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When the tomato plants are established, (2-3 weeks) apply a fertilizer low in Nitrogen and high in Phosphorous. Too much Nitrogen will produce lots of green foliage, but very little fruit. Fertilize at least once a month thereafter. (Never fertilize a plant that is wilting or is stressed from lack of moisture. Water the plant thoroughly, then wait a few hours and then fertilize.) Keep in mind that Tomatoes need one inch of water per week and slightly more in extreme heat.
Blossom-end rot occurs on tomatoes from a lack of calcium in the developing fruit. This results from slowed growth and damaged roots caused by any of the following factors.

Blossom-end rot
Mays Greenhouse 2010