I don't suppose anyone guessed(or cared?) but selenium is not a plant nutrient.

Selenate anion is not an essential element for plants(atleast for the time being), but it is very essential for animals including humans.

Plants however take selenium into them and through them we animals get it by eating. Selenium is a trace element and we don't need much of it, but our hearts will eventually seize to operate if we don't get enough.

The tricky thing is that, like almost every substance in the world, too much selenium is toxic. We end up having areas in the worlds where soils have too little available selenate to plants so that animals suffer from deficiencies and some soils where there's so much selenate ends up in plants that animals suffer from toxicities.