Yes crushed eggshell work also. They and oystershell and small limestone all provide some buffering of pH in the mix. Dolomitic limestone contains some magnesium as well as calcium. All are slow acting. Calcium nitrate is quick and the nitrate is good also. Magnesium sulfate also suppiles a little sulfur along with the Mg. Rough proportion is 2:1 or 3:2 - CA:MG Both help the plant to produce chlorophyll. If you mix the two don't put them in solution at high concentration. You can get a preciptate of Ca SO4 (gypsum). When dissolving fert I use 1/4 teaspoon normal fert + 1/8 t. of Calcium nitrate per gal. after that is dissolved then I add a little less than 1/8 t. of the epsoms salt and mix again. Often you can find Calcium Nitrate (small BB sized pellets)in 50 lb quantities for landscaping fert or used as an additive to concrete to keep it from freezing when pouring in cold weather.