The Henchman wrote:
I'm in Southern Ontario Zone 5b (Niagara escarpment)

I have tomatoes in pots, two plants per pot, staked. Pots are 12" x
12" x 12" deep. A bit of a gravel bed on the bottom, maybe 3/4"
thick. They face south and get 7 hours of sun a day. Soil is
triple mix topsoil with 1/3 peat moss, and drainage holes at bottom
of pots. These plants were planted by children in a school then we
inherited the plants when school was out for summer. They flowered
nice.
The bottoms of some of the tomatoes have gone black. A quick Google
search suggests this is prolly due to lack of calcium but we have 25
hardness hard water. Our garden water is not run thru the water
softener.
How can I determine if it's calcium defiency or not? What other
possibilities can this be? The leaves never wilt and growth and
foliage seem healthy. No bugs are eating the leaves. I've never
grown tomotes before. Our herb gardens and flower beds are fantastic
and are watered at the same intervals as our tomatoes.

If somebody wants pictures I can through up on the net if so.

Blossom end rot is a complex problem. Some highly qualified authorities are
not sure exactly how it happens although they seem to agree it is related to
calcium nutrition. There was a long thread on this at rec.gardens a week or
two ago. To summarise:

- BER is an issue with calcium mobility which occurs most often when the
plants are young and growing quickly. It often goes away without
intervention as the plants mature.
- It can be caused by lack of calcium in the soil or uneven watering which
can interfere with nutrient mobility.
- Some cultivars are more susceptible to it than others.
- If adding lime to supply calcium do not overdose as this may raise the pH
too far which will cause other nutrition problems.

David