# Hydroponic salts explained: what each fertiliser salt supplies

> Almost every mineral cannabis feed is built from about a dozen salts: calcium nitrate, potassium nitrate, monopotassium phosphate (MKP), magnesium sulfate, potassium sulfate, a few others, plus chelated iron and trace salts. Each one supplies known percentages of two elements, so 1 g/L of a salt gives (percent x 10) mg/L of each. Calcium must be kept apart from sulfate and phosphate in concentrated stocks, which is why feeds come as A and B.

- Canonical: https://genetixbay.com/knowledge/nutrients/hydroponic-salts.html
- Category: Nutrients · Level: Intermediate
- Evidence: Established · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Any stage · Systems: DWC, RDWC, NFT, Ebb and flow, Drip, Coco, Rockwool, Aeroponics
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What does calcium nitrate supply?** Greenhouse-grade calcium nitrate supplies about 15.5% nitrogen, roughly 1.1% of it as ammonium, and about 19% calcium. At 1 g/L it gives about 155 mg/L nitrogen and 190 mg/L calcium. It is the main calcium source in mineral cannabis feeds and belongs in tank A only, because it precipitates with sulfate and [phosphate](https://genetixbay.com/knowledge/glossary/phosphate.md) in concentrated stocks.

**How many ppm does 1 gram per litre of MKP give?** One gram per litre of monopotassium phosphate (MKP) gives about 228 ppm phosphorus and 287 ppm potassium, because MKP is 22.8% phosphorus and 28.7% potassium and 1 g/L of any salt supplies its percentage times 10 in mg/L. MKP is mildly acidic and goes in tank B. Check the bag label, which overrides typical values.

**Why are nutrients split into part A and part B?** Hydroponic nutrients are split into part A and part B because calcium forms insoluble calcium sulfate (gypsum) and calcium phosphate when concentrated with sulfate or phosphate. At [feed strength](https://genetixbay.com/knowledge/glossary/input-ec.md) they stay dissolved, but at 100x concentrate they precipitate. Tank A holds calcium nitrate and [iron chelate](https://genetixbay.com/knowledge/glossary/iron-chelate.md); tank B holds MKP, magnesium sulfate, potassium sulfate and trace salts.

**Which salts do I need to make my own hydro nutrients?** A home-made cannabis hydro feed needs about a dozen salts: calcium nitrate, potassium nitrate, monopotassium phosphate, magnesium sulfate, potassium sulfate, often magnesium nitrate, an iron chelate such as Fe-DTPA, and trace salts for manganese, zinc, copper, boron and molybdenum. Use greenhouse or hydroponic grade, fully soluble salts, since agricultural grades can carry insoluble fillers or heavy metals.

**What is the difference between EDTA and DTPA iron?** The difference between EDTA and DTPA iron chelates is pH stability. Fe-EDTA, about 13% iron, stays stable only up to about pH 6.5, while Fe-DTPA, about 11% iron, holds iron available up to about pH 7.5, making it the usual choice for cannabis in hydro and coco. Fe-EDDHA, about 6% iron, is stable to about pH 9.

**Is Epsom salt the same as magnesium sulfate?** Yes, Epsom salt is magnesium sulfate heptahydrate, the standard magnesium source in hydroponic feeds. It contains about 9.9% magnesium and 13.0% sulfur, so 1 g/L supplies about 99 mg/L magnesium and 130 mg/L sulfur. It goes in tank B and must never share a concentrate with calcium nitrate, because gypsum would precipitate.

**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](https://genetixbay.com/knowledge/nutrients/chlorine.md)) |
| 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](https://genetixbay.com/knowledge/glossary/manganese-sulfate.md) monohydrate | ≈32.5% Mn | 325 Mn | Trace |
| [Zinc sulfate](https://genetixbay.com/knowledge/glossary/zinc-sulfate.md) heptahydrate | ≈22.7% Zn | 227 Zn | Trace |
| [Copper sulfate](https://genetixbay.com/knowledge/glossary/copper-sulfate.md) pentahydrate | ≈25.5% Cu | 255 Cu | Trace |
| [Boric acid](https://genetixbay.com/knowledge/glossary/boric-acid.md) (H₃BO₃) | ≈17.5% B | 175 B | Trace |
| [Sodium molybdate dihydrate](https://genetixbay.com/knowledge/glossary/sodium-molybdate.md) | ≈39.7% Mo | 397 Mo | Trace |
| [Potassium silicate](https://genetixbay.com/knowledge/glossary/potassium-silicate.md) | Varies (Si and K) | Label | Add first; very alkaline (see [silicon](https://genetixbay.com/knowledge/nutrients/silicon.md)) |

**Acids and bases are nutrients too.** [Nitric acid](https://genetixbay.com/knowledge/glossary/nitric-acid.md) adds N, [phosphoric acid](https://genetixbay.com/knowledge/glossary/phosphoric-acid.md) 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](https://genetixbay.com/knowledge/nutrients/chlorine.md)) or calcium sulfate (gypsum) dissolved directly in the final solution. See [nitrogen](https://genetixbay.com/knowledge/nutrients/nitrogen.md) 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](https://genetixbay.com/knowledge/glossary/micronutrient.md) 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](https://genetixbay.com/knowledge/nutrients/feed-by-stage.md) 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].

## Sources

1. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6
2. Hoagland DR, Arnon DI. 1950. The water-culture method for growing plants without soil. California Agricultural Experiment Station Circular 347. https://archive.org/details/watercultureme3450hoag/
3. HortScience. 2025. Iron chelates (Fe-EDTA, Fe-DTPA, Fe-EDDHA) in high-pH soilless media. HortScience 60(3):404. https://journals.ashs.org/view/journals/hortsci/60/3/article-p404.xml
4. NC State Extension. Producing conventional tobacco transplants in greenhouses: water quality. https://content.ces.ncsu.edu/producing-conventional-tobacco-transplants-in-greenhouses-water-quality
5. Cockson P, et al. 2019. Applied Sciences 9(20):4432. https://doi.org/10.3390/app9204432