Short answer. Nearly every mineral cannabis feed, bottled or home-made, is built from about a dozen salts. Each supplies fixed percentages of two elements, so 1 g/L of a salt gives (percent × 10) mg/L of each element. Calcium nitrate supplies Ca and N; potassium nitrate supplies K and N; MKP supplies P and K; magnesium sulfate supplies Mg and S; potassium sulfate supplies K and S; chelates supply iron and other metals [1]. Calcium must be kept apart from sulfates and phosphates in concentrated stocks, which is why feeds come as part A and part B.
The salts
Percentages are for pure or typical greenhouse-grade material; check your bag's label, which overrides this table.
| Salt (formula) | Supplies | 1 g/L gives (mg/L) | Notes |
|---|---|---|---|
| Calcium nitrate, greenhouse grade (5Ca(NO₃)₂·NH₄NO₃·10H₂O) | 15.5% N (≈1.1% as NH₄), ≈19% Ca | 155 N, 190 Ca | Main Ca source. Tank A only |
| Potassium nitrate (KNO₃) | 13.9% N, 38.7% K | 139 N, 387 K | Either tank |
| Monopotassium phosphate, MKP (KH₂PO₄) | 22.8% P, 28.7% K | 228 P, 287 K | Tank B; mildly acidic |
| Magnesium sulfate heptahydrate, Epsom salt (MgSO₄·7H₂O) | 9.9% Mg, 13.0% S | 99 Mg, 130 S | Tank B |
| Potassium sulfate, SOP (K₂SO₄) | 41.5–44.9% K, 17–18.4% S | ~415–449 K, ~170–184 S | Tank B; dissolves slowly |
| Magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O) | 9.5% Mg, 10.9% N | 95 Mg, 109 N | Mg without S; either tank |
| Monoammonium phosphate, MAP (NH₄H₂PO₄) | 12.2% N (all NH₄), 26.9% P | 122 N, 269 P | Adds ammonium; acidifies |
| Calcium chloride dihydrate (CaCl₂·2H₂O) | 27.3% Ca, 48.2% Cl | 273 Ca, 482 Cl | Small doses only (see chloride) |
| Fe-DTPA | ≈11% Fe | 110 Fe | Stable to ~pH 7.5 [3]; hydro and coco |
| Fe-EDTA | ≈13% Fe | 130 Fe | Stable to ~pH 6.5 [3] |
| Fe-EDDHA | ≈6% Fe | 60 Fe | Stable to ~pH 9 [3]; soil and alkaline water |
| Manganese sulfate monohydrate | ≈32.5% Mn | 325 Mn | Trace |
| Zinc sulfate heptahydrate | ≈22.7% Zn | 227 Zn | Trace |
| Copper sulfate pentahydrate | ≈25.5% Cu | 255 Cu | Trace |
| Boric acid (H₃BO₃) | ≈17.5% B | 175 B | Trace |
| Sodium molybdate dihydrate | ≈39.7% Mo | 397 Mo | Trace |
| Potassium silicate | Varies (Si and K) | Label | Add first; very alkaline (see silicon) |
Acids and bases are nutrients too. Nitric acid adds N, phosphoric acid adds P, sulfuric acid adds S (about 14, 31 and 16 mg/L respectively per meq/L of acid used); potassium hydroxide pH-up adds K [4]. Count them.
Tapering N in late flower. Single salts make this possible: drop calcium nitrate and potassium nitrate, take K from potassium sulfate and MKP, and switch the Ca source to an N-free salt such as calcium chloride (small doses only: 50 mg/L Cl brings just ~28 mg/L Ca, so keep Cl under ~50–70 mg/L; see chloride) or calcium sulfate (gypsum) dissolved directly in the final solution. See nitrogen for the ramp.
Why part A and part B
Calcium forms insoluble calcium sulfate (gypsum) and calcium phosphate when concentrated with sulfate or phosphate. At feed strength they stay dissolved; at 100× concentrate they do not [1]. So:
- Tank A: calcium nitrate, (potassium nitrate), iron chelate.
- Tank B: MKP, magnesium sulfate, potassium sulfate, (potassium nitrate), trace salts.
- Add A to the water, mix, then B. Never mix A and B concentrates directly.
A worked micro example
To hit 2.5 mg/L Fe from Fe-DTPA 11%: 2.5 ÷ (11 × 10) = 0.0227 g/L, i.e. 2.27 g per 100 L. Trace elements are easier with a 1:1000 stock solution because the weights are tiny.
Reference: a classic solution
The modified Hoagland all-nitrate solution used in cannabis deficiency research supplied, in mg/L: N 210, P 31, K 235, Ca 200, Mg 49, S 64, Fe 4.0, Mn 1.0, B 0.49, Zn 0.20, Cu 0.19, Mo 0.01 [5] (converted from the mM/µM values in the paper; based on [2]). See building a feed by stage for cannabis targets.
Safety
- Nitrate salts are oxidisers; store away from fuels and organic material, keep dry.
- Acids: gloves and eye protection; always add acid to water, never water to acid.
- Boric acid, copper and nickel salts are toxic if swallowed; keep away from food and children.
Common mistakes
- Using agricultural-grade salts with insoluble fillers or heavy-metal contamination in hydro. Use greenhouse or hydroponic (fully soluble) grade.
- Forgetting the ammonium in calcium nitrate and MAP when calculating NH₄ share.
- Mixing calcium nitrate with Epsom salt in the same concentrate.
- Ignoring what the water already supplies.
Evidence
- Element percentages: Established (stoichiometry; label values vary by grade).
- A/B separation: Established [1].
- Chelate pH ceilings: Established [3].
