Blossom-end rot is a calcium-transport failure driven by uneven soil moisture, not a shortage of calcium in the soil.
Well established · suboptimal if ignored → advise
How it works
Calcium reaches the fruit in the water the roots draw up: it is carried by the xylem along with that stream. When the supply of water is interrupted the supply of calcium is interrupted with it, and the enlarging fruit runs short at its tip - even where soil calcium is adequate.
What to do about it
Keep the soil evenly moist rather than letting it dry out and then soaking it, and mulch to hold that moisture steady. We are not naming a calcium amendment or a rate: the sources disagree on whether adding calcium helps at all, and any amendment number would need a soil test first.
How much it matters: Not quantified by any source read. All three describe the disorder and its prevention without a rate, an incidence figure or a yield effect, and none states how much moisture variation is enough to cause it. Deliberately left unquantified rather than estimated.
The evidence
Evidence: confirmed against 3 sources in the research-based literature.
the 3 sources
Iowa State Univ. Extension, Yard & Garden, 'Blossom End Rot', yardandgarden.extension.iastate.edu/encyclopedia/blossom-end-rot (fetched in-container 2026-08-02, READ AND CONFIRMED BY THE MAINTAINER 2026-08-02): 'This is a problem born of calcium deficiency, most often the result of erratic watering.' On the amendment question, which is why this rule refuses to name one: 'Adding calcium to the soil is generally ineffective.' And the remedy: 'Mulch the area around tomato plants to conserve and maintain uniform soil moisture levels.' [read]
Univ. of Maryland Extension, 'Blossom End Rot on Vegetables', extension.umd.edu/resource/blossom-end-rot-vegetables (fetched in-container 2026-08-02, READ AND CONFIRMED BY THE MAINTAINER 2026-08-02): THE SOURCE OF THE CROP LIST - 'tomato, pepper, eggplant, pumpkin, squash, and watermelon' - which is why `ber_susceptible` is a species field and not a family gate: that set spans two families and excludes potato. Cause: 'a shortage of calcium in enlarging fruits', linked to 'inconsistent watering, shallow watering or droughty conditions'. WHAT THIS SOURCE DOES NOT BUY, recorded so the rule cannot grow into it: it also says 'Mix in 1/4 cup of gypsum (calcium sulfate) with the soil from each planting hole' and that a calcium chloride spray 'may offer some temporary relief'. That is an amendment quantity with no soil test behind it, which invariant 3 forbids regardless of whether the agronomy is right - and the other two sources read contradict it outright. [read]
Clemson HGIC, 'Gardening Myths: Fix Blossom End Rot with Calcium Sprays', hgic.clemson.edu/gardening-myths-fix-blossom-end-rot-with-calcium-sprays (fetched in-container 2026-08-02, READ AND CONFIRMED BY THE MAINTAINER 2026-08-02): the mechanism this rule states, in the source's own words - 'Calcium is taken up from the soil through the plants' roots and is transported by the xylem, along with water.' On sprays: 'Foliar calcium sprays are not effective', because 'the skin of the fruit is not able to directly absorb foliar-applied calcium.' And it lands where another rule here already stood, independently: it recommends conducting a soil test before planting. [read]