It's quite normal for high-class planets to not have extra space for low-tier soil enhancements.  You'll probably get a few as you unlock higher technology.

Quoting 00zim00,

reply 3

In this case might be nice to have a tool tip come up when u hover over an empty square telling u in advance if it can or cannot have upgrades later. Maybe even a toggle function to display it. Seeing as adjacent bonuses are kinda important, just my 2 cents

That would be useful.

That's what I've been doing ...

Is that comment really Necessary ??

MMMmmm lets see  STFU = Star Trek Federation University

The problem here is, for what ever reason, the tile improvements unlocked by the first levels of terraforming techs (which are now two sepearate improvements to help speed up early game planet quality) will not disappear if there are no tiles to be upgraded.  This can be a pain on "paradise worlds" which usually have few or no low quality tiles to improve.

Ironically, many, if not most, lower quality planets will eventually leave the "paradise worlds" in the dust when ultra-terraformers come into play.

Yeah it seems kind of odd that I actually look forward to colonizing the tier 9 ish planets more than the tier 15+ planets, because I know I'm probably going to end up with way more space to work with from the initially lower quality planets after I get all the terraforming techs researched.