What kind of soil is in the saltwater biome?

It depends on the area in question; in a kelp forest, the soil is Excellent, in a place like the Challenger Deep, while there are nutrients pressent, they are usually swept up to the surface. For example: a red tide is caused by a surge of nutrients coming in from deeper waters into where a species of dinoflagellates resides, causing their numbers to skyrocket, causing them to shake the ecosystem. In estuaries, the soil is also Excellent, as the nutrients brought in by the tides keep on coming in at high tide.