Calcium nitrate in hydroponics and greenhouse nutrition: targets, stock tanks and how much to dose
Calcium nitrate (15.5-0-0 plus about 19% calcium) is the standard calcium source in hydroponics because it is highly soluble and supplies calcium and nitrate nitrogen together; its companion ion is nitrate, a nutrient, rather than sulphate or chloride.[1][19] University recipes put the finished nutrient solution at roughly 90 mg/L Ca and 150 mg/L N for lettuce,[8] 144–200 mg/L Ca and 90–190 mg/L nitrate-N for tomato,[2] and about 52 mg/L Ca and 77 mg/L N for strawberry.[12] Calcium nitrate must be kept in a separate concentrate tank from sulphates and phosphates, otherwise calcium sulphate and calcium phosphate precipitate.[4][7][11] As a worked example, 4 mmol/L Ca (160 mg/L) needs about 84–87 g of calcium nitrate per 100 L of final solution, assuming 18.5–19% Ca.
Key facts at a glance
| Item | Value / answer |
|---|---|
| Fertilizer-grade analysis | 15.5-0-0 with 19% Ca, sold as the double salt 5Ca(NO3)2·NH4NO3·10H2O[1][19] |
| Dry grade used in UF/IFAS calculations | 15.5% N and 19% Ca (UF worked example); UF’s product table lists dry calcium nitrate as 15% N, 19% Ca[3][4] |
| Solubility | Tetrahydrate dissolves at about 1,290 g/L at 20 °C[19] |
| Tomato targets (OSU, 3 stages) | Ca 144 / 160 / 200 mg/L; nitrate-N 90 / 120 / 190 mg/L[2] |
| Tomato, cucumber, pepper (Arizona CEAC recipe) | 170 mg/L Ca and 189 mg/L N for the mature (week 12+) tomato recipe, used by Arizona for all three crops[7][11] |
| Lettuce and leafy greens (Cornell) | Sonneveld-type recipe: 150 mg/L N, 90 mg/L Ca, EC about 1.5, pH 5.5–6.0[7][8] |
| Strawberry (modified Yamazaki) | 77 mg/L N (74 nitrate + 3 ammonium), 52 mg/L Ca, EC 0.75–1.25 dS/m, pH 5.5–6.5[12] |
| Tomato EC and pH (OSU) | EC about 2.0 (stage 1) to 2.4 dS/m (stage 3); pH 5.5–6.5[2] |
| Stock-tank rule | Keep calcium apart from phosphate and sulphate; concentrates above about 50× form insoluble precipitates[4][11] |
| Dilution used in UF/IFAS recipes | 1 part stock to 100 parts water; 150 mg/L Ca needs 79 g/L calcium nitrate in a 100× stock[4] |
| Quick yield per gram (calculated) | 1 g of 18.5–19% product per 100 L gives about 1.85–1.9 mg/L Ca and 1.55 mg/L N |
| Bicarbonate warning level | Above about 30–60 ppm HCO3 raises pH over time; with high Ca and Mg it can precipitate carbonates and clog emitters[4] |
Why is calcium nitrate the standard calcium source in hydroponics?
A hydroponic crop needs calcium at a high concentration, about 90–220 mg/L in the recipes below (roughly 4.5–11 meq/L, our conversion),[2][3][7][8][17] and it needs nitrogen mostly as nitrate. Calcium nitrate supplies both from one salt. Its calcium is released as free Ca2+ without a companion anion the plant does not need: the nitrate is itself a nutrient, unlike the sulphate in gypsum or the chloride in calcium chloride.[3][18] It is also very soluble (about 1,290 g/L for the tetrahydrate at 20 °C), so concentrated stock tanks are practical.[19] For these reasons Wikipedia describes it as popular in the greenhouse and hydroponics trades,[19] and university recipes from Florida, Cornell, Ohio State and Arizona all rely on it as the calcium source.[2][3][4][7][11]
What is in fertilizer-grade calcium nitrate?
