the state factor model is again a good conceptual guide. Thus, it is best to pick landscape positions off-site, where all of the state factors are similar to the sampling loci on the site. Multiple control locations may be necessary. In many situations (e.g., modern or ancient urban areas), finding a location that has not been previously utilized or occupied or that you know has not been utilized or occupied is difficult but should be attempted. Certainly a number of authors have advocated using control samples or have effectively used control samples in their research (see Proudfoot, 1976; White, 1978; Bakkevig, 1980; Carr, 1982; Sandor, 1992; Entwistle et al., 2000; Wells et al., 2000; Holliday, 2004a). In addition all samples should be analyzed using the same techniques/procedures and by the same laboratory to reduce unnecessary sources of error and uncertainty (see Holliday and Stein, 1989; Holliday et al., 2004c).

In many geoarchaeological investigations that use soil chemistry, a suite of chemical analyses is used to address research questions. For this reason geoarchaeological applications will follow the discussion of each of the chemical techniques.

Carbon/organic matter

Sources and transformations in soils and deposits

Carbon occurs in soils in organic and inorganic forms (Stevenson and Coles, 1999). Organic forms occur as living plants and animals and as the by-products of the decomposition of plants and animals referred to collectively as the soil’s organic matter fraction (SSSA, 1997). Inorganic forms can also be added to the soil by plants that contain crystals of calcium oxalate or opaline silica (Weiner et al., 2002; Piperno, 2006; Prychid et al., 2008). Calcium oxalates would contribute some carbon to a total carbon assay. However, most inorganic carbon is derived from the parent material (carbonate rocks and dust) (Birkeland, 1984; Nelson and Sommers, 1982). In non-calcareous soils almost all of the carbon is in the organic fraction of the soil (Nelson and Sommers, 1982). Carbon is a part of organic matter that is introduced into the soil by natural process and anthropogenically as plant tissue with a more minor contribution from animal tissue. Plant residue consists of 25 % solids that are made up of carbon, oxygen, hydrogen, and ash (Brady, 1974). The ash contains the macronutrients (phosphorus, potassium, calcium, magnesium, and sulfur) and micronutrients (zinc, iron, copper, boron, manganese, and molybdenum) as well as minor trace elements (Brady, 1974). These are relevant to studies of soil chemistry at archaeological sites as they form part of the anthropogenic and natural chemical load in soils and deposits. As soon as organic matter is added to the soil, it begins to decay. The rate of decay and the products of decomposition depend on the soil environment (Brady, 1974). In turn, the nature and strength of any anthropogenic anomaly depend on the nature and intensity of occupation and the soil-forming environment (Carr, 1982).

Anthropogenic additions, subtractions, and transformations

Human populations are major players in cycling organic material in the environment. The organic carbon fraction is of interest in geoarchaeological studies because it is a component of building material (wood and adobe), food, waste, and a by-product of food preparation, material processing, and heating (e.g., charcoal) at human habitations and ultimately in archaeological deposits. It is continually moved from place to place in the process of food production, settlement construction, and waste disposal. As a result, it is added to the soil, directly and indirectly, in the form of waste from a variety of activities in and around settlements, for example, the dark earths in Amazonia (McCann et al., 2001) and Europe (Chapter “FTIR” in Courty et al., 1989). And it is removed from the soil in places where farming or resource extraction (removal of tress or crops), for example, occurs. The most significant anthropogenic transformation of organic matter is by burning. This reduces organic matter to the much more decay-resistant and carbon-rich charcoal. In chemical analyses charcoal is measured as a part of the organic matter or total carbon fraction of the soil. It can also be used, for example, to determine the species (Asouti and Austin, 2005; Marguerie and Hunot, 2007) of wood being exploited for fuel and building material or if the wood was collected dead or alive (Moskal-del Hoyo et al., 2010). Charcoal is only relatively stable. It can be degraded and disseminated into small particles in alkaline soils (Dufraisse, 2006; Braadbaart et al., 2009) and can be attached by soil fauna and flora (Thery-Parisot et al., 2010). Reduced particle size has implication for site formation processes and chronology as the charcoal is more mobile in the soil profile. Stein (1992) provides a general summary of organic matter in archaeological contexts.

