If you check over on the Growing Tomatoes forum here you'll find many discussions about this problem. It isn't fertilizer related, it is a watering issue that causes splitting.

Yet another reason why picking at color break is the standard recommendation rather than leaving the fruit on the vine.

Dave

Here is a link that might be useful: tomato splitting discussions

Marg, It is not necessary to water every day, even in a hot, dry spell as we had in June/July. Water deeper but less often (2-3 times per week.) Use mulch such as straw (not hay) to keep the moisture in the soil longer.

I would suggest you apply several inches of mature compost this fall. This will improve the soil and help hold moisture, and you can stop using chemical fertilizers.

Thanks for the help. We have heavy clay soil that we amend with composted straw each fall. We spread it on the garden and let it sit for the winter and then till it in. The only fertilizer we use is Aggrand, which we use as a foliar spray.