Planetary soil upgrade, can't be placed

I've seen this before. I think what happens that each hex has a land quality value to it. It think this has something to do with how much of the tile looks like land, and how much looks like water. Different terraforming techs seem to require a minimum land quality in order to be applied. I think the problem you are dealing with on your world is that it has no tile that meets this minimum rating for soil enhancement, so you can't build it.

Yeah, the terraforming type placements are set to certain land % required.  It happens semi-frequently to me.  Ideally, if there are no tiles which meet the minimum land for a particular terraforming type, then that improvement should not appear on the list.  

The problem i had yesterday was that soil enhancement didnt appear on the list, after i used it once.

When i placed it the first time i had a choice of 3 tiles to place it on, so i thought i would be able to do it 2 more times after the first. But it never reapperaed on the list.

Many of the terraforming projects can only be used once per planet. It would likely be beneficial for the developers to communicate this more clearly to the players. It would also be beneficial for the terraforming projects to be revised, since under the current model there are some repeatable terraforming projects and some one-per-planet terraforming projects, and the one-per-planet terraforming projects can, if I'm not mistaken, be wasted on tiles which could be terraformed with repeatable terraforming projects.

But this is undoubtedly part of the efforts by the developers to "improve" the game by giving the players "meaningful choices," such as "which four extreme world types do I want to be useless to me for the rest of the game," complete with the back door of trading for the other supposedly mutually exclusive technologies to remove any semblance of sense from the technologies being mutually exclusive and reduce the choices from {(delay advanced tech to get these things) or (trade for them) or (don't bother with them until I have nothing better to spend my production on)} to {(trade for them) or (don't bother with them)}. Clearly the latter choice is far more meaningful than the former.

All terraforming tile improvements EXCEPT for Ultra-Terraformer (which is based on the lowest level of tiles that can be fixed) can be used exactly once per planet.  This is by design so players won't build Super Planets too early.  It's something I personally disagree with, but I realized long ago this was a lost battle and moved on.

As it is, making it so all the terraforming tiles can be used as many times as one wants is trivial to mod.  Literally all it requires is removing one line from each of the improvements in the XMLs.  Alternatively, one can make buildings available that mimic the Ultra-Terraformer for the "higher" levels of terraformable tiles via modifiying the tech tree and creating new planetary improvements.  That requires slightly more work, however.  Not too much work.  But some nonetheless.

Thank you for this information.

Thinking about it i am ok with this, but it should be made clear in the buildings description ... in big red letters.