If the wood used for the subfloor is so soft that you can dent it with your fingernail, I think you're likely to be disappointed with its performance no matter what you do, especially in a high traffic area like the kitchen. Multiple coats of polyurethane will help, but the truth is subfloors are not intended to be the finished surface. The effort refinishing would take will never be rewarded with a good floor surface.

What you are describing is probably underlayment. The subfloor should be below that and on top of the floor framing.

Underlayment is often 1x4 T&G fir boards. When sanded it can be beautiful and quite serviceable but it will wear fast in a kitchen especially near the sink.

I am using my Douglas Fir subfloor and it is so beautiful! You can't find the old growth wood they used back then to make such a beautiful floor now. I think you should go for it, and if it doesn't wear well, you can put a harder floor on top in 3 years or 5 years or 10 years, or however long it takes to look bad. My main doorway floor is pretty scratched after 5 years of my dogs tracking in dirt, but it still looks WAY better than any newer fake stuff you could buy. And although it will never be "perfect" I definitely think it has a couple more refinishes left. Advice I would give, though, is definitely keep a rug over the main walkway, my floor would look much better if I had.

Carla in Sac

You haven't confirmed that the material is fir but if it is I would finish it with an oil-modified, solvent-based (commonly called oil-based) varnish. Polyurethane Varnish will provide a hard finish and Spar Varnish will provide a softer less brittle finish which won't resist abrasion as well but it should take dents better without separating from the wood.

I'm not sure if it's fir or not, but I will plan on using the polyurethane varnish - and rugs for the sink area and in front of the door! Thanks for the help, everyone.