Too much selenium causes loss of hair and nail loss in humans. Too much selenium is toxic to livestock. Selenium occurs in a wide variety of rocks, volcanic materials, fossil fuels, soils, plant materials and waters. Toxic soils have high amounts of soluble Se than can be used by plants. Acid soils rarely produce plants that contain toxic traces of Se.
Alkaline soils can lead to toxic concentrations.
Selenium and Vitamin E protect biological membranes from oxidative breakdown . Se depends upon species and Se in soils. Accumulators, converters, indicators. A less toxic form would be good. Calcium selenate is easier to handle.