Crushed eggshells in the planting hole prevent blossom-end rot.
Eggshells in the planting hole won't stop blossom-end rot. BER is a calcium-transport problem caused by uneven watering. It's rarely a soil-calcium shortage, and adding calcium to the soil is generally ineffective. Water evenly, mulch to steady the moisture, and soil-test before adding anything.
What is actually known
Blossom end rot is rarely a soil-calcium shortage. It is usually a calcium-transport failure inside the plant, driven by uneven watering (calcium rides the water stream, so erratic moisture starves the fruit tip even when soil calcium is fine; see the belief that Epsom salts prevent blossom-end rot). The real levers are even watering and mulch, and a soil test before adding any amendment.
The evidence
Evidence: checked against 1 source in the research-based literature.
The source
This is a problem born of calcium deficiency, most often the result of erratic watering
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Milpa Gardens. “Crushed eggshells in the planting hole prevent blossom-end rot.” Evidence grade: Refuted. https://milpa.garden/beliefs/eggshells-blossom-end-rot-eggshells-tomatoes/
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