I've been adding a lot of baking soda as a buffer. I've been checking
the salt level in my pond, and it doesn't show any change. I don't
want to kill my plants.

Does anybody know whether most salt test kits test for sodium,
chloride, or both? Is it the sodium in salt that kills plants?

Any reason I can't use powdered magnesium/calcium carbonate to
increase alkalinity as long as my general hardness is also low?