Selenium in seleniferous environments

Selenium is biologically important because (i) it is essential in animal and possibly plant metabolism, (ii) in many areas diets do not contain sufficient Se to meet animals' needs, and (iii) in other areas it is toxic to animals when it occurrs in high concentrations in soil, water, plants, fly ash, or in aerosols. Animals require 0.05 to 0.1 mg Se/kg in their diets to prevent Se deficiency but suffer Se toxicosis when dietary levels exceed 5 to 15 mg Se/kg. The earth's crustal materials generally contain <0.1 mg Se/kg. Higher concentrations are found in Cretaceous shales. The Se-accumulator plants growing on the seleniferous soils may contain hundreds or even thousands of mg Se/kg. However, the nonaccumulator grasses and forbs seldom accumulate >50 mg Se/kg and more often contain