Answer: Blossom-end rot is caused by a calcium deficiency aggravated by widely fluctuating soil moisture and high temperatures. It occurs because there's too little calcium in the soil, too much nitrogen applied as fertilizer; very wet or very dry soils which interferes with calcium uptake; or combinations of these factors. To avoid Blossom-end rot, apply lime to adjust the soil pH to 6.8 - 7.2, fertilize only moderately, plant where drainage is good, and water to wet all soil in the root zone every 7-10 days. Eggshells are a form of calcium, but the minerals are released too slowly to be of much use to the plant's rootzone.
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