The fertilizer-grade product is not pure Ca(NO3)2. It is a double salt with ammonium nitrate and water, 5Ca(NO3)2·NH4NO3·10H2O, graded 15.5-0-0 with 19% calcium.[19] Working the stoichiometry from that formula (molar mass about 1,080.6 g/mol) gives 18.5% Ca and 15.55% N, of which about 14.3 points are nitrate-N and about 1.3 points ammonium-N. This is our own calculation, and it explains why suppliers quote 18.5–19% Ca. Liquid calcium nitrate (7-0-0-11) also exists and is sometimes preferred because it is already clear.[3][4]
Recipes are often written for a different form of the salt, so check which one before you weigh anything:
| Form | Approx. Ca content | Note |
|---|---|---|
| Anhydrous Ca(NO3)2 (164.09 g/mol) | 24.4% | Calculated from molar mass[16][19] |
| Tetrahydrate Ca(NO3)2·4H2O (236.15 g/mol) | 17.0% | Calculated from molar mass[19] |
| Fertilizer-grade double salt (15.5-0-0) | 18.5–19% | The form used in horticulture[3][4][11][19] |
To convert a recipe, multiply the weight it lists by (Ca% of the listed form ÷ Ca% of your product). A recipe written for the tetrahydrate needs about 8–10% less of an 18.5–19% double salt for the same calcium.
How much calcium and nitrate does each crop need?
Targets are final concentrations in the solution delivered to the plant, in mg/L (ppm). They vary with crop stage, light, climate and system, so treat the numbers as published starting points, not fixed requirements.[3][7]
| Crop | Calcium (mg/L) | Nitrogen (mg/L) | Source |
|---|---|---|---|
| Tomato, Ohio State | 144 / 160 / 200 (stages 1–3) | 90 / 120 / 190 nitrate-N | Kroggel & Kubota[2] |
| Tomato, Florida | 150 at all five stages | 70, 80, 100, 120, 150 total N | Hochmuth & Hochmuth[3] |
| Tomato, cucumber, pepper (single recipe) | about 170 | about 189 | Arizona CEAC, via Cornell[7] |
| Cucumber | see single recipe above | production target 175 N (Cornell feed-rate table) | Mattson & Peters[7] |
| Tomato / cucumber / pepper (Agrodynamics species table) | 185 / 175 / 150 | 200 / 230 / 175 | Univ. of Arizona CEAC course notes[11] |
| Bell pepper (vendor recipe) | 220 (0–6 weeks), 200 (mature) | NO3 200 / 180 ppm, i.e. about 45 / 41 mg/L nitrate-N (÷4.43), plus 7 / 15 ppm NH4 | Hort Americas trade blog[17] |
| Lettuce (Sonneveld-type) | 90 | 150 | Cornell CEA[8] |
| Leafy greens and herbs, three Cornell recipes | 38–139 | 150 (heavy feeders 175–200) | Mattson & Peters[7] |
| Strawberry (modified Yamazaki) | 52 | 77 | Hutchinson et al. 2025[12] |
Two cautions on the table. The vendor pepper recipe is quoted as nitrate (NO3), not nitrogen, so it is not directly comparable with the N column until divided by 4.43; it also comes from a trade blog, not a university.[17] And strawberry is a much lighter feeder than tomato: in the cited trial the reservoir EC was held at 0.75–1.25 dS/m.[12] Research strawberry work in China has also used calcium nitrate as the main N source, with an optimum range reported as 22.28–24.61 g Ca(NO3)2·4H2O per plant, but that is a per-plant fertigation design, not a recirculating-solution recipe.[15]
What EC and pH should a calcium nitrate nutrient solution have?
- Tomato: EC about 2.0 dS/m in the first stage, rising to about 2.4 dS/m in stage 3; pH 5.5–6.5.[2] The Arizona CEAC course notes give a typical EC of about 2.0–3.5 mS/cm (range 1.2–4) for fruiting crops and a pH of about 6.[11]
- Lettuce: EC about 1.5 and pH 5.5–6.0 (Cornell leafy-greens recipe); the Cornell lettuce handbook calls pH 5.8 optimum with 5.6–6.0 acceptable.[8][9] UF/IFAS cites 1.2 dS/m for germination and 1.5–1.8 dS/m in production for NFT lettuce.[5]
- Strawberry: EC 0.75–1.25 dS/m, pH 5.5–6.5.[12]
EC is only a measure of total salts. Florida extension warns that holding EC constant while switching to a different fertilizer can hide a deficiency, and that raising EC raises every nutrient at once when only one may be short.[4] Subtract your source water’s own EC when you read the meter.[9]
Why must calcium nitrate go in a separate tank (Tank A/B)?