Analytical methods

Total carbon in soils can be determined by wet or dry combustion techniques (Nelson and Sommers, 1982). Note this technique measures all forms of both the organic and inorganic carbon in the soil. The basic principle is to drive off and capture the CO2 and then measure the amount captured gravimetrically or titrimetrically. This is generally done with automated laboratory instruments designed for carbon analysis (see Nelson and Sommers (1982) for examples and procedures). Another measure of soil organic matter is near-infrared reflectance spectroscopy (see entry “Anthrosols” by Woods this volume).

The most commonly used procedures to determine organic carbon are Walkley-Black (Nelson and Sommers, 1982; Singer and Janitzky, 1986) and loss-on-ignition (Dean, 1974) techniques. With Walkley-Black the sample is digested in dichromate and sulfuric acid, and the amount of carbon is determined by titration or colorimetrically. This procedure uses strong acids and needs a laboratory setup to do the digestion.

Loss-on-ignition is a simpler procedure, is as accurate (Dean, 1974) as Walkley-Black, and can also be used to determine carbonate in the sample. The procedure consists of placing oven-dried soil in a small pre-weighed crucible and heating it in a muffle furnace to 550 °C, cool to room temperature in a desiccator and reweighed. The difference is the amount of organic carbon ignited. The sample and crucible are placed in the oven and reheated to a higher temperature to determine the carbonate content (see section on carbonates below). The number of samples that can be done at one time is only limited by the size of the muffle furnace. Loss-on-ignition can also be done using automated thermogravimetric analyzers, which can process many samples at one time with direct computerized calculations, producing immediate tables and plots of results.

Nitrogen

Most nitrogen in the soil is associated with organic matter or soil humus (Brady, 1974) that can be slowly released by the actions of microorganisms and made available to plants. The soluble ammonium and nitrate is readily available to plants but is also easily leached from the soil. Because nitrogen compounds are rapidly fixed (unavailable to plants) and mobile (available but easily leached), heavily cropped soils need a constant artificial supply of nitrogen fertilizer especially in modern mechanized agricultural systems.

Sources and transformations in soils and deposits

Inputs of nitrogen to the soil come from addition of organic matter during the process of plant growth and decay, fixed by microorganisms from the atmosphere, and brought in to the soil in the form of ammonium and nitrate salts by precipitation (Brady, 1974). Once in the soil nitrogen is generally immobile or fixed except for small amounts of inorganic nitrogen in the form of nitrates and ammonium nitrates. Some ammonium nitrogen is also fixed in the lattices of clay minerals where it is very slowly available to plants during weathering. These later forms are available to plants and are mobile in soil water. Most nitrogen is rapidly cycled (Stevenson and Coles, 1999), a process whose rate depends on soil conditions (factors) especially climate.

Anthropogenic additions, subtractions, and transformations

Human activity alters the nitrogen cycle by adding organic matter (waste and garbage) or fertilizer/manure in some places and removing it in others (movement of plants and building material to settlements). Because nitrogen is added to the soil along with carbon and other elements when disposing of plant or animal waste or fertilizing agricultural fields, it creates an anomaly that is closely associated with organic matter (carbon) anomalies. As organic matter breaks down, much of the nitrogen is rapidly volatilized and lost to the atmosphere or becomes mobile in the soil water (Brady, 1974). The remaining nitrogen is fixed by clay mineral or combines with soil organic matter. Because nitrogen cycles rapidly, it may not maintain a anthropogenic anomaly over long time spans, so it is not a good indicator of anthropogenic load (Holliday, 2004a) except, perhaps, on young archaeological sites (Woods, 1982) or in arid areas (Homberg et al., 2005).

