Every bottled nutrient is water plus a small mass of soluble salts. If you know the analysis of the raw salts and can do one line of arithmetic, you can work out how much of each one you need for a target concentration. This article covers that arithmetic, a complete worked recipe, chelated micronutrients, concentrated A/B stocks and the precipitation rules that decide whether a home mix works. It also gives an honest view of who should do this and who should keep buying a formulated product.
1. Who this is for, stated up front
We distribute formulated nutrient lines. So we will be direct about where this article sits.
Mixing from raw salts makes sense in a narrow set of cases:
- Large-scale commercial operations where nutrient cost is a meaningful line item and where a technician can run the mixing and verification properly.
- Research and trial work where you need to vary one element independently, which a formulated product cannot do.
- Growers who genuinely cannot source formulated product and need to work with what is locally available.
- Anyone who wants to understand what is in the bottle, which is the reason most of this article exists.
It does not make sense for most growers, and the reasons are not commercial ones:
- A formulated line is a solved problem. The ratios, the chelation, the micronutrient package and the pH behaviour have been worked out and tested. Reproducing that from raw salts is real work.
- Salt quality and consistency vary. Agricultural-grade raw salts are not always what the label says, and you have no easy way to check.
- Precipitation is a real failure mode. Calcium and phosphate, or calcium and sulphate, will precipitate out of a concentrated mix. This is why every commercial line is split into two or three parts that are never combined undiluted.
- The savings are smaller than they look once you account for your time, the meters, the failed batches and the crop risk.
Read this to understand your fertiliser. Mix from salts only if you fit one of the four cases above.
2. EC, CF and ppm in one paragraph
The meter fundamentals are covered in topic 392 (pH & EC), so here is only the short version. EC (electrical conductivity) is what your meter physically measures, in mS/cm (1 mS/cm = 1000 µS/cm). CF is the same reading ×10 (1 mS/cm = CF 10). TDS in ppm is a mass, meaning mg of dissolved solids per litre. To measure it properly you would evaporate a filtered sample and weigh the residue: 50 ml of a 700 ppm solution leaves 35 mg. No handheld “ppm pen” measures TDS. Every ppm meter measures EC and multiplies it by a conversion factor. That factor is an approximation, because the meter cannot know what is in your solution.
3. The conversion-factor trap, calibration and EC targets
Three conversion factors are in common use:
| Convention | Conversion |
|---|---|
| US (500 scale) | 1 mS/cm (EC 1.0, CF 10) = 500 ppm |
| European (640 scale) | 1 mS/cm = 640 ppm |
| Australian (700 scale) | 1 mS/cm = 700 ppm |
Which scale a meter uses depends on the model, not just the brand. Check your manual: many meters let you set the factor yourself.
The consequence: the same solution reads 500, 640 or 700 ppm depending on which meter you pick up. A “700 ppm” recipe written for a 700-scale meter is a “500 ppm” solution on a 500-scale meter. A grower who targets 700 on a 500-scale meter is feeding 40% stronger than intended.
Three rules that follow:
- Know your meter’s conversion factor. If you cannot find it, that is a reason to buy a different meter.
- Prefer EC over ppm for recording and communicating. EC is a physical measurement with no convention attached.
- When you see a ppm figure anywhere, including on this forum, ask which scale it is on. If nobody says, treat it as approximate.
Meter specification: you want a meter that reads at least 0–2000 ppm (or 0–1999). A 0–999 meter can be stretched by diluting the sample with an equal volume of clean water and doubling the reading, but the result is only rough. Waterproof meters cost more and are worth it.
Calibration: check the meter regularly (weekly is a sensible habit) against a conductivity standard. 1.413 mS/cm (1413 µS/cm at 25 °C) is the most widely used, and 2.77 mS/cm standards are also sold for hydroponic meters. Calibrate in EC. If your meter only shows ppm, the number it should display depends on its scale. At 1.413 mS/cm, a linear 500-scale meter shows about 707 ppm and a 700-scale meter about 990 ppm.
EC targets for the feed solution. These are typical ranges only. The right value depends on cultivar, medium, climate and the stage of growth:
| Stage | Typical feed EC (mS/cm) |
|---|---|
| Seedlings / clones | 0.4–0.8 |
| Vegetative | 1.0–1.6 |
| Flowering | 1.4–2.2 |
Hydro and coco growers often run the upper part of the flowering range. Start lower and raise it according to how the plants respond. Separately, a runoff or root-zone reading of around 4.0 mS/cm is far above any normal feed. It indicates salt build-up in the medium, and you should leach the medium with a weaker solution or plain water.
