Stan, one of my great landscape lenses (really a great all around'er) is the Sigma 15~30mm EX - this lens satisfies my wide angle to semi-wide angle needs by covering the 15mm wide end and everything in between through 28mm and up to 30. This lens is very very [VERY] sharp, as sharp as pentax [star] glass and resolves extremely high. Another bonus with this lens is it is a FF lens so it is only one of a couple of wide lenses that work with FF bodies for pentax. Looking at the reviews here on the forum, it is rated a 9.67 (puts it near the top of all lenses in the database here) and that is not including my review of "10" which is not in there yet.
I probably got you all pumped up for it, now for the downside - there were very few of them produced for K-mount and it is pretty hard to find.
I guess I am with Canada_Rockies on this one, maybe pick up something that covers both focal-lengths you mentioned.