Magnesium Deficiency in Hydroponics — Symptoms & Fix
Interveinal yellowing on older leaves means magnesium deficiency. Diagnose K antagonism and RO water — fix with Epsom salt at 1 g/L.
BY ROOTLESS FARM
Quick answer
Yellow leaves with green veins on the older lower foliage = magnesium deficiency. Two causes dominate: RO water without remineralization (no Mg in the input) or high K crowding Mg out of uptake. Add Epsom salt at 1 g/L for 100 ppm Mg, hold K below 300 ppm, and keep pH at 5.8–6.2. New leaves green within a week.
Symptoms
- Interveinal chlorosis (yellow between green veins) on older leaves first
- Yellowing starts at leaf tips and works inward
- Leaf edges may curl upward
- Older leaves drop in severe cases
- Newer top growth stays green initially
- Reduced fruit sweetness and overall yield [OSU-NUT-01]
Cause
Magnesium is phloem-mobile and sits at the center of every chlorophyll molecule — when supply runs short, the plant strips Mg from older leaves to feed new growth, producing the characteristic interveinal pattern. The veins stay green because the bundle sheath holds Mg longest. Two failure modes account for most cases. First, reverse-osmosis or distilled source water has zero Mg, and many hydroponic recipes assume 15–25 ppm from tap; the RO grower is silently 20% under-spec from day one [OSU-NUT-01]. Second, K and Mg share root transport channels, and a fruiting-stage K bump to 350 ppm pushes Mg out of uptake even when solution Mg looks fine.
Diagnose
| Check | Target | Deficiency signal |
|---|---|---|
| Solution Mg | 40–60 ppm | < 30 ppm |
| Solution K | 200–300 ppm | > 350 ppm (antagonism) |
| pH | 5.8–6.2 | < 5.5 (uptake drop) |
| Source water | tap or remineralized RO | pure RO without supplement |
| Leaf pattern | green | interveinal yellow on old leaves |
Differentiate from iron deficiency (same pattern but on young leaves) and from nitrogen deficiency (uniform yellowing, no veins retained green). Tissue test confirms: leaf Mg below 0.25% dry weight is deficient.
Fix
- Add Epsom salt (MgSO₄·7H₂O) at 1 g/L to the reservoir — this delivers ~100 ppm Mg and ~130 ppm sulfate [OSU-NUT-01].
- Drop K to 250 ppm if you are mid-fruiting and saw deficiency creep in — relieves antagonism.
- Hold pH at 5.8–6.2. Below 5.5, Mg uptake drops sharply.
- For RO users, remineralize the source water before nutrient addition: add Cal-Mag or equivalent to reach 150 ppm Ca and 50 ppm Mg in the base water.
- Foliar rescue for fast recovery: 2% Epsom salt spray, evening, on affected foliage. Visible greening within 3 days.
Prevention
If your source is RO or distilled, build remineralization into the recipe — never run nutrients in pure RO. Test source water annually with a lab kit so you know what you are working from. Calibrate the EC meter monthly; a drifting probe masks Mg depletion as the reservoir ages. Keep K:Mg ratio at roughly 5:1 by mass, never above 7:1.
FAQ
4 entries- Q01How do I add magnesium to my hydroponic reservoir?
- Epsom salt (magnesium sulfate) at 1 g/L delivers roughly 100 ppm Mg. Stir until fully dissolved and verify EC change matches expectations.
- Q02Why do RO users get magnesium deficiency?
- Reverse osmosis strips Mg out of source water. If your nutrient mix assumes 20 ppm Mg from tap, RO leaves you 20 ppm short. Add Epsom salt.
- Q03How fast does magnesium deficiency reverse?
- New growth greens within 5–7 days once Mg is added and pH is in range. Yellowed older leaves do not recover.
- Q04Does too much potassium block magnesium?
- Yes. K and Mg compete for the same root channels. Above K 350 ppm with Mg below 40 ppm, expect deficiency within two weeks.