Justus von Liebig (1842) introduced soil fertilization with nitrogen and improved the importation of Chile saltpetre, which contained iodine up to 1 g/kg and increased the iodine content of the food. World War I stopped nitrogen fertilization in Germany, resulting in iodine deficiency. After World War II, East Germany imported fish meal from Peru, which satisfies the iodine requirement of farm animals and - via milk and eggs - that of man. After the end of fish meal importation, iodine deficiency reappeared. The iodine contents of most rocks vary from 0.01 - 0.50 mg/kg in the ultramafic rocks to 2 - 6 mg/kg in shales. Worldwide, the iodine concentration in soils varies between < 0.1 and 10 mg I/kg dry matter (DM), with an average of 2.8 mg/kg DM. Ocean water is rich in iodine, which gets back to the continents with the rain as diiodine methane, 8-10 µg/L near the coast and < 1 µg/L in the European Alps. The vegetation of the Holocene riverside soils is relatively rich in iodine, whereas that growing on Triassic sediments is extremely iodine-poor. Iodine is delivered to the food chain of animals and man through contamination of feeds and foods. The iodine concentration of the flora decreases with increasing age of the annual plants. Flowers, fruits, seeds, pulps and all starch- and sugar-rich parts of plants are poor in iodine. All baked goods contain low iodine amounts.
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