User blog comment:Unownshipper/Metallurgists Wanted: Rapture's Natural Resources/@comment-24819226-20140810131157/@comment-75.36.142.53-20140810153433

The added minerals near certain land volcanos is what truns into rich soil, but those same minerals would go into solution into an endless supply of sea water and flow away.

Now as far as 'soil' for growing things inside Rapture, you usually add various nutrients in alot of modern farming anyway. So sterile seabottom materials only need be ground up and then nutrients like mulch from sea kelp and processed nitrogen fertilizers (a high energy process to create it). Same thing without soil at all in hydroponics which can maximize space usage (tunnels in bedrock if you want lots of volume) and can be significantly automated (those things on the BaS 'Pneumo Lines' look to be more useful for that then the games contrivance).

Fortunately you can recycle Iron alot and with (expensive)  high energy processes aluminium/magnesium can be got from many ocean rocks.

Biggest mass for building construction is probably still going to be cement (added to plenty of igneous rock aggregate)  or as mentioned elsewhere some ceramic alternative.