Analytical methods

There are two types of analysis that deal with total nitrogen: Kjeldahl wet combustion and Dumas dry combustion (Bremner and Mulvaney, 1982). In the Kjeldahl analysis the nitrogen in the samples is converted to ammonia (NH4+−N) by heating in sulfuric acid in the presence of catalysts. The amount of nitrogen is determined by measuring the amount of NH3 liberated from the digest when distilled in an alkali. Dumas analysis involves heating the sample with CuO and exposing the liberated gas to hot Cu to reduce the nitrogen oxides and then to CuO to convert the CO to CO2. The N2–CO2 mixture is then collected and exposed to a concentrated alkali that removes the CO2, and then the volume of N2 is measured. Both methods are complex and have recovery problems that researchers should be aware of before choosing a procedure (see Bremner and Mulvaney, 1982). Automated N analyzers are capable of processing samples relatively rapidly and produce results comparable to the wet chemistry methods (Thomas et al., 1967; Schuman et al., 1972).

pH

The measure of the activity of ionized H (H+) in the soil solution is called pH (Mc Lean, 1982). It is one of the most indicative measures of soil chemistry (Boul et al., 1989) and is important in determining (after Mc Lean, 1982) the (1) solubility and hence mobility of compounds in the soil, (2) the bonding of ions to exchange sites, (3) activity of microorganisms, and (4) availability of plant nutrients. The pH scale ranges from 1 (most acidic) to 14 (basic), 7 being neutral.

Sources and transformations in soils and deposits

Soil pH is not an element or compound that can be added or subtracted from the soil but instead is a condition of the aqueous phase of the soil environment that is very dependent on the interaction and evolution of the soil-forming factors. Many chemical reactions, weathering trajectories, and soil–plant relationships are pH dependent. Because soil water system is open, external inputs of water (including its dissolved constituents) and organic and inorganic particles – both natural and anthropogenic – can rapidly change the soil pH (Sposito, 1989) and therefore the pedogenic trajectory and the maintenance of the anthropogenic anomaly.

Anthropogenic additions, subtractions, and transformations

The degree to which soil pH is modified by anthropogenic additions depends on the initial soil pH, buffering, and pedogenic context. Anthropogenic modifications of pH are direct and indirect. Direct addition of wood ash, limestone (especially burnt), and shell maintains alkalinity (Cook and Heizer, 1965). Addition of organic matter indirectly lowers pH because the decay of OM produces acids (Brady, 1974). Soil pH is an important parameter for predicting the degree of bone preservation, including bone proteins used in DNA analysis, and metal and charcoal preservation in archaeological deposits (Tylecote, 1979; Gordon and Buikstra, 1981; Pate and Hutton, 1988; Nielsen-Marsh et al., 2007; Braadbaart et al., 2009; Adler et al., 2011). Sheppard and Pavlish (1992) have shown that among other soil chemical variables, pH is important in the weathering of chert. Soil pH is also one factor in determining the potential for preservation of phytoliths (see Piperno 2006; Cabanes et al., 2011). In most geoarchaeological investigations that use soil chemistry, pH is one of a suite of chemical analyses used to characterize the soil as background for interpretations.

Analytical methods

Determination of pH is accomplished using either colorimetric or electrometric techniques (Mc Lean, 1982). Colorimetric techniques use dyes or acid–base indictors that react by changing color in different pH environments. In its simplest form the electrometric technique consists of a glass electrode that measures the hydrogen ion activity and a reference electrode that completes a circuit so voltage can be measured (Mc Lean, 1982). The pH is typically measured in a 1:1 soil–water mixture (see Janitzky 1986 or Mc Lean, 1982). Many portable colorimetric and electrometric systems are available for field measurement of pH.

Phosphorus

The most widely used soil chemical technique in archaeological research is certainly the analysis of phosphorus. This is because humans are very proficient at concentrating P in and around places where they live and much of the P added to soils is considered fixed (Brady, 1974; Walker and Syers, 1976). The source of the P is the plant and animal remains and waste left at sites that ultimately ends up in the soil. The association of high soil phosphorus levels and human settlements was first documented in the 1920s by Swedish soil scientist, G. Arrhenius (see Eidt, 1985 or Wells et al., 2000 for brief history). Since that time, P analysis has been used in many archaeological contexts to aid in determining site and feature boundaries, intra-site activity areas, intensity of occupation, and types of land use (for recent studies, see Barba et al., 1996; Parnell et al., 2001; Fernández et al. 2002; Barba, 2007; Middleton et al., 2010; Roos and Nolan, 2012).

