Answer: Blossum-end rot is common on peppers, tomatoes, squash and watermelons, and is caused by a lack of calcium in the developing fruits. This lack can be caused by several things: fluctuations from too dry to too wet; rapid early growth followed by extreme dry periods; excess salts in the soil; as well as cultivating too close to the plant.
The solutions require that you limit those factors from affecting your plants! Keep the moisture level even; avoid use of high-nitrogen fertilizers and fresh manures; be sure the soil is well drained; water heavily when you do
water, in order to leach out the salts in the soil; and last, don't cultivate within a foot of the plant, or deeper than one inch. It's also a good idea to mulch plants with a 3 or 4" layer of straw, to help keep soil moist.
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