Calcium and buffers

Just a thought, but wouldn't calcium carbonate be a better buffer for a
planted tank than sodium bicarbonate, since sodium isn't all that
desirable, and someone noted recently that some plants (especially
Echinodorus sp, right?) are often calcium deficient?

Boy oh boy am I having a rough time getting my water properly buffered.  I
am having to dose about every other day with some source of KH to keep up
with my yeast CO2 (producing a little more than 1 bubble per second).  I'm
thinking of putting a piece of shell in the tank.  Thoughts?