The Secondary Three: Calcium, Magnesium & Sulfur
Why Ca and Mg are the make-or-break elements on RO water and in coco, target concentrations, and the transpiration link most growers miss.
6 min read
Calcium, magnesium and sulfur are called secondary nutrients because the plant uses less of them than N-P-K — not because they matter less. On soft/RO source water and in coco coir, Ca and Mg are the single most common thing growers get wrong, and the failure looks exactly like a dozen other problems until you understand the mechanism.
| Element | Role | Mobility / where it shows | Target | Why it goes short |
|---|---|---|---|---|
| Calcium (Ca) | Cell-wall structure, membrane integrity, new-growth stability | Immobile → new/upper growth (tip burn, hooked leaves) | ~150–200 mg/L | RO water lacks it; coco's CEC binds it |
| Magnesium (Mg) | Central atom of chlorophyll; enzyme cofactor | Mobile → interveinal yellowing on lower leaves | ~50–75 mg/L | RO water lacks it; coco binds it early |
| Sulfur (S) | Amino acids, proteins, enzyme systems | Immobile-ish → pale young leaves | Supplied via sulfates in Part B | Rarely deficient with a normal base |
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