Watering with water-softened water contributes to blossom end rot
I found that watering tomatoes using softened water (from a water softener) can contribute to blossom end rot. Softeners reduce calcium (among other agents). After years and years of having the blossom end rot problem, I started bypassing the water from the softener. Eureka, it worked. Best crop ever.
Tomato Dirt note: You can use these natural products specifically developed to treat, prevent, and slow blossom end rot in tomatoes: Enz-Rot Blossom End Rot (a concentrate that can make up to 8 gallons) and Tomato Rot-Stop in a ready-to-use spray bottle.) Follow package directions for application.