A couple of thoughts Frolf..
First, Minnesota is interesting as parts of the state seem to have good seleniumlevels while other areas are low..It's right on the line. Here is a US gov. interactive map, you could check your county and see what
local soil levels are.
selenium map
If you have livestock and are feeding them a mineral supplement, then the composted manure can be a source for micronutrients for gradual soil building. (Or maybe you know someone who has..) Organic sources are absorbed best by animals.
Interestingly, with livestock there can be problems from selenium deficiency in some regions and at the same time high-selenium toxicity issues in other areas.
People are increasingly finding they have low selenium levels as industrial agriculture gets bigger and more concentrated...lots of feedlot animals are fed up on grains that are grown on selenium deficient soils. If you eat meat, you might want to just source out a local farmer who pays attention to herd nutrition...maybe you can score some naturally raised grass-fed
beef