When the tomatoes are growing rapidly, they can "outgrow" the plant's ability to keep the growing fruit supplied with enough minerals, especially calcium,

One of the critical factors is moisture to keep the nutrients flowing towards the fruit and within the fruit ... keep it even. Mulch thickly. But don't drown the roots.

pH affects the availability of soil minerals - within a broad range. I add sulfur to lower the pH (AZ alkaline dirt), where someone else might have to add lime to raise it.

THEN, after you have watering and pH adjusted, try adding calcium and magnesium (they interact) ... but not too much.

It's a bit of an art.

lazygardens hit the nail on the head... also get your soil tested by a pro.. not a test kit that you by a job lot or agway..

After battling BER for the last few years on my Super Marzanos, I switched to resistant varieties. This is the time of year when I'd normally see it and so far, not a spec. We'll see!

Check out FAQ on the tomato forum. Good article there.