Analytical Methods:Quantitative Phosphate

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Phosphate analysis
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. Other possible sources of phosphate though include the geology, modern fertilisers, and animal dung and urine.

Scale
Phosphate analysis is usually a laboratory based technique although methods are available for qualitative field analysis, and with the necessary analytical equipment it is possible to analyse samples on site.

Archaeological questions
Phosphate analysis is a good general indicator of human activity; hence it is one of the most commonly used soil analyses in archaeology. It has been used to address the following questions:
 * Site prospection
 * Identify and interpret space use and activity areas
 * Identify the extent of human activity around sites
 * Identify buried soils
 * Identify graves where the bone and organic remains have decayed
 * Studies of land management and soil fertility

Sampling and sample preparation
Although individually phosphate analyses may be relatively low cost, large numbers of samples are often required. It is important that a suitable sampling strategy is in place, as too many samples may be costly, whilst too few may not provide the the data needed to answer the archaeological question. A soil specialist can advise on appropriate sampling schemes as long as they have a clear understanding of the questions being asked and the nature of the archaeological context.

Bulk samples are required for phosphate analysis; often these are taken using a grid pattern. For site prospection this will be a grid over topsoils, for answering specific questions the grid will be over features of interest. It is important that the grid extends not just over the feature of interest but that reference samples from outside of this area are also collected. The size of the grid used will depend on the size of the feature being studied and the need to collect suffieicnt sample numbers to account for the background variability in phosphate concentrations. It is important that the sampling scheme minimizes the risk of missing features and allows isolated hotspots of phosphate concentration to be distinguished from the statistaically more important average enhancement of the soil matrix.

In some cases samples will be taken from vertical sections rather than horizontal grids, here it is important that samples are taken from a range of contexts in the section, not just the one of interest. Replicate samples from each context may also be needed to ensure that the data shows the average phosphate enhancement of that context.

Analysis
Whatever the source, phosphate rapidly reacts with other compounds in the soils, particularly iron, aluminium and calcium, and so can exist in the soil in many different molecular forms. As a result there are many different methods of phosphate analysis which measure concentrations of different forms of phosphate. None of these fractions directly measures "archaeologically-derived" phosphate. Those fractions that have commonly been analysed in archaeology are:
 * Total phosphate - Total phosphate concentrations include both the anthropogenically (archaeological and modern) and geologically derived phosphate in both its organic and inorganic forms. The different extraction methods use a range of aggresive chemicals such as sodium hydroxide (Smith and Bain, 1982) or sodium hypobromite and sulphuric acid (Dick and Tabatabai, 1977; Crowther, 1997) to break down the soil components releasing all, or almost all, the phosphorus they contain.
 * Inorganic phosphate - Even when phosphorus is added to the soil in organic materials such as dung it may become mineralised, often as a chemical precipitate of orthophosphate or adsorbed onto the surface of minerals (Holliday and Gartner, 2007). Methods for the extraction of inorganic phosphorus include hydrochloric acid, ammonium chloride, and sodium citrate and sodium dithionite. Inorganic phosphate analysis is often used in archaeology as it tends to correlate well with total phosphate whilst the extraction process is typically simpler (Crowther, 2006).
 * Organic phosphate - Organic phosphates includeesters, nucleic acids and phospho-lipids. These may be bound to organic matter and clay mineral particles, which make them very stable over long periods of time. A two stage organic/inorganic phosphate extraction is sometimes used in archaeology, usually when there is a need to understand the origin of phosphates in specific context(s). Extraction methods usually involve a standard extraction for inorganic phosphorus, followed by oxidation and extraction of the organic phosphate. For example, 1Ml HCl for inorganic phosphate then a sodium hypobromite oxidation and dilute sulphuric acid extraction for organic phosphate as used by Crowther (2006). Alternatively two samples from the same soil, one ignited to oxidise the organic matter and one not ignited may be analysed using a standard method for inorganic phosphate the unignited sample provides a measure of inorganic phosphate and the difference between the two sample provides a measure of the organic phosphate.
 * Plant-available or exchangeable phosphate - The Mehlich II extraction is a weak acid extraction method that measures plant available phosphate. Whilst this is likely to be only a small proportion of the anthropogenically derived phosphate and will not correlate directly with total phosphate, this technique has been used in field situations (Terry et al. 2000) as a more quantitative alternative to the field spot test. However, as a laboratory assay plant-available fractions are of less interest to archaeology as they are strongly influenced by soil conditions and are not directly related to the amount of phosphate in the soil derived from archaeological processes.

Data and interpretation
Phosphate data is usually expressed as concentrations: parts per million (ppm) or more correctly mg kg-1 or mg l-1.

Data may be presented as tabulated average concentrations (with standard deviation), bar charts and/or concentration isomaps. Statistical analysis may also be applied to show if apparent differences in phosphate concentrations between different areas are real (significant).

Interpretation of phosphate data should take into account:
 * The extraction method used and what fraction of soil phosphate is being analysed.
 * 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, for example, acid sandy soils are usually less able to retain phosphate than neutral or alkaline soils, or those containing a significant proportion of clay and/ or organic matter.

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

Related Techniques

 * Field-based phosphate analysis
 * Elemental analysis
 * Multi-element analysis

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