Calcium reacts with sulphate to form calcium sulphate and with phosphate to form calcium phosphates. At the strength of a stock tank this produces precipitate that clogs emitters and removes nutrients from the crop.[1][4] The Arizona CEAC puts the threshold at concentrates stronger than about 50×.[11] In the dilute final solution, the ions stay in solution, so the two stocks can safely meet only after injection.[10]
Typical split, from the published recipes:
- Tank A (calcium side): calcium nitrate, often with potassium nitrate and iron chelate.[4][7][11]
- Tank B (everything else): the phosphate source, magnesium sulphate and micronutrients, sometimes with potassium chloride or part of the potassium nitrate.[4][7]
- Acid: if source water needs acidifying, Cornell advises adding acid to the tank without calcium nitrate, or using a separate acid tank.[7][11]
Calcium can also be lost in the root zone when pH drifts too high: Sambo et al. note that phosphate availability falls above about pH 7 through precipitation with Ca (minerals such as hydroxylapatite), that Ca-sulphate minerals can limit sulphur, and that Ca and Mg can precipitate as carbonates at high pH and high dissolved CO2.[14]
How do you make a calcium nitrate stock solution?
The maths is a straight ratio. UF/IFAS shows it for 150 mg/L Ca: 150 mg Ca ÷ 0.19 = 790 mg calcium nitrate per litre of final solution; × 100 for a 100× stock = 79 g per litre of stock, or 16.5 lb in a 25-gallon tank.[4] The Arizona CEAC does the same sum for 141 mg/L Ca (170 target minus 29 from source water) at 19% Ca.[11] The University of Hawaii Kratky recipe is a simpler small-scale case: 1 lb/gal calcium nitrate as Stock B, 25 lb in 25 gallons, added in equal volume to Stock A to reach EC 1.5 mS.[10]
How much calcium nitrate per 100 litres? A worked example
Assumption: HRSU grade, 18.5–19% Ca and 15.5% min N. Target: 4.0 mmol/L Ca, soft or RO water with no Ca of its own, 100 L of final solution.
- Calcium in mg/L: 4.0 mmol/L × 40.08 mg/mmol = 160.3 mg/L.
- Calcium needed for 100 L: 160.3 mg/L × 100 L = 16.03 g Ca.
- Product at 19% Ca: 16.03 g ÷ 0.19 = 84.4 g. At 18.5% Ca: 16.03 ÷ 0.185 = 86.7 g.
- Nitrogen delivered at 15.5% N: about 131–134 mg/L total N, of which roughly 92% (about 120–124 mg/L) is nitrate-N.
- For a 1:100 stock, dissolve the same weight per 1 litre of stock: about 84–87 g/L, well inside the salt’s solubility.[19]
If your water already contains calcium, subtract it first. With 40 mg/L Ca in the source water, the 160 mg/L target needs only 120 mg/L from the fertilizer, or about 63–65 g per 100 L. Florida reports well-water Ca of 40–60 ppm, though not all of it may be available to the crop.[3] This method matches the UF/IFAS worked example (150 mg/L Ca at 19% = 790 mg/L of product).[4] Ratio N:Ca of about 0.8 from this salt alone is why a separate nitrate source such as potassium nitrate is often added to reach 150–190 mg/L N.[4][7]
| Target Ca | mg/L | Product per 100 L at 19% Ca | at 18.5% Ca |
|---|---|---|---|
| 1.3 mmol/L (about strawberry) | 52 | 27 g | 28 g |
| 2.5 mmol/L | 100 | 53 g | 54 g |
| 3.74 mmol/L (Florida tomato) | 150 | 79 g | 81 g |
| 4.0 mmol/L (worked example) | 160 | 84 g | 87 g |
These figures are our own arithmetic from molar mass and the stated Ca percentages; confirm against the Ca% on the batch COA you receive.
How does calcium actually reach the plant?
Calcium moves mostly with water in the transpiration stream, up the xylem, and does not move back out of leaves to feed new growth.[6][7] At night, when transpiration is low, root pressure can push calcium into fruits.[6] Pepper fruits transpire less than leaves because of their waxy skin, so leaves compete with fruit for calcium.[6] Two practical results follow:
- Humidity and airflow matter as much as the recipe. UF/IFAS notes that periods of high humidity can cause lettuce tipburn because leaves do not transpire fast enough to move calcium to the expanding leaf edges.[4] Cornell uses downward paddle fans to raise transpiration and prevent tipburn at high light; without airflow their lettuce had to be grown at a lower light integral.[9]
- Antagonism from potassium. High K can interfere with Ca uptake and cause blossom-end rot, especially early in the season and when well-water Ca is under about 50 ppm.[3]
Foliar calcium chloride is a rescue tool, not a substitute for correct root-zone calcium: a HortScience trial on NFT lettuce (Ohio State University with USDA-ARS) tested foliar CaCl2 at 0, 200, 400 and 800 mg/L Ca. Twice-weekly sprays at 400 or 800 mg/L gave good tipburn control with minor yield impact, whereas lowering solution EC to 1.4 mS/cm also reduced tipburn in ‘Green Butter’ but cut its yield by 75 g per head compared with EC 1.8.[13]
What are the alternatives to calcium nitrate?