4. The arithmetic
Start from the definition. 1 gram of any substance dissolved in 1 litre of water = 1000 mg/L = 1000 ppm of that substance.
Now account for the fact that a fertiliser salt is not pure nutrient. If a salt is P% of a given element by mass, then 1 g/L of that salt delivers:
1000 × (P ÷ 100) = 10 × P ppm of the element.
Invert to get what you actually want:
grams per litre = target ppm ÷ (10 × P)
where P is the percentage of that element in the salt.
Worked example. You want 150 ppm of nitrogen, using calcium nitrate at 15.5% N:
grams per litre = 150 ÷ (10 × 15.5) = 150 ÷ 155 = 0.97 g/L
Check it: 0.97 g/L × 15.5% = 0.15 g/L of N = 150 mg/L = 150 ppm. Correct.
A second example. You want 50 ppm of magnesium from Epsom salt (magnesium sulphate heptahydrate), which is about 9.8% Mg:
grams per litre = 50 ÷ (10 × 9.8) = 0.51 g/L
Two things this formula does not do, and both matter:
It works on one element at a time. Real salts carry more than one element. Calcium nitrate supplies both calcium and nitrogen, and potassium nitrate supplies both potassium and nitrogen. You have to build the recipe step by step: pick the salt that supplies your most constrained element, calculate it, subtract what it adds to the other elements, and continue. Section 5 shows the result.
Fertiliser labels are not all on the same basis. The classic trap: NPK labels conventionally express phosphorus as P₂O₅ and potassium as K₂O, not as elemental P and K. To convert: P = P₂O₅ × 0.436, and K = K₂O × 0.830. If you put a P₂O₅ percentage into the formula and treat the answer as elemental phosphorus, you will be out by more than a factor of two. Check which basis your salt’s analysis uses before you calculate anything.
5. A complete worked recipe
This is an illustrative general-purpose solution to show the method. It is not a cannabis-optimised feed chart. The percentages are typical label analyses converted to elemental form. Use the figures on your own bags.
| Salt (analysis used) | g/L | N | P | K | Ca | Mg | S |
|---|---|---|---|---|---|---|---|
| Calcium nitrate (15.5% N, 19% Ca) | 0.80 | 124 | – | – | 152 | – | – |
| Monopotassium phosphate, MKP (22.7% P, 28.2% K) | 0.22 | – | 50 | 62 | – | – | – |
| Potassium nitrate (13% N, 38% K) | 0.20 | 26 | – | 76 | – | – | – |
| Magnesium sulphate heptahydrate (9.8% Mg, 13% S) | 0.50 | – | – | – | – | 49 | 65 |
| Potassium sulphate (41.5% K, 18% S) | 0.15 | – | – | 62 | – | – | 27 |
| Total (ppm) | 1.87 | 150 | 50 | 200 | 152 | 49 | 92 |
Each cell is simply g/L × 10 × P. For example, MKP gives 0.22 × 10 × 22.7 = 50 ppm P. Micronutrients (section 6) are added on top.
Estimating EC before you mix. Convert each major ion to milliequivalents per litre (ppm ÷ equivalent weight). A widely used rule of thumb is EC (mS/cm) ≈ total cations in meq/L ÷ 10. For this recipe: K 200 ÷ 39.1 ≈ 5.1, Ca 152 ÷ 20.0 ≈ 7.6, Mg 49 ÷ 12.2 ≈ 4.0, plus about 0.6 from the small ammonium fraction in calcium nitrate. That makes about 17.3 meq/L, so roughly 1.7 mS/cm on top of whatever your source water reads. The anions balance at about the same total (nitrate ≈ 10.1, sulphate ≈ 5.7, phosphate ≈ 1.6 meq/L), which is a useful check that the arithmetic is right. Real EC varies with the salt mix, so this is only an estimate. The meter has the final say.
6. Micronutrients and chelated iron
Iron, manganese, zinc, boron, copper and molybdenum are needed in tiny amounts. As a rough guide, typical hydroponic recipes run around Fe 1–3, Mn 0.3–0.8, Zn 0.05–0.3, B 0.2–0.5, Cu 0.02–0.1 and Mo 0.01–0.05 ppm. Weighing those quantities for a small batch is not practical without an analytical balance. Most home mixers buy a ready-made chelated micronutrient blend and dose it with the same formula.
Iron must be chelated. Unchelated iron salts precipitate as the pH rises. Which chelate you choose depends on your pH:
| Chelate | Holds iron available up to about |
|---|---|
| Fe-EDTA | pH 6.5 (fine for hydro run at 5.5–6.0; cheapest) |
| Fe-DTPA | pH 7.5 |
| Fe-EDDHA | pH 9 (for high-pH media or hard water; most expensive) |
Published cut-offs vary somewhat between sources, but the order is consistent. Example with the formula: a 13% Fe-EDTA at 2 ppm Fe needs 2 ÷ (10 × 13) = 0.015 g/L. Iron chelates are broken down by light, which is one more reason to keep the reservoir dark.