There are a number of reviews of archaeological/geoarchaeological research using phosphorus that should be consulted as an initial source before developing a research strategy that includes P analysis. The most recent and most thorough reviews can be found in Holliday (2004b) and Holliday and Gartner (2007). They cover basic chemistry, common methods of extracting and measuring soil P, and the use of soil P in chronosequence studies. Proudfoot (1976) provides a general review of the extraction procedures and chemistry of P in soils, anthropogenic additions, and sampling issues as well as an example of P analysis from an archaeological site in Britain. Bakkevig (1980) provides more of a cautionary tale pointing out the importance of understanding the natural P background and the geomorphic context of any sampling site. White (1978) also stresses the importance of having background data.

Sources and transformations in soils and deposits

The chemistry of soil P is complicated, in part because the P anions can bind with a number of cations in the soil to form compounds where the P bond varies in strength. P chemistry is strongly pH dependent which in turn is dependent on the soil factors at a particular site and on the natural and anthropogenic evolution of the site. A detailed explanation of P chemistry is beyond the scope of this entry, so P will be covered in a simple way under the heading of additions, subtractions, and transformation.

Almost all of the phosphorus in the soil system ultimately came from weathering of the inorganic P minerals (primarily apatite) in the soil parent material (Walker and Syers, 1976). The soluble P is taken up by plants, and upon death, they add organic matter to the soil. Once the system is established, most of the soil phosphorus is contained in soil organic matter (Brady, 1974). Soil microorganisms mineralize the organic forms of P to soluble inorganic forms (H2PO4−, HPO4−) that are available to plants and can be leached out of the soil with the soil water. These latter processes are ways P can leave the soil, although on landscapes that are not cropped, the P is recycled. Of course weathering continues and small amounts of P still enter the soil from that source.

Most of the phosphate anions that enter the soil quickly form calcium (Ca), aluminum (Al), or iron (Fe) phosphates. Which compounds form depends on the soil pH and the amount and kind of each cation present. Brady (1974) divides P compounds in the soil into three major groups: (1) readily available phosphates that are generally water soluble (non-occluded P); (2) slowly available P including newly formed Al, Fe, and Mn phosphates, Ca phosphates, and mineralized organic phosphates; and (3) very slowly available phosphates of Fe, Al, and Mn, apatites and stable organic phosphates. His view of P is from the perspective of agronomy and soil science, where most basic research on P in soils has taken place. Laboratory analyses designed to study P in soils reflect the kinds of P found in soil (see below). Anthropogenic addition of P to the soil is also held at different location in the soil so to detect the anthropogenic anomalies P must be extracted either totally or differentially by chemically targeting the different phosphate compounds.

Soil phosphorus is only relatively stable over time because as soil factors change in response to environmental change and pedogenic processes adjust, P can be removed from the soil system or reorganized within the soil (see Walker and Syers, 1976; Tiessen et al., 1984; Roberts et al., 1985). On geomorphically unstable landscape facets, where erosion or deposition is occurring, the retention of P and the post-depositional evolution of the any anthropogenic P anomaly change.

Anthropogenic additions, subtractions, and transformations

Anthropogenic sources of P come from domestic refuse, food waste, plant and animal remains, human bodies (especially bones), human and animal excrement, and wood ash (Cook and Heizer, 1965; Carr, 1982; Woods, 1982). Human populations are a factor in the P cycle and as such alter the process of P cycling. These alterations can be detected in the soils on archaeological sites.

Analytical methods

Analysis of P in soils has two stages. The first stage is extracting the P from the soil (Olsen and Sommers, 1982; Meixner, 1986a). The extractant used depends on which form or forms of P are being targeted. The second stage is the determination of the amount of P in the extractant. This is accomplished by using a colorimetric method. In P fractionation multiple extractants are used in sequence to determine the different forms of P in the soils.

Spot test or ring test is a qualitative measure of P that can be done in the field with simple tools and reagents (Gundlach, 1961; Eidt, 1973; Woods, 1975). The test uses a weak acid extractant to measure the available P. Color is developed on filter paper based on a qualitative scheme (see Eidt, 1973). The advantage of the spot test is fast, low-cost results, but the disadvantages are qualitative non-reproducible results (Holliday, 2004b).

