I kept blaming “humidity” when new leaves felt flimsy and creased while unfurling. The fix turned out smaller: add a little calcium and magnesium to my soft water and keep the light and airflow steady. Two leaves later, the difference was obvious.
“Once I measured TDS and dosed Ca/Mg to a target, the plant stopped acting fragile—and I stopped chasing 75% RH for the whole room.”
Quick answer
- Targets (at the leaf): water TDS ~100–150 ppm, with roughly 20–40 mg/L Ca and 5–15 mg/L Mg.
- Why: Calcium = cell wall strength in new growth; magnesium = chlorophyll core. In very soft water, both can be near zero.
- How: Start with RO/rain, add CaCl₂·2H₂O + MgSO₄·7H₂O (Epsom salt) using the recipe card below.
- Judge success on the next leaf. Old leaves won’t thicken after the fact.
For humidity and airflow that support clean unfurls (without fogging the whole room), see Cloud Forest Plant Care & Humidity at Home.
Background in plain English
Soft water (TDS often <60 ppm) is kind to pots but can leave aroids under-mineralised, especially during active growth. You’ll see thin, crease-prone new leaves and occasional leaf-edge oddities that look like “humidity problems.” A tiny Ca/Mg buffer plus steady light/air usually stabilises things.
“Do I need a fancy GH test?” A simple TDS pen is enough to mix reproducibly at home; it won’t tell Ca vs Mg, but it keeps your batch repeatable.
Simple Ca/Mg recipe (mg/L → grams)
Goal band (per litre of finished water): 30 mg/L Ca and 10 mg/L Mg (lands most homes near 120–150 ppm TDS from RO/rain).
Use food/analytical-grade salts: Calcium chloride dihydrate (CaCl₂·2H₂O) and Magnesium sulfate heptahydrate (MgSO₄·7H₂O, Epsom salt).
Per 1 L finished water
- CaCl₂·2H₂O: 0.11 g → ~30 mg/L Ca
- MgSO₄·7H₂O: 0.10 g → ~10 mg/L Mg
Per 5 L finished water
- CaCl₂·2H₂O: 0.55 g
- MgSO₄·7H₂O: 0.50 g
Per US gallon (3.8 L)
- CaCl₂·2H₂O: 0.42 g
- MgSO₄·7H₂O: 0.38 g
Method note (why these grams?): Ca fraction in CaCl₂·2H₂O ≈ 27.3%; Mg fraction in MgSO₄·7H₂O ≈ 9.9%. The gram values above back-calculate from the mg/L targets and keep the math reproducible at home.
How to mix (repeatable in any kitchen)
- Start with RO/rain. Measure out your final volume (e.g., 5 L).
- Dissolve salts separately. In two cups of warm water, dissolve the Ca salt and the Mg salt fully.
- Combine into the main jug, stir gently, and top up to volume.
- Measure TDS. You should land near 120–150 ppm; adjust next batch by ±0.05 g if needed.
- Label & store capped for up to 1 week at room temp; shake before use.
Where this helps most (and where it doesn’t)
- Helps: soft-water homes, new growth that creases/tears, plants pushing frequent leaves (philodendrons, anthuriums, monsteras).
- Doesn’t fix: variegation colour (that’s light genetics), chronic overwatering, or true nutrient lockout. Pair this water with bright, even light and gentle airflow. For practical light targets, see Best Light Conditions for Rare Tropical Orchids Indoors—the lux logic translates well to aroids.
Dosing table (tweak for your tap)
| Your source water | Quick read | What to do | Expected result |
|---|---|---|---|
| RO/rain (TDS <10) | basically zero | Add full recipe above | Ends ~120–150 ppm; Ca 30 / Mg 10 mg/L |
| Very soft tap (TDS 20–60) | nearly zero | Add ¾ recipe; re-check TDS | Ends ~100–150 ppm |
| Moderate tap (TDS 120–200) | has minerals | Skip recipe; just leach monthly | Avoid stacking salts |
| Softened by sodium | Na-softened | Avoid for plants; prefer RO/rain | Prevents sodium stress |
Safety note: Household sodium softeners swap Ca/Mg for Na⁺; rainwater is preferred for houseplants. The Royal Horticultural Society puts it plainly and recommends not using softened water for watering plants—see their guidance: RHS — Water: using softened and other types.
What I actually do in my apartment
I mix RO: 5 L with 0.55 g CaCl₂·2H₂O + 0.50 g MgSO₄·7H₂O, which lands ~130–140 ppm TDS. During active growth I water with this every other time and use plain RO/rain in between, flushing pots every 6–8 weeks.
Common pitfalls (and quick fixes)
- Over-dosing “for strength”: Keep Ca in the 20–40 mg/L band, Mg 5–15 mg/L. Higher isn’t better indoors.
- Chasing room-wide 70% RH: Stabilise leaf-level humidity instead; micro-zones beat whole-room fogging. See Cloud Forest Plant Care & Humidity at Home.
- Using softened tap: If your kitchen tap is softened, don’t use it for mixing—collect bathroom cold tap (often unsoftened) or use RO/rain.
FAQs
Will Ca/Mg bring back variegation?
No—variegation is genetic and light-driven. Ca/Mg improves leaf integrity, not colour.
Can I use a bottled “Cal-Mag” instead of salts?
Yes—dose by label to land near 120–150 ppm TDS in your finished water. If the product lists NPK, remember you’re also adding nitrogen—keep fertiliser light.
How fast will I see change?
On the next leaf (typically 3–6 weeks in active growth). Old leaves won’t thicken after dosing.
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