More ferts and light= better mangrove growth, reductive sediments etc help.
I have been using clay loams which are typical in rice paddies with good results.
Marine bacteria will infest the soil and after about 1-2 months,m the soil is prepped and ready to go. Typically, I'll just add some tank water to a 1-2 liter cup and set it outside with a mangrove in that. Not quite full sun etc, just partial shade. Top off for evaporation etc.