Radioactivity is present everywhere, and has been since the formation of the Earth. Natural radioactivity detected in soil is predominantly due to the following four natural radioisotopes: K, Ra, U, and Th.  In one kilogram of soil, the potassium-40 amounts to an average 370 Bq of radiation, with a typical range of 100–700 Bq; the others each contribute some 25 Bq, with typical ranges of 10–50 Bq (7–50 Bq for the Th). Some soils may vary greatly from these norms.