7. Making 100× A/B stocks
Weighing grams for every reservoir change is slow, and the error is worst on small batches. The standard solution is to make two concentrated stocks once:
- Part A: the calcium nitrate, usually together with the iron chelate.
- Part B: the phosphate and sulphate salts (MKP, potassium nitrate, magnesium sulphate, potassium sulphate) and the other micronutrients.
For a 100× stock of the recipe above, multiply each g/L figure by 100 and dissolve it in 1 litre of stock. Part A: 80 g calcium nitrate (+ 1.5 g Fe-EDTA). Part B: 22 g MKP, 20 g potassium nitrate, 50 g magnesium sulphate, 15 g potassium sulphate. To use, add 10 ml of A and 10 ml of B per litre of final solution (1 litre of each per 100 litres). Potassium sulphate is the least soluble salt in Part B, so check solubility before pushing the concentration factor higher. Label both containers clearly, store them dark and cool, and never combine A and B with each other undiluted.
8. Mixing procedure
- Dissolve every solid completely. Warm water speeds dissolving. Make sure every solid is fully dissolved before you measure, because undissolved salt at the bottom of the reservoir makes the meter reading meaningless.
- Add each part separately to a full volume of water, and stir between parts. Never mix concentrates together undiluted, especially calcium with phosphate or sulphate. A submersible pump keeps the reservoir mixed while you work.
- Main salts first, supplements last. Organic additives make EC readings unreliable, so add them only after the salt readings are where you want them.
- Set EC first, then pH. pH takes longer to stabilise than EC, so mix thoroughly before reading. Aim for roughly 5.5–6.0 in hydro (details in topic 392). Do not chase decimals.
- Dilute pH UP/DOWN and add slowly while stirring. A local high-pH zone can precipitate calcium phosphate and iron.
- Start low and increase gradually. Every cultivar responds a little differently.
- Keep it simple. Do not try to add everything.
- Replace the solution rather than chasing a deficiency. After several days of feeding, the balance of a reservoir drifts. The usual correct response to a suspected nutrient problem is to replace the solution, not to add to it (see topic 1337, Nutrient Lockout).
- Keep the reservoir at 20–22 °C and keep light out. That makes algae much less of a problem. Warmer water holds less dissolved oxygen, and bubbling can only bring it up to saturation, not beyond.
That last point is the one that matters most in this region. A reservoir in an unconditioned Southeast Asian room will not sit at 20–22 °C on its own. Warm nutrient solution holds less oxygen, which is exactly the condition in which Pythium and root rot establish. Reservoir chilling is not optional here. At minimum, insulate the reservoir, keep it off a warm floor and keep it out of the light. It is the single largest difference between running hydroponics in a temperate climate and running it in a tropical one.
9. If you are going to do this properly
- An EC meter you have calibrated, whose conversion factor you know.
- A pH meter, calibrated.
- A scale accurate to 0.01 g for the major salts, and a pre-blended micronutrient mix unless you have an analytical balance.
- Salts with a stated guaranteed analysis, from a supplier who will provide it.
- Separate A and B stocks, never combined undiluted.
- A written recipe with the calculation shown, so you can check it and somebody else can reproduce it.
- Records: batch, date, target and measured EC and pH, and what the plants did.
The failure modes in home salt mixing are rarely arithmetic. They are undissolved salt, precipitated calcium, a meter on the wrong scale and a mislabelled bag.
For anyone in the region who has actually mixed from raw salts: what could you source, and what was the analysis quality like? The arithmetic on this page is straightforward. The real question is whether you can buy a bag of calcium nitrate in Thailand or Vietnam and trust the number on it, and only people who have tried can answer that.
Related reading on the forum: topic 392 (pH & EC), topic 268 (General Nutrients), topic 1337 (Nutrient Lockout), and the brand feeding-schedule articles for CANNA, Grotek, Advanced Nutrients, House & Garden and BioBizz.
This article is technical horticultural information for readers operating legally in their jurisdiction. Fertiliser salts are chemicals: store them safely, keep them away from children and follow the supplier’s safety data sheet. Cannabis is regulated differently in every country. In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis) B.E. 2568. It may be dispensed only against a prescription on form PT 33, may not be sold or advertised online, and may not be used recreationally. Cultivation and trade are subject to Ministry of Public Health licensing rules. The Cannabis Control Act is still a draft. Nothing here is legal or medical advice.