Adding potting soil to the gritty mix would defeat the purpose of the gritty mix (porosity and durability). If you're looking for a compromise between the gritty mix and a typical potting soil, you should look at something like the 5-1-1 mix. It would be great for ferns since they generally prefer more moisture than other plants. Here's an excellent thread documenting the growth and repotting of a fern in 5-1-1.

Giant Chainfern potted in 5-1-1 (pics)

-Chris

Chris, thanks for the link. It's pretty much what I've been mixing. I just want a mix that won't pack down, and this one won't. I'm experimenting with a gritty mix to see what will do well in it, but if other people have already done it with plants that I'm thinking about, I'll learn from their experience, rather than risk a crash!

Thanks for linking, Chris!

Eahamel, a bark-based mix with perlite and peat makes for a happy fern, in my experience.
The bark and perlite should give you 2 - 3 years before the mix begins to degrade...but by that time,
the fern will most likely need re-potting anyway.

Josh

Josh, I went to a local nursery that specializes in tropicals. The owner has a collection of ferns of his own, and told me he uses a non-organic mix similar to what I'm using, and adds about 50% Sunshine Mix, which has a lot of peat in it. I repotted a couple that I'd just put into hanging baskets (wire with coir), and added more of the soil, so they won't dry out so fast. I hate watering every day.

Now I get to work on repotting a really big Boston fern that's in a 10" plastic basket.