Just because the shell fragments are too small for you to see and differentiate from other parts of the soil doesn't mean that they've decomposed into their constituent elements. They can do so, but the soil needs to be fairly acidic (have a low pH) in order to react with the calcium carbonate and free up the calcium in any sort of reasonable time frame.
That's what I am reading from people who are lots smarter than me on this issue. Well said.