I'd go with Melgyver on this. Most of the airstrips seem to have been almost totally devoid of vegetation except on the outer fringes.
The type of "soil" really depends upon what island the airstrip is on and where on that island the airstrip is.
If you don't care about any of this, just do what you want and don't worry about what "should and shouldn't be".
As far as the grass, static grass is my fave. For dirt, I use dirt. Real dirt, sifted, is really good stuff. I usually brush some thinned white glue onto the base and cover it with dirt. Let it dry and then shake, tap and blow off the excess. Touch up wherever necessary. If you want to change the color, either mix in some powdered pastels, or mix some flat water-based paint in with the glue. The dirt dissolves into the glue some, and will mix with the pigment.
As for the grass I, again, brush on some thinned glue and then sprinkle a ton of static grass onto the appropriate areas. You can blow horizontally across the top of the base to make the stuff stand up. Let it dry, then remove the excess as with the dirt. Make sure to try and save as much of this excess as you can, there will be alot of it and you can re-use it.
I usually paint the grass when it's dry, staring off with a dark green and then working my way up to light greens, yellows, browns... whatever look you're trying to achieve.
This ain't the best photo, and it ain't my best example of grass, but I developed the techniques I described above while doing this base.
Click on the picture for a larger image.
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