Phosphate surveying of coastal settlements

The first person to discover that phosphates in the ground might be an indication of archaeological occupation levels was Olof Arrhenius. He performed a large number of phosphate analyses in order to map out the phosphate requirements of arable land in Sweden, mainly in regard to the cultivation of sugar beets. He soon noted that both prehistoric and medieval settlements were quite conspicuous on his phosphate maps, because of their enhanced phosphate content. He reported his findings in a series of papers and pointed out the potential of phosphate surveys as an aid to archaeology.

Since Arrhenius' interests mainly concerned the cultivation of sugar beets, he concentrated his work on extracting easily dissolvable phosphates available to crops. Briefly, his method involved shaking a soil sample with diluted citric acid to release the phosphates. […]

The citric acid method is comparatively time-consuming; partly because the actual shaking process takes time, and partly as samples must be sent to a laboratory and one must wait for results. […] sufficient phosphates can be obtained in solution to be able to show the differences in phosphate content between a settlement area and its surroundings, as well as between different levels in a cross-section of an occupational layer.

The simplest and without doubt cheapest method is the so-called spot test method. The principle here is that phosphates are extracted from a small sample of soil by means of a strong acid on a piece of filter paper, and indicated by ammonium molybdate and ascorbic acid. A blue spot then spreads over the paper; its size will depend on the phosphate content. […]

Both the methods mentioned above - the citric acid method in laboratory conditions, and the spot test method carried out in the field - serve archaeological purposes equally well. […]

Phosphate surveys are particularly valuable when searching for archaeological occupation levels, which lack visible traces above the ground surface, as well as areas where ground exploitation renders other forms of survey impossible. The information provided by phosphate surveys is also of great significance in planning archaeological excavations, as it can help to decrease costs.[…]