| Alternative | What it gives | Limits in hydroponics |
|---|---|---|
| Calcium chloride (about 36% Ca) | Ca with no nitrogen; UF/IFAS adds it to the calcium stock to raise Ca where N is already high. 1 lb in 30 gal of stock adds about 14 ppm Ca.[3] | Chloride can accumulate in recirculating systems; Cornell wants source-water chloride under about 70 ppm. Supplies no nitrate.[7] |
| Gypsum (calcium sulphate) | Ca and S with minimal pH effect, no N. Solubility about 2.4 g/L at 20 °C, so up to roughly 550 ppm Ca before precipitation.[18] | At that solubility a concentrated stock is not practical (our inference), so it can only go into the final solution. Calcium sulphate is also exactly what Tank A/B separation avoids.[4] |
| Calcium ammonium nitrate (CAN) | Fertilizer-grade CAN is roughly 8% Ca and 21–27% N.[20] | Much less calcium per unit N; one common grade is made with powdered limestone, so confirm full solubility before use in drippers.[20] |
| Liquid calcium nitrate (7-0-0-11) | Same chemistry, already dissolved and clear.[3][4] | Less Ca per kg shipped; UF/IFAS notes clear liquid avoids the sludge seen in dry products.[4] |
| Chelated or organic calcium | Used mainly as foliar products. | Evidence is thinner than for simple salts; one tomato comparison found CaCl2 clearly better than Ca-citrate.[21] |
For a head-to-head view see our guides to calcium nitrate vs calcium chloride and calcium nitrate vs CAN.
How does water quality change the recipe?
- Test first. Cornell says to begin with a lab analysis for alkalinity, EC and specific elements.[7] Florida recommends testing pH, carbonates, S, Mg, Ca and Fe.[3]
- Alkalinity. Reported as ppm CaCO3, it can range from near 0 (RO water) to over 300 ppm; the higher it is, the more the solution pH rises.[7] UF/IFAS flags bicarbonate above about 30–60 ppm and recommends acidifying to pH 5.6–6.0 with nitric, phosphoric or sulphuric acid.[4]
- Existing calcium and magnesium. Count them. Arizona subtracts source-water Ca before dosing,[11] and Cornell advises supplying Ca, Mg and S only if the water does not already cover the recipe.[7]
- Closed systems need clean water. Cornell suggests source EC below 0.25 mS/cm and sodium and chloride ideally below 50 and 70 ppm, which often means reverse osmosis.[7]
- Carbonate precipitation. At high pH and high dissolved CO2, Ca and Mg can precipitate as carbonates.[14]
What are the most common mistakes?
- Mixing the concentrates. Putting calcium nitrate and magnesium sulphate or phosphate in one concentrate tank.[4][11]
- Ignoring water calcium and alkalinity, which leads to over- or under-dosing and pH drift.[4][11]
- Too much nitrogen early. Keeping tomato N near 60–70 ppm early prevents bullish growth, which can bring blossom-end rot.[3]
- Too much potassium relative to calcium, particularly with pre-mixes.[3]
- Managing by EC alone, instead of testing individual nutrients; Florida suggests flushing sump tanks weekly and rebuilding the solution.[4]
- Letting sludge build up in the calcium tank; anti-caking additives in some dry fertilizers are insoluble and settle out, so rinse tanks periodically.[4]
- Expecting nutrient fixes alone to cure tipburn or BER. Humidity, airflow and irrigation consistency matter.[4][6][9]
When calcium nitrate is NOT the right choice
- Do not mix calcium nitrate with sulphate or phosphate fertilizers in a concentrate. They form calcium sulphate and calcium phosphate precipitate that blocks emitters.[4][11]
- It is not a complete fertilizer. It gives Ca and N only, with no P, K, Mg, S or micronutrients; the Ca:N ratio of the salt also cannot be changed, so a second N source is usually needed.[4][7]
- Do not rely on it for tipburn or blossom-end rot alone. Calcium reaches growing tissue by transpiration, so humidity, airflow, root health and EC strategy all affect it.[4][6][9]
- The recipe numbers are starting points, not requirements. UF/IFAS notes that no single formulation suits every system and climate, and that nutrient concentrations must rise when irrigation is less frequent.[3]
- Handle it as an oxidiser and store it dry. It is hygroscopic and absorbs moisture from air.[19] See the safety, handling and storage guide.