Available P refers to techniques that extract the water-soluble P and weakly held P fractions (Olsen and Somers, 1982). This involves extracting the P with weak acid and developing color intensity that can be read in a spectrometer. Available P types are often referred to by the name of the person who developed them such as Olsen P, Bray 1, or Mehlich II tests. They are differentiated because they use different extractants. Available P can also be done in the field with a portable spectrometer (see Terry et al., 2000).

Phosphate fractionation is the process of sequentially extracting P beginning with the most weakly bound P using extractants that target specific P compounds (Olsen and Somers, 1982; Meixner, 1986b). As many as eight different fractions, grouped into non-occluded P (three extractions), occluded P (three extractions), calcium bond P (one extraction), and organic P (one extraction), can be involved (see Meixner, 1986b). Most P fractionations in geoarchaeological applications use a three-fraction extraction sequence developed by Eidt (1977). This is an intensive wet chemistry procedure that targets the weakly bound Fe and Al-P and the reabsorbed Ca-P as fraction I (Eidt, 1977). Occluded P is fraction II and calcium P and apatite are fraction III.

Total P can be determined by using very strong acids to completely digest the soil, and P is measured colorimetrically (Olsen and Somers, 1982; Meixner 1986c). Total P can also be measured using ICP spectrometry, usually as one of a suite of elements. The ICP measures the P content so the type of P measured still depends on the extraction procedure.

Carbonates

The origin of carbonate in soils is either from eolian sources, inherited from calcareous parent material, or weathered from non-calcareous parent material (Birkeland, 1984). Pedogenesis results in carbonate accumulations in soils in arid and semiarid climate zones and in its removal from the soil system in humid and tropical climatic zones (Birkeland, 1984; Boul et al., 1989). Carbonates are often measured as a part of soil/deposit characterization by determining the presence or absence of free carbonate using a few drops of HCl or less often by laboratory analysis. Results are used to determine the presence or absence of an anthropogenic carbonate load by comparison of control samples or other regional soil data and, if carried a step further, to interpret the anthropogenic changes in the pedogenic trajectory relative to site formation processes. For example, carbonates can dominate the soil chemistry in part by their effect on pH which in turn affects artifact preservation, especially bone and shell, and the post-depositional evolution of any anthropogenic additions (e.g., see Weiner et al., 2002).

Sources and transformations in soils and deposits

The source of the carbonates in soil is atmospheric dust containing carbonate and Ca2+ ions (Machette, 1986; Birkeland, 1984) and carbonate in parent materials (limestone, gypsum, dolostone, loess, glacial deposits from carbonate terrain). Parent material weathering in non-calcareous soils cannot account for the large amount of carbonate in arid and semiarid soils (Birkeland, 1984). In arid and semiarid regions, pedogenic processes form calcic soil horizons (K horizons) (Birkeland, 1984; Machette, 1986). In humid and tropical soils with lower pH, the carbonate is disassociated and is leached out of the soil system or accumulates as minor secondary carbonates in the C horizon (Boul et al., 1989).

Anthropogenic additions, subtractions, and transformations

Anthropogenic additions that may increase the carbonate content in soils or lead to the formation of secondary carbonates are limestone and dolostone for cooking and, in some cases, pottery manufacturing and/or food processing, building material (plaster and stone), wood ash, and shell (Cook and Heizer, 1965; Woods, 1982; Schiegl et al., 1996). The age of the archaeological site, soil conditions, and landscape position are some factors that affect the post-depositional modification of anthropogenic carbonate additions. For example, physical and chemical processes during pedogenesis may destroy or fragment shell or carbonate rock adding secondary carbonate to the soil or removing it from the soil system entirely. Soil – geomorphic and stratigraphic – studies at archaeological sites record the soil carbonate status for characterization purposes with little geoarchaeological interpretations. Woods (1982) interprets the high carbonate levels in a midden in Illinois to be the result of the addition of ash to the midden. Indirectly human activity (e.g., land clearing and agriculture) that causes geomorphic instability may result in wind erosion, which could also add carbonate to soil.

Analytical methods

The simplest measure of the presence of carbonate in soil is to observe the strength of soil reaction to 10 % HCl. The strength of the reaction is measured by the violence of the effervescence. The more carbonate, the more violent the reaction.

