Persistence of organic phosphates in buried soils

Studies of the occurrence of phosphate in soil on archaeological sites have sometimes measured total phosphate 1 , but more frequently the inorganic or ‘available’ phosphate 2,3 —that extractable by brief treatment with acid or by inorganic solutions in the cold. The nature of the organic phosphate component does not seem to have been examined. Here we present an analysis of total phosphate and inorganic phosphate in 50 samples of soil taken from various levels of a site with a continuous settlement record from 1,600 to 6,800 yr BP. We could detect no mineralization of organic phosphate, at least over the period 1,600–5,200 yr BP. But in the upper soils, corresponding to the interval 1600–2800 BP, there was much more organic phosphate than in the lower samples, the transition being quite sharp. We tentatively conclude that the lower soils belonged to an island site relatively sparsely inhabited and often covered with natural vegetation. The upper soils corresponded to a more dense settlement in which organic phosphate, possibly phytic acid, was deposited on the transient surface and was not incorporated into the soil phosphate cycle.