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✕Avoid
Epsom salt for tomatoes
Blossom end rot is a calcium delivery problem. Epsom salt is magnesium, and adding it competes with the calcium.
What it is
Adding magnesium sulfate to the planting hole or as a foliar spray, usually to prevent blossom end rot, sometimes for general vigor.
The numbers
- Blossom end rot is a calcium problem, and almost never a calcium *deficiency in the soil* — it's a delivery failure caused by uneven watering, since calcium moves with water.
- Magnesium and calcium compete for uptake. Adding magnesium to fix a calcium delivery problem pushes in the wrong direction.
- Most soils have ample magnesium already, and a $15 soil test will tell you whether yours is one of the rare ones that doesn't.
Why it's here
Because the fix that actually works is both free and automatable, which makes this a satisfying one to get right.
The catch
The real fix is consistent soil moisture — mulch heavily, water deeply on a schedule rather than in panic bursts, and if you want the automated version, a soil moisture probe driving a drip valve will end blossom end rot in most gardens by itself. Deal with the water, and the calcium takes care of itself.