Bottom rot or Blossom End Rot. Too late to fix it this year-I had that
problem last year. The soil needs some chemicals-I stand remember which
ones, but ask your local garden store and till them into the soil early
next year. One note: if you also grow cukes(squash, etc.) in that
garden, they do not like the chemicals for tomatoes. Keep that soil
separate.

Sly boots 9 wrote:

my crop of tomatoes is not so great. Some of the fruit has black, flat bottoms.
What is this, what causes this and can I prevent it. Any help, thanks