K₂SO₄ and MgSO₄ explained
Most dry-fertilizer kits include four salts. KNO₃ and KH₂PO₄ are the core; K₂SO₄ and MgSO₄ fill specific gaps.
Potassium sulfate (K₂SO₄)
K₂SO₄ is 44.9 % potassium by weight and brings no nitrogen or phosphorus. You need it when the potassium that rides along with KNO₃ (38.7 % K) and KH₂PO₄ (28.7 % K) doesn't reach your K target.
With EI's 22.5 ppm NO₃ a week, KNO₃ alone supplies about 14 ppm K and KH₂PO₄ another 1.2 ppm. To reach 30 ppm K you add roughly 15 ppm from K₂SO₄. The calculator does this automatically and tells you how short K was before K₂SO₄.
You can skip it if your tap water is high in potassium, you run a lean or low-tech plan, or you use a potassium-rich all-in-one.
Magnesium sulfate heptahydrate (MgSO₄·7H₂O, Epsom salt)
Only 9.9 % of Epsom salt is magnesium; the rest is sulfate and crystal water. Plants need magnesium for chlorophyll, and it is part of general hardness (GH).
Do you need it? If your tap or remineralized RO water already has moderate GH with magnesium in it, probably not. Soft water and pure RO usually need it.
Where does MgSO₄ go?
- Macro bottle: the most common choice; it dissolves easily.
- Dry, after the water change: sensible because it replaces hardness the change removed.
- Never in the iron/trace bottle.
Use the checkboxes in the calculator to turn either salt off and see the effect: EI without K₂SO₄.
Open the dosing calculatorMore guides
Sources
- Potassium sulfate: composition and solubility
- Magnesium sulfate: heptahydrate composition
- IUPAC standard atomic weights
- r/PlantedTank: "Do I need K2SO4 / where does MgSO4 go"
- Seachem Flourish Potassium: K from potassium sulfate
Written in our own words from the sources above. Numbers are educational starting points; check product labels and your water.