Designation (also microelement) for chemical elements that occur only in low concentrations or traces. If the concentration is extremely low, the term ultra-trace element is also used. As a rule, in geochemistry, constituents and accompanying elements of rocks and minerals in concentrations below 0.1% or 1000 ppm (millionths) are referred to as trace elements. In analytical chemistry the threshold value is 100 ppm or 100 µg/g (= 100 mg/kg) or 0.01 %. In biology, essential trace elements are chemical elements that are essential for a living being (colloquially mostly related to humans) and occur in the organism in mass fractions of less than 50 mg/kg. These elements include arsenic (As), chromium (Cr), cobalt (Co), iron (Fe), iodine (I), copper (Cu), manganese (Mn), molybdenum (Mo), nickel (Ni), selenium (Se), silicon (Si), vanadium (V), zinc (Zn) and tin (Sn). They are necessary for the formation of enzymes. In viticulture, trace elements that may be required are added by fertilization according to the soil analysis. Trace elements can also be found in wine; see under ashes, total extract and nutrients.