To prevent blossom rot, I put a handful of egg shells in the hole with the tomato plant when setting them. This gives the tomatoes calcium and it works.
For 2 dozen tomato plants, I use 2 gallons of crushed egg shells. It takes me a year to save up that many.
Whenever I use eggs, I rinse the sticky stuff out of the shells and set them on a paper towel to drain. After they dry, they are ready to crush and add to the bag. I usually let them build up in the bag and then put them in a bowl and crush with my hands and fingers. (This also buffs my rough skin and makes it smooth.) I store the bag in the pantry until spring planting time.
If you want to try this, you need to start saving your egg shells.
Smart! The spray calcium you buy to prevent blossom end rot is expensive. Egg shells are free. They release the calcium over a long period of time.