Apparent Blossom End Rot?
I am growing a variety of tomatoes. The plants are in large pots and growing like crazy. However, several varieties are producing some tomatoes with black and/or deformed bottoms on the fruit. I included oyster shell in the soil initially and use a fertilizer that contains calcium. What more should I do???
This is a condition known as "blossom end rot". Oyster shell does contain calcium, but it would break down extremely slowly and the calcium would not be available to your plants this year (or even enough for next year.)
Lime is used to guarantee that plants have enough calcium. There are other factors that can contribute to your plants not having enough calcium (such as cold weather or drought), so please read over this info from our website which comes from our Grow It Eat It section: http://extension.umd.edu/growit/blossom-end-rot-vegetables It gives you a list of steps to take.
The article mentions using lime in the soil at planting time. Can I apply now to my existing plants to ensure adequate calcium?
Your best bet at this point is to visit your local farm store or well-stocked garden center and look for a product called 'Blossom End Rot Stop'. This is a calcium supplement in an aerosol container. The spray should be applied directly into each developed flower.