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"Paul M. Cook" wrote:

Just how much more can one do?
Not to be too terribly contrarian here, but you've done everything
except the very _first_ thing to do when you notice symtoms of mineral
imbalance: Check and correct soil pH! Second to water, pH is the major
determinant of nutrients' availability to plants. In my view, the first
course of action when signs of chronic mineral deprivation should be to
adjust pH to neutral-to-mildly-acidic (7.0-6.5, say) and keep it there
for at least 3-to-4 days -- preferrably a week -- before doing any
further soil tests or adding amendments. Remember to adjust the water in
which you place your soil sample to pH neutral and, while you're at it,
check the pH of your irrigation water. Certainly don't simply
gratuitously add stuff to the dirt ;-)

Soil pH tests 6.6 and my tap water tests 7.8.

You're finding out the hard way that nutrient and mineral
deficiencies are more easily prevented than corrected because most (but
not all) "organic" and/or "natural" sources of nutrients are relatively
low-proof and slow-release. However, to directly address your present
dilemma this site,
http://aob.oxfordjournals.org/cgi/content/full/95/4/571 -- cited
elsewhere in this thread -- along with advocacy of further investigation
into minimum levels of Ca relative to BER and of research into genetic
predisposition to BER, offers this recommendation (although, it provides
no evidence of efficacy):

...increasing the apoplastic Ca concentration in susceptible fruit tissue
should provide a
simple and reliable, practical solution for the prevention of BER in
tomatoes. It is
suggested that current horticultural practices, such as the manipulation
of the mineral
composition of the feed or the growth environment, are not completely
effective in
reducing BER because they affect apoplastic Ca concentration in fruit
tissue indirectly.
Therefore, spraying Ca directly onto young fruits is recommended for the
prevention of
BER.
I have seen liquid preparations offered as foliar spray Ca
supplements but have never noted their constituents. I assume the
calcium is present in an ionic form that migrates easily through foliar
and/or fruit cell walls but you never know.... It might be time to
retire to a garden center and read some labels.

I got this product called Foli-Cal. It is calcium acetate. The garden
center recommended it. I mixed it up according to instructions and doused
each plant with 16 ounces. Then I got the BER a week later. So today I
mixed up a batch and watered the roots with the mix.

Paul