Selenium-deficient soil is a consequence of the parent material from which
the soil was formed, not a consequence of human activity. The problem of
selenium deficiency in the human diet is not common, for two reasons. First,
selenium deficiency in soil occurs only in certain locations. Second, even
if you live in such a location, your diet is not going to be deficient in
selenium unless you get most of your food from the immediate area. Drawing
one's food from various sources, as opposed to getting it all from within a
local "foodshed," for example, evens out local soil-related nutrient
problems. The value of geographically diverse sources of food in overcoming
selenium deficiency is a counterexample to the conventional wisdom that
eating locally necessarily means eating more healthfully.
The other side of the selenium story -- one that is of much greater concern --
is the problem of excessive selenium in certain irrigated areas, most notably
the San Joaquin Valley. The selenium originates in the parent material, but
it becomes an environmental problem as a result of human activity, namely
irrigation, and with it the need to "flush" the soil to avoid salinization.
In the San Joaquin case, the drainage water used to be sent to the Kesterson
Wildlife Refuge, but the indigenous waterfowl started acting a little funny,
and some dropped dead. So selenium is yet another example of a substance for
which both too much or too little is a problem.