Blossom-end rot comes from uneven watering
Blossom-end rot comes from uneven watering, which stops calcium reaching the fruit. It is usually not caused by too little calcium in the soil.
As recorded: Blossom-end rot is a calcium-transport failure driven by uneven soil moisture. A shortage of calcium in the soil is usually not the cause.
Well established A person has checked it against its sources. If you skip this, it still grows, just less well. The planner suggests it.
How it works
Calcium reaches the fruit in the water the roots draw up, 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, without 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, because 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. No estimate is made.
The evidence
Evidence: checked against 3 sources in the research-based literature.
The 3 sources
This is a problem born of calcium deficiency, most often the result of erratic watering.
Mix in 1/4 cup of gypsum (calcium sulfate) with the soil from each planting hole
Calcium is taken up from the soil through the plants
Plus one working note on how the sources were read. Every note is in the data download.
Plants it applies to · 7
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How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- Details updated
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Milpa Gardens. “Blossom-end rot comes from uneven watering, which stops calcium reaching the fruit. It is usually not caused by too little calcium in the soil.” Evidence grade: Well established. https://milpa.garden/rules/blossom-end-rot-calcium-transport-failure-driven/
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How we grade evidence
- Well established
- The rule names its mechanism and is settled, by sources a person has read and checked or by plain arithmetic and geometry.
- Promising
- The mechanism is named and the literature backs it. A few questions are still open.
- A good hunch
- Plausible by its mechanism, but field studies are thin, so treat it as a lean.
- Contested
- The evidence points both ways, or no study has tested it, so the planner does not enforce it.
- Refuted
- The claim did not survive its sources. It is kept so the folklore can be answered.