According to the website... by: Jacki Cammidge, Certified Horticulturist
According to the Sea Soil website, this soil is supposed to be complete, and doesn't need anything else added to it.
What kinds of plants are you growing? Are they in the same soil for many seasons? Or is it just the one? What makes you think the soil needs to be richer? I need a LOT more information to be able to make a suggestion.