Analytical Methods:Field Phosphate

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Phosphate Analysis (field techniques)
Elevated concentrations of phosphate are a generalised signature of human activity in soils. Concentrations occur because of the addition of cess, manure, bone, cooking and food processing residues, and other animal and plant residues.

Questions
Field phosphate analysis is usually used to answer questions that will inform subsequent excavation or sampling. Site prospection is one common application using phosphate concentrations in the topsoil, but care should be taken that this does not reflect recent land management practices such as manuring. Multi-element analysis could be considered where time allows for laboratory analysis. Because phosphate concentration reflects such a wide range of human activities it is also used to help identify the spatial extent of activity around a site.

Phosphate analysis has also been used to identify grave sites and burial locations in situations where the body has decayed away completely. It is also used to identify specifc activity areas associated with high phosphate concentrations such as middens, rubbish pits and animal houses. Again post-excavation multi-element analysis may be used to aid interpretation of functional areas.

Examples of case studies where field based phosphate analysis has been successfully used include:
 * Rapid identification of middens
 * Site prospection
 * Grave analysis

Samples
Field analysis is generally used to answer individual context-specific questions or to provide results on a time-scale that allows them to be used to inform excavation priorites. However it is usually necessary to analyse a wider range of soils to establish the off-site background level of phosphate and the on-site normal (average) phospahte concentration. Because phosphate concentrations can be quite variable across very short distances replicates may be necessary to ensure that differences reflect real enhancements of soil phosphate rather than very localised variations.

The sampling scheme used will depend on the question being asked.
 * Identification of a buried soil requires sampling from the entire section, not just from the context in question. This should include as a minimum: multiple replicates (3 or more samples) from the natural, the modern topsoil, the context in question, the contexts above and below that in question, and further samples from representative contexts through the entire sequence.
 * Site prospection usually requires samples to be taken over a grid of appropriate size. It is often advisable to randomise or stagger the sampling location within each of the grid squares so that linear features are not missed. More detailed sampling can be undertaken if necessary where high concentrations are identified.
 * Functional area identification often involves analysis of specific features identified during excavation, such as floor layer, pits, graves, or dumps of material. In this situation it is necessary to not only take samples from the specific feature, but also more widely from on-site, and even off-site if possible, to provide a site average against which to compare the phosphate levels of the feature in question.

No specialist equipment is required for sampling. Bulk samples are required and can be collected using a trowel or spoon from surfaces or sections, or using an auger or other coring device from topsoils. Care should be taken not to touch or otherwise contaminate the sample and the sampling implement should be wiped clean between samples.

Fresh samples are used for qualitative analysis (see below). For quantitative analysis samples should be air-dried and sieved to less than 2 mm. Sieved air-dried samples may also be stored for subsequent post-excavation analysis if required.

Analysis
Field analysis can be undertaken at two levels.
 * Very quick, qualitative analysis using basic equipment and chemicals. This technique will provide a quick answer to context specific questions but may well require additional post-excavation analysis to confirm the results and allow for statistical analysis. More information on this method can be found here.
 * Quantitative phosphate analysis using mild acid extractions such as the Mehlich II method and more specialist equipment (colorimieter) have also been used in the field (Terry et al. 2000). This technique analyses plant-available forms of phosphate and post-excavation laboratory analysis may be necessary to establish a correlation with total and archaeologically derived phosphate forms.

Data and interpretation
Interpretation of field based phosphate data should take into account:
 * Possible background variability in phosphate concentrations linked to soil type, geology, and land use.
 * The possibility of later phosphate additions, for example as manure and fertiliser application.
 * The local soil type (in particular texture, pH, organic matter content) and the effect this may have had on soil phosphate retention.

If more than one phosphate dataset is to be compared, the same method of extraction and analysis must have been used.

Related techniques

 * Laboratory phosphate analysis
 * Field texture analysis
 * Field pH analysis
 * Field organic matter analysis
 * Multi-element analysis

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