GenetixBayEST. 2009
Nutrients · Intermediate · 4 min read

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.

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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].

Questions this article answers

What does calcium nitrate supply?

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.

How many ppm does 1 gram per litre of MKP give?

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.

Why are nutrients split into part A and part B?

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.

Which salts do I need to make my own hydro nutrients?

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.

What is the difference between EDTA and DTPA iron?

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.

Is Epsom salt the same as magnesium sulfate?

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.

Sources

  1. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. 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. archive.org/details/watercultureme3450hoag/
  3. HortScience. 2025. Iron chelates (Fe-EDTA, Fe-DTPA, Fe-EDDHA) in high-pH soilless media. HortScience 60(3):404. journals.ashs.org/view/journals/hortsci/60/3/article-p404.xm
  4. NC State Extension. Producing conventional tobacco transplants in greenhouses: water quality. content.ces.ncsu.edu/producing-conventional-tobacco-transpla
  5. Cockson P, et al. 2019. Applied Sciences 9(20):4432. doi.org/10.3390/app9204432

Related

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Building a cannabis feed by stage: ppm targets per element and how to hit them

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