The Q&A Archives: Using Calcium Nitrate to Control Blossom End Rot on Tomatoes

Question: We grow tomatoes in large containers on our deck. Some varieties, especially Italian plum types, have a tendency to develop blossom end rot. I know that we have to be vigilant with watering, but occasionally things get away from us. I recently read a recommendation for adding calcium nitrate to the containers as a preventative measure. Where can I find calcium nitrate? Will bone meal serve just as well? What should be the proportion of either to water per plant? MaryAnne Spinner Chicago,IL

Answer: Blossom end rot is the result of a calcium deficiency in the cell wall of developing fruit, caused by moisture fluctuation. If you have trouble keeping up with watering your plants, mulch the soil surface with straw or other organic material to help maintain soil moisture, and reduce the plants' exposure to drying winds. Also, consider using self-watering pots, or set up a small drip irrigation system for your containers. Adding calcium as a foliar spray may help prevent the problem in fruit that develops later this season. Calcium nitrate may be available at local garden centers, or try a calcium product called Calcium 25 from Peaceful Valley Farm Supply, P.O. Box 2209, Grass Valley, CA 95945, ph# 916-272-4769. Application instructions will be listed on product labels. Hope this helps!

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