A bottled nutrient is water plus a small weight of dissolved fertiliser salts. If you know what each raw salt contains, one line of arithmetic tells you how many grams to weigh for a target concentration. Part 1 covers who should mix from raw salts, what your EC meter really measures, the three ppm scales and the arithmetic. Part 2 puts the method to work. It gives a complete recipe, chelated micronutrients, concentrated A/B stocks and the mixing order that keeps salts from turning solid in the tank.
Who should mix from raw salts, and who should not
A “raw salt” here means a single fertiliser chemical sold by the bag, such as calcium nitrate or magnesium sulphate. Mixing your own solution from these salts makes sense in four cases:
- Large commercial farms. Here fertiliser is a real cost, and a trained technician can run the mixing and the checks properly.
- Research and trials. Here you need to change one nutrient while keeping the others fixed. A bottled product cannot do this, because all its nutrients change together.
- Growers who cannot buy a formulated product. They have to work with whatever is sold locally.
- Anyone who wants to understand what is in the bottle. This is the main reason this article exists.
For most growers, mixing from salts does not make sense. The reasons are practical, not commercial:
- A formulated line has already done the hard work. Its maker has worked out and tested the ratios, the chelated micronutrients (explained in Part 2) and how the mix behaves on pH. Doing all of that again from raw salts takes real effort.
- Raw salt quality varies. Agricultural-grade salts do not always contain what the label says. You have no easy way to check this at home.
- Salts can turn solid in the tank. In a concentrated mix, calcium reacts with phosphate or sulphate and forms solid particles that settle out. This is called precipitation. It is why every commercial line comes in two or three parts that are never combined undiluted.
- The savings are smaller than they look. You also have to count your time, the meters, the batches that fail and the risk to the crop.
What to do: read this article to understand what is in your fertiliser. Mix from salts only if one of the four cases above fits you.
What your meter actually measures
The basics of pH and EC meters are covered in pH & EC: The Nutritional Foundation Every Grower Needs to Know. Here is the short version.
EC (electrical conductivity) shows how easily the solution carries an electric current. More dissolved salt gives a higher EC. EC is what your meter physically measures. The unit is mS/cm (millisiemens per centimetre), and 1 mS/cm = 1000 µS/cm.
CF (conductivity factor) is the same reading multiplied by 10. So 1 mS/cm = CF 10.
TDS (total dissolved solids) is a weight, given in ppm (parts per million). One ppm means one milligram of dissolved material per litre. To measure TDS properly, you filter a sample, evaporate the water and weigh what is left. For example, 50 ml of a 700 ppm solution leaves 35 mg of residue.
No handheld “ppm pen” measures TDS. Every ppm meter measures EC and multiplies it by a fixed conversion factor. That factor is only an approximation, because the meter cannot know which salts are in your solution.
What to do: check which unit your meter displays. If it shows ppm, use the next section to find its conversion factor before you follow any recipe.
The three ppm scales: one solution, three different numbers
Three conversion factors are in common use:
| Scale | 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 |
The scale depends on the meter model, not only on the brand. Check the manual. Many meters let you set the factor yourself.
What this means in practice. The same solution reads 500, 640 or 700 ppm, depending on which meter you use. A “700 ppm” recipe written for a 700-scale meter is only 500 ppm on a 500-scale meter. If you aim for 700 on a 500-scale meter, you feed 40% stronger than