I don't like the idea of selenium on the garden. If in years to come someone grows cabbages or one of the many other types of brassica on this plot, then these will draw the selenium out of the soil and concentrate it in a food crop. Brassicas are used in this way to clean up old industrial sites. In certain places (Stroud in the UK for instance, although it is cadmium in this case), residents are advised not to grow these crops for fear of concentrating naturally occurring poisonous metals in their food crop. The problem is that this kind of contamination can remain in the soil for years.