In spite of its abundant occurrence in the earth's crust (310 mg Rb/kg) and its composition of a stable (72.2%) and a radioactive (27.8%) isotope, rubidium (Rb) belongs to the forgotten ultratrace elements. The interest in this ultratrace element grew considerably after Rb deficiency experiments with goats had shown that their growth was depressed, that > 80% of them aborted their kids. The geological origin of the site takes significant effect on the Rb content of the flora. Granite and gneiss weathering soils produce the by far Rb-richest plant populations, and drinking water. The water of the gneiss sites contained 18 mu g Rb/l, that of diluvial sands 3 mu g/l. Herbivores store most Rb whereas carnivores and omnivores accumulate significantly less Rb. The analysis of 137 foodstuffs and beverages in 15-fold repetition showed that the starch-and sugar-rich cereals, pasta, bread and confectionary are poor in Rb (1 mg/kg dry mater (DM)). Fruit and vegetables accumulate between 5 and > 60 mg Rb/kg (asparagus). Boiling drastically reduces the Rb content of vegetables. Animal foodstuffs are relatively poor in Rb. Poultry meat as well as freshwater fish are relatively rich in Rb. Coffee (40 mg/kg DM) and black tea (100 mg Rb/kg DM) store much Rb, 85% of which pass into the beverage.
