Answer: Actually, all plants require some calcium as a micronutrient. Most soils contain sufficient calcium and most balanced fertilizers will also contain it. All fertilizers must list their composition on the label. (I'll include some basic info on nutrients and fertilizers below.) Calcium becomes a factor for tomatoes with a condition called blossom end rot. BER is basically an imbalance of calcium in the soil, often aggravated by varied watering schedules. Tomatoes are moisture sensitive and need a regular supply of water to thrive and produce fruit.
There are several factors which can lead to blossom end rot: insufficient available calcium in the soil, rapid early season growth followed by extended dry period, excessive rain which smothers root hairs, excessive soil salts which "lock up" calcium uptake (usually caused by a fertilizer which is too high in nitrogen or is applied too often and nitrogen builds up), and, cultivating too close to the plant which kills rootlets.
Keep blossom end rot at bay by providing uniform soil moisture, avoid high nitrogen fertilizers (and, follow application rates carefully no matter what you use), plant in well drained soil, and, when cultivating within 1' of the plant, do not cultivate deeper than 1". When fertilizing, look for a reliable brand, high in calcium or an amendment such as lime, gypsum, or bone meal will help.
Here's some basic info on fertilizer and nutrients that plants require. The 3 numbers on a fertilizer bag refer to the percentage of N (nitrogen), P (phosphorous), and K (potassium) in the bag. There are different formulations for different purposes. In general terms, nitrogen produces lush green growth, phosphorous helps strengthen stems and produce flowers, and potassium keeps the root system healthy. If you're applying fertilizer to fruiting (e.g., tomatoes) or flowering plants, you're not as interested in the plant developing leaves as you are in it flowers and fruit, so you'd use a formulation lower in nitrogen and higher in phosphorous, such as Miracle-Gro's Plant Food at 15-30-15. Bone meal is an organic source of phosphorous. Other nutrients that plants need, but in lesser amounts, are referred to as micronutrients and include: calcium, magnesium, sulfur, boron, chlorine, copper, iron, molybdenum, and zinc. These are usually available in the soil in sufficient amounts.
You can remove the affected tomatoes and then keep the soil evenly moist to prevent future tomato fruits from developing the same problem.
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