The examination of phosphates from decayed organic matter; a technique for detecting the presence of phosphate in soil and for using phosphorus concentrations to determine human settlements and activity within sites. Phosphate is a natural constituent of soil, however, it is concentrated by animals' bones, excrement, and food refuse. The technique has been employed particularly in the study of cave deposits (to show human or animal occupation), settlement sites (to identify the uses to which different areas were put) and burials (to show the former existence of bodies completely decayed). Once phosphate is in the soil, it is usually converted into an insoluble form, so that it does not tend to move down profile nor to be redistributed sideways in the soil. For this reason, settlements and farms tend to leave high concentrations of phosphate in the soil, which often remain stable over long periods, sometimes thousands of years. Much preliminary work must be done on the distribution and range of naturally occurring phosphorus because variations are caused by vegetation abundance and type and by soil horizon.