The chemistry in plain English
When you dissolve Epsom salt (MgSO₄) you get Mg²⁺ and SO₄²⁻ ions in solution.
When you dissolve calcium acetate (from eggshells + vinegar) you get Ca²⁺ and acetate⁻ (CH₃COO⁻).
Ca²⁺ + SO₄²⁻ → CaSO₄, and calcium sulfate has limited solubility. At typical concentrations it can (and often will) come out of solution as a solid (gypsum). That’s exactly the “fail” you observed.
In short: sulfate + calcium → possible precipitate depending on concentrations and water chemistry. That problem is independent of whether the sulfate came from Epsom salt or another sulfate source.
Your options (pros/cons)
- Make two solutions and apply separately (best reliable fix)
Keep a Ca concentrate and a Mg concentrate separate, then apply them separately or mix only after heavy dilution.
This is exactly what commercial hydroponic nutrient makers do (two-part systems) to avoid precipitation.
PRO: avoids precipitation entirely. CON: slightly more fiddly.
- Use soluble chlorides (CaCl₂ and MgCl₂) and mix into one solution
Chloride salts of Ca and Mg are both very soluble, so CaCl₂ + MgCl₂ won’t precipitate CaSO₄ (there’s no sulfate) and you can have a single bottle.
PRO: simple single-bottle calmag, reliable.
CON: adds chloride to your feed — many plants tolerate modest chloride, but sensitive plants (some leafy greens, chloride-sensitive varieties) or high long-term use can cause issues. Also CaCl₂ can be warm to dissolve and is very hygroscopic (store carefully).
- Use carbonates (CaCO₃, MgCO₃)
Carbonates are mostly insoluble in neutral water. They only dissolve if you add acid (vinegar, etc.). So they’re not a convenient one-bottle water-soluble supplement unless you deliberately acidify.
PRO: slow release in soil if you use as an amendment (e.g., lime). CON: not for easy soluble stock solutions.
- Use acetate (calcium acetate) + chloride magnesium
Calcium acetate is soluble and acetate is generally plant-safe at low levels. Mixing calcium acetate with MgCl₂ is fine (no sulfate to make CaSO₄).
PRO: avoids sulfate precipitation. CON: acetate can affect pH and soil microbes if used concentrated; in small, dilute doses it’s okay.
- Buy a commercial chelated product
There are chelated calcium products but calcium is hard to chelate strongly (EDTA works less well for Ca than for Fe). These are pricier but avoid precipitation issues.
- Soil amendments instead of soluble solutions
For in-ground garden beds use dolomitic lime (CaCO₃·MgCO₃) or gypsum (CaSO₄) depending on your soil needs. These are slow-release, don’t require mixing, and avoid solution precipitation issues.
Is it OK to use human-grade MgCl₂ or MgCO₃?
Yes — the “human therapy” label just means it’s food/medical grade. Chemically, MgCl₂ or MgCO₃ are the same compounds whether sold as food/medical or horticultural grade. Just watch purity and any additives on the label.
Practical recommendations (what I’d try, in order)
- If you want one bottle and simple: buy calcium chloride dihydrate (CaCl₂·2H₂O) and magnesium chloride (MgCl₂) (food grade is fine). Dissolve both in water at a reasonable concentration for a stock solution and dilute to working strength when needed. Store sealed.
Keep concentrations modest; avoid making an extremely concentrated stock that you expect to keep forever (some precipitation can still form over long storage or with impurities).
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If you want foolproof and best chemistry practice: make two concentrated stock solutions — one Ca and one Mg (e.g., CaCl₂ stock and MgCl₂ or MgSO₄ stock) — and add them separately when you dilute to your final watering volume. That eliminates any precipitation risk and is what pros do.
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If you prefer “natural”/cheap DIY: use the vinegar → eggshell method to make calcium acetate, and use MgCl₂ rather than Epsom salt. That gives soluble Ca and Mg without sulfate. Just be cautious about vinegar leftovers (don’t make it too acidic).
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If you’re feeding soil (not hydroponics): consider soil amendments (dolomite, gypsum) rather than soluble calmag — slower but very safe and simple.
Safety & handling notes
Calcium chloride dissolving is exothermic (it releases heat). Add the salt to water slowly and don’t use boiling water.
Measure by weight (a kitchen scale) for consistency.
Keep stock solutions sealed (hygroscopic salts absorb moisture).
Don’t use high concentrations of any salt directly on plants (always dilute to working strength).
A few extra tips
Test your water and soil if you can (EC, pH). High background chloride or sodium can change what’s safe.
If you ever get a white flaky precipitate, it’s probably a sulfate or carbonate salt — toss that concentrate and remake it with a different pairing (or split into two bottles).
For many gardeners, occasional light feeding of a chlorides-based calmag at recommended dilution causes no trouble; problems show up with chronic overuse or in sensitive crops.