You can amend the soil, yes. Depending on the selenium source in the supplement, some of it will pass through the horse undigested. The problem is that if it's difficult to digest in the horse, the soil organisms will have difficulty digesting it too. It will happen, but it will take quite some time.

It isn't as simple as dumping some mineral on the soil and calling it good. Call your local farm adviser or university, get a soil test done, and ask for advice on your specific problem areas and what's normal for your geographic location. Many minerals work in concert, and fixing one deficiency may not be enough for truly healthy soil.