What HRSU supplies — and what it does not
HRSU Indore supplies fertilizer-grade calcium nitrate powder: 18.5–19% calcium, 15.5% minimum nitrogen, 100% water-soluble with 0% insoluble residue, in 25 kg and 50 kg bags with a batch Certificate of Analysis, and TDS and SDS on request. Complete solubility is the property that matters for stock tanks, emitters and injectors. HRSU does not claim any organic, food, pharmaceutical or other special-purpose certification, and the figures on this page come from the cited university sources, not from HRSU testing. Every worked example above assumes 18.5–19% Ca; check the Ca% on your batch COA, and run a small-batch trial on your own water and injector before switching a production tank.
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Related guides
- Calcium nitrate in drip irrigation and fertigation
- Calcium nitrate for blossom-end rot and bitter pit
- Calcium nitrate vs calcium chloride
- Uses of calcium nitrate: the complete guide
References
- Hydroponics Systems: Using the Two Basic Equations to Calculate a Nutrient Solution Recipe — Penn State Extension. University extension
- Hydroponic Nutrient Solution for Optimized Greenhouse Tomato Production (Kroggel and Kubota, 2018) — The Ohio State University, CFAES. University extension
- HS796/CV216: Nutrient Solution Formulation for Hydroponic (Perlite, Rockwool, NFT) Tomatoes in Florida (Hochmuth and Hochmuth) — UF/IFAS Extension. University extension
- HS787/CV265: Fertilizer Management for Greenhouse Vegetables, Florida Greenhouse Vegetable Production Handbook, Vol. 3 (Hochmuth) — UF/IFAS Extension. University extension
- AE610: How Efficient Are Hydroponic Lettuce Production Systems in Nutrient Usage Under Low and High Salinity? (Vought, Bayabil, Martin-Ryals) — UF/IFAS Extension, 2025. University extension
- SS497: Blossom-End Rot in Bell Pepper: Causes and Prevention (Hochmuth and Hochmuth) — UF/IFAS Extension. University extension
- A Recipe for Hydroponic Success (Mattson and Peters) — Cornell University, Department of Horticulture (greenhouse extension). University extension
- Growing Hydroponic Leafy Greens (Mattson), Greenhouse Product News — Cornell Controlled Environment Agriculture, October 2016. University extension
- Hydroponic Lettuce Handbook (Brechner and Both) — Cornell Controlled Environment Agriculture. University extension
- Fertilizer Stock Solutions for Lettuce (Kratky) — University of Hawaii at Manoa, CTAHR. University extension
- Chapter 10: Fertigation Systems and Nutrient Solutions — University of Arizona, Controlled Environment Agriculture Center. University course material
- Substrate system outperforms water-culture systems for hydroponic strawberry production (Hutchinson et al.) — Frontiers in Plant Science, 2025. Peer-reviewed
- Effects of Electrical Conductivity, pH, and Foliar Application of Calcium Chloride on Yield and Tipburn of Lactuca sativa Grown Using the Nutrient-Film Technique (Samarakoon, Palmer, Ling, Altland) — HortScience, 2020, via USDA Agricultural Research Service. Peer-reviewed
- Hydroponic Solutions for Soilless Production Systems: Issues and Opportunities in a Smart Agriculture Perspective (Sambo et al.) — Frontiers in Plant Science, 2019. Peer-reviewed
- Optimal fertigation for high yield and fruit quality of greenhouse strawberry (Wu et al.) — PLoS ONE, 2020. Peer-reviewed
- Calcium nitrate (PubChem compound summary) — U.S. National Library of Medicine / NCBI. Government database
- Nutrient recipe for hydroponic greenhouse bell peppers — Hort Americas (trade blog). Trade
- Using calcium sulfate in hydroponics — Science in Hydroponics. Technical blog
- Calcium nitrate — Wikipedia. Encyclopedia
- Calcium ammonium nitrate — Wikipedia. Encyclopedia
- Foliar calcium in hydroponics — Science in Hydroponics. Technical blog