I know this is kind of old, but for anyone just looking here for the first time. . .
I've learned through trial and error, research, and a friend that worked with Purdue University in Ag that calcium is rarely lacking so much as the plants ability to use it is. The most common causes are supposedly hot reservoir temperatures which affects the water's ability to hold oxygen and too high of EC which inhibits hydration. Calcium doesn't move without the water and it has to be constant since calcium is not mobile. Once it's in place the plant can't recruit it in reserve. Low oxygen also means low uptake. Can't recall the technical explanation, so if someone knows then please jump in.
Now, I'm not an expert and what I wrote there may not be completely accurate. I didn't write the stuff down like I should have. Just remembered the fix. See, the reason I did so much research was because I was having serious issues with deficiency. My tomatoes were in waterfarm type systems and the buckets were exposed to direct light outside. Plus, behind them was a white wall. The water temps always stayed high. Consequently, the CalMag I kept putting in did no good. I was at a loss. Now I realize that it didn't matter how much CalMag I used, the plants simply wouldn't have ever been able to take the stuff up. I cooled the reservoirs by shading with insulation left over from putting up siding. Guess what, the blossom end rot went away. . . The tomatoes were San Marzano Romas, by the way.
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