Database of the iodine content of soils populated with data from published literature

A database of results for the iodine content of soils has been prepared for the DFID funded project looking at "Environmental Controls in Iodine Deficiency Disorders". It has been populated with literature citations of iodine results for soils and contains 2262 records. The average result for the iodine content of soils is 5.1 μg/g (based on screened data of 2151 cited results). Given the skewed nature of the distribution of results the geometric mean of 3.0 μg/g is a more suitable value to quote for the level of iodine in soils. Using a classification based mainly on texture the order for levels of iodine in soil has been determined: peat (7.0) > clay (4.3) > silt (3.0) > sand (2.2). The figure in brackets represents the geometric mean value (μg/g I) for the screened data. There is no simple correlation between the iodine content of the soil and the distance of the sample site from the sea. However, in the coastal zone (0-50 km) the highest levels of iodine in soils are recorded with a large range of results (0.8 - 150 μg/g) and a geometric mean of 11.6 μg/g. Inland, at distances greater than 50 km, there is a much narrower range of results (0.4 - 14 μg/g) with a lower geometric mean value (2.6 μg/g). There is no great difference between results for soils one hundred kilometres inland compared with those samples from several thousand kilometres, although the lowest values are recorded in continental interiors. This suggests that the mechanism of direct deposition from the atmosphere (either as wet or dry deposition) is most significant in the coastal zone and further inland other mechanisms such as volatilisation from the soil-plant system must become more important. Unconsolidated and texturally coarse parent materials appear to produce low iodine soils, as do metamorphic bedrocks.