Just realized that I should of had this posted in the Pest and Disease forum.

This forum is fine since BER is neither a pest or a disease. However it isn't primarily a lack of calcium issue as a review of the numerous discussions here about it will show. The search will pull them up for you to browse through.

Aside from the fact that some varieties (primarily paste types) are more prone to it, it is primarily a soil moisture, a hydration issue. Consistent soil moisture levels that can insure adequate delivery of the available calcium to the fruit at crucial development points. For container grown plants that consistency can be difficult to get. And for outdoor plants during early spring unstable weather is can also be quite difficult.

But many studies show that once the plant matures and develops a good root system and the weather stabilizes most outdoor plants will out grow it. Container plants, not so much.

http://www.gardenweb.com/gardenweb/query/BER/topic=tomato

Dave

Just to add to what Dave said, BER is often due a lack of calcium within the plant, but not necessarily a lack of calcium in the soil. It's about poor calcium uptake by the plant. Too much nitrogen in the soil can interfere with that uptake, as can other stresses. So loading the soil with calcium may not fix the problem. True, if your soil is highly deficient in calcium, the plant won't be able to uptake much, but most soil has plenty. I'm sitting on top of limestone caliche, and my soil has HUGE amounts of calcium. But my neighbors have still gotten BER.