The loss-on-ignition (LOI) (Dean, 1974) method is used to determine both OM and carbonate. A sample is placed in a furnace first to 550 °C to destroy the organic matter, cooled and weighed, then put back in the furnace and heated to 1,000 °C to drive off the CO2 in the carbonate. The sample is cooled and weighed again to determine the percent carbonate. Dean (1974) compared the LOI method with acid extractions and then titration and with determination of total Ca with atomic absorption and found that they yielded very similar results. Thermogravimetric analyzers have now completely automated the above procedure.

In the acid neutralization method, the carbonates are dissolved in acid and the amount of carbonate is determined by titration (Nelson, 1982). The gravimetric method uses a Chittick apparatus to determine the volume of CO2 evolved during acid digestion (Machette, 1986).

Geoarchaeological applications

The section on applications begins with examples of the use of phosphorus in geoarchaeological studies. Phosphorus data have been used as a tool in geoarchaeological investigations for nearly a century, and the literature is relatively extensive (see Eidt, 1985; Wells and Terry, 2007). The treatment below is not comprehensive and attempts to group the investigations by type. Chemical analyses, including phosphorus, are a part of a suite of measures used in the study of the Amazonian dark earths and are not included here (Glaser and Woods, 2004; see Woods this volume). Most investigations focus on the spatial distribution of P anomalies on the landscape surface within and around sites. The goal of these studies is to find site boundaries or to identify activity areas within sites. This involves examining both positive and negative P anomalies.

Skinner (1986) investigated P levels at five archaeological sites in Ohio. The goal of the investigation is to determine if P can identify anthropic soils and locate site boundaries determined by artifact distributions. Three different extraction techniques are compared for available P and one for total P. The conclusion is that the reliability of P as an anthrosol indicator depends on the geomorphic and pedogenic context specifically whether or not a soil/site was subject to inundation (i.e., located on a floodplain).

Roos and Nolan (2012) used available P (Mehlich II extraction) levels from 131 samples at a late prehistoric village site in Ohio to map intra-site activity areas. They were able to identify a ring midden and plaza using P data supported by magnetic data and artifact distributions.

Schuldenrein (1995) used soil chemistry (pH, OM, K, Ca, Mg) including available P, total P, and P fractionation, to detect activity areas at two sites in contrasting environments: the semiarid plains and humid temperate woodlands, both in the USA. Comparisons of control sample series with on-site and feature sample series indicate anthropogenic anomalies are present at both sites and is most strongly characterized by levels of P and K or P and selected other measures depending on the physical and cultural context. Plots of the three P fraction loadings on ternary diagrams are proposed as a graphic means of differentiating types of activity areas.

Woods (1982) found the following chemical trends at archaeological sites in Illinois. Carbon (organic matter) and nitrogen level were higher in midden soils than in control soils and both decreased in magnitude with depth. He found pH levels to be significantly more alkaline than control samples due to the large amount of wood ash in the middens that in effect neutralizes the acidifying effect of decaying organic matter. He also attributed carbonates in the middens to the addition of wood ash in an alkaline environment. P is high in the middens and absolute levels correlate with soil texture with P levels higher in clayey soils.

A number of interdisciplinary investigations have been conducted at the Piedras Negras site and surrounding modern settlements in Guatemala. These studies all use a field test procedure based on a Mehlich II acid extraction and measurement with colorimetry modified for use in relatively primitive field conditions (Terry et al., 2000; Wells et al., 2000; Parnell et al., 2001). The investigation identified a good correlation between P levels, density of ceramics, and boundaries of disposal areas. Fernandez et al., (2002) and Terry et al., (2004) investigated soil chemical signatures in modern settlements and a Mayan archaeological site to explore the relationship between chemical data (P, pH, Mg, Na, K, and trace elements) and household human activities. Phosphorus was high in areas of food processing, consumption, and disposal. Food preparation areas had high levels of P, Mg, and K and were more alkaline, while food consumption areas had high P and Na and were more acid. Traffic lanes had low P and refuse disposal areas have high P.

Dunning (1993) used total P to distinguish different types of land use and P fractionation to differentiate between agricultural and nonagricultural soils. High P levels are interpreted as areas that were gardens and likely fertilized and areas with depleted P as places of more intensive field agriculture.

Sandor (1992) compared the morphological and chemical characteristic of terraced cultivated soils and uncultivated soils at a 1,000–1,500-year-old prehistoric site in New Mexico, USA. Cultivated soils lost organic matter, N, and phosphorus (total and moderately available) and lowered pH. In contrast soils in terraced fields in Peru have elevated levels, relative to uncultivated soils, of total and available P, nitrate nitrogen (NO2-N), total nitrogen, and organic carbon. Soil pH tended to be more acidic due to the increased organic matter. The chemical data, supplemented the archaeological evidence and soil morphological data, indicating the agricultural soils in New Mexico were not amended or fertilized and the agricultural soils in Peru were amended and fertilized. More recently similar methods including chemical analysis of soils was applied on a more regional scale to Native American agricultural system in the American southwest (Sandor et al., 2007) and more specifically to prehistoric Zuni agricultural systems (Homberg et al., 2005). Note these studies are among only a few that measured any form of nitrogen in geoarchaeological contexts (also see Woods, 1982).

Cavanagh et al. (1988) used HCl-extractable P data to map boundaries of sites in Greece. A positive correlation was found between high pottery sherd densities and high P levels.

The following investigations examine P distributions stratigraphically. Lippi (1988) used stratigraphic data (including artifacts), obtained from cores, and P data, obtained using the field ring test, to map paleosols and activity areas at the Nambillo site in Ecuador. The strata and soil description and P data provided an excellent framework for planning excavations and for making interpretations of land use on the buried landscape surfaces.

Katina (1992) used fractionation to test Eidt’s ideas about the correlation of total P with intensity of land use and the use of fraction II/I ratio to determine relative time elapsed since phosphate enrichment. Results of the fractionation were very difficult to interpret because of the land-use palimpsest, but the total P and fraction II/I ratio was used to support soil landscape degradation during the Bronze Age followed by less intensive use during the Middle Ages.

Davidson (1973) used total P (fused with sodium carbonate and measure colorimetrically) from a tell stratigraphic sequence to measure intensity of occupation. P indicates that the (1) intensity of occupation increased up section and (2) the tell sediments have higher P than the local alluvium. He concluded that “phosphorus analysis confirms what might be expected-the tell evolved as a result of occupation and thus the activities of people who occupied the site…. accounts for the growth of the tell” (Davidson 1973, 146).

Bakkevig (1980) claims to get good results from the spot test in part because large numbers of samples can be processed quickly allowing a researcher to obtain data from a large area. The research questions involved correlation of land use with P levels and identifying cattle trails.

Ahler (1973) investigated the distribution of total P (perchloric acid/nitric acid digestion), available P (Brays Strong P test), OM (Walkley-Black), and pH from a stratigraphic sequence at the Rogers Rockshelter in Missouri. Results of the chemical analysis are compared with the distribution of lithic debris and micro-debris (sand-sized material of cultural origin). Ahler’s results point out the importance of context for interpreting the chemical data. There is a strong correlation among lithic debris, micro-debris, and total P throughout the sequence and a strong correlation with available P and total P in the lower part of the sequence. The difference between the upper and lower stratum is due to higher sedimentation rates during the accumulation of the lower stratum not allowing pedogenesis to alter the distribution of the available P. It is concluded that total P is more useful for locating intra-site activity areas and available P is more useful for subsurface detection of sites and buried soils especially in strata with pHs similar to those at Rogers shelter.

Conclusions

This brief overview of the uses of carbon, nitrogen, pH, phosphorus, and carbonate analysis in geoarchaeological investigation is far from exhaustive but hopefully illustrated the potential such analyses have for answering archaeological questions. When formulating research questions that involve data generated by chemical analysis, the plan should always have some type of control sampling and an understanding of the physical context of the samples. Control samples are necessary because all of the elements, compounds, and measures covered in this overview occur naturally without any anthropogenic input. So by default the analysis has to be comparative. Context is always important but it is particularly important for chemical analysis because of the multiple physical (stratigraphic/pedogenic), chemical, and anthropogenic transformations that occur during and after human occupations. In many cases the evidences for some types of human activity are all or in part chemical signatures and as such are a valuable tool for targeted geoarchaeological investigations.