Unless you have a soil test indicating calcium deficiency, BER is more often due to erratic watering. First make sure you are watering deeply and not often.

Also, BER tends to go away on its own as the season goes on.

You could try bone meal or limestone dust.

Yes, as lazy implies, I am unaware of a Ca fert that is NOT organic by most measures. You can also save your expired antacids and put a tablet in the bottom of the planting hole, but I can't speak to whether that is organic enough for some.

DS

Lime if your soil is acid and gypsum if it's alkaline. Both supply all the calcium you will need. Strictly speaking I guess you could say they are not organic but both are mined from the soil and not chemically altered in any way. If you insist on applying an organic form I suppose oyster shell is the best way. Maybe you can talk your chickens into lending you some. As already noted egg shells also supply calcium but it would take an awful lot of them to make much difference.

Blossom End Rot is caused by a lack of Calcium at fruit set. Why there is a lack of Calcium can have many reasons stzting with the soil and the only way to know if your soil is deficient in Ca is with a good reliable soil test. The soil may have adequate levels of Ca but the plants cannot properly utilize it because there is a deficiency of Magnesium. Both Ca and Mg move through a plants vascular system with difficulty which is why an evenly moist soil is necessary so if the soil is allowed to dry quite a bit between watering then possibly there is a growth spurt when the soil is doused with water and neither the Ca of Mg move up quick enough to be present when the fruit is set resulting in the Blossom End Rot.
Putting egg shells or other sources of Calcium now will not help much since the Ca from those sources take a considerable amount of time to become available to the plants. Organic acceptable lime is ground limestone, hydrated lime is not.

If you have "Typical" soil in the deep south -- you are likely deficient in both Calcium and Magnesium. Dolomite lime is the traditional recommendation... It's just ground up limestone....

My own practice is to throw a tablespoon or two of dolomite lime under each tomato plant when you see the next round of flowers starting.... If you don't -- you get BER.....

Why? Glad you asked....

In the "Humid" south east -- our soils are famous for being very deficient in almost every mineral that there is... It's a consequence of our high rainfall and high temperatures.... No avoiding it... Minerals just leach out of the soil down into the subsoil....

There is a strong body of agricultural research going back past 1800 that indicates the soils of the deep south almost always need Lime.... There were even instances of calves eating plaster off of walls to get the lime inside the walls...

Thanks

John

The best all round trace mineral supplier in organic form is kelp meal but it's a bit pricey. It supplies trace minerals of practically any mineral on earth since sooner or later all minerals end up in the sea. There are no doubt many trace minerals that benefit plants that even botanists do not yet know are necessary.
That being said I do not hestitate to use Ironite spread over the soil when I till the ground. Minerals are minerals and the organic matter in your soil will convert them to organic quickly. As you may gather I believe in organic gardening but am not a hard nose purist like the trout fishermen that think the only trout worth having is one caught on a dry fly.