Last year I experienced 1 container where are my tomatoes had blossom end rot. The cause for this is suspected to be the irregular watering, which causes the plant not to absorb calcium. I feel that it could be just from a lack of calcium in the soil. Researching for a solution I found some interesting treatments to reduce or eliminate this problem. 1 method is to give the plant milk which incidentally is also a fertilizer. They use powdered skin milk out of ratio of 1 part powdered skim milk to 9 parts water and just use it to water your plant. The claim is this is one of the fastest ways to get calcium into the plant because powdered milk dissolvesfast. Another method is the place a Tums inside the hole where you plant your tomatoes. You can also crush the Tums and top dress the plant. If you have any suggestions I'd be glad to hear it.
Calcium chloride solution is made for just this problem. See here if you use Amazon. You can find it in your local garden stores as well. It is best to stop it before it starts, so make sure you have adequately limed (or use gypsum if you have soil with high pH), or use a Cal-Mag fertilizer.
So far the tomatoes I have this year are showing no signs blossom end rot. I had some hard spackle that I pulverized and mixed in what the potting soil I used for the tomato plants, maybe it's working.
By the time they show blossom end rot, it's too late to stop it. I usually put Epson salt under my plants and don't get blossom end rot. I didn't do it last year and lost a lot of my tomatoes. I Binged Epsom Salt for tomatoes and found this. "Epsom salt is simply magnesium sulfate, a pure, naturally occurring compound with dozens of uses— Studies show that magnesium and sulfur, two naturally occurring minerals that are major components of Epsom salt, have been known to help plants grow greener with higher yields and more blooms. Grow bigger, better plants with Epsom salt. Experts recommend gardeners use Epsom salt to prepare soil for planting and to treat and feed specific plants, including peppers, tomatoes, fruit and roses. Epsom salt can also help keep house plants, lawns, trees and shrubs healthy. Tomatoes Dilute 1 tablespoon Epsom salt in water for every foot of plant height per plant. Apply every 2 weeks." I didn't know this. I'm going to try it. Weed your garden with a chemical and toxin-free solution including Epsom salt. You can safely create a chemical-free and natural weed killing solution by mixing two cups of Epsom salt, one gallon of white vinegar and a quarter cup of gentle washing liquid into a spray bottle. Ensure you soak the entire weed with the mixture and wait for a day to inspect and repeat until all the weeds are eliminated. How to use Epsom salt with vegetable plants In general, Epsom salt can benefit the majority of vegetable plants, but it particularly works wonders for tomatoes and peppers. This is mainly because both these plants are naturally deficient in magnesium. Magnesium sulfate can help prevent rot and enables plants to establish a deeper root system and produce a higher yield. Incorporate the salt into the soil while planting the veggies so they can access the necessary minerals. After that, keep spraying the plants with the Epsom salt solution every two weeks or so, to provide them with a consistent supply. Actually, I don't add it regularly. I do mix it with water and water the plants about midway of the summer.
I have used Epsom salt I have like you watered plants with it. When I see small fruit I use it. I find it works great on Pepper plants.
Magnesium is required to develop color, so tomatoes need it to ripen as do peppers. I posted an experiment I read about in another post some years ago. If you plant flowers, use Epsom salt on some of them and not on others, then compare the vibrancy of color. According to one source, the magnesium will brighten the colors on many, but not all, flowers. Give it a try if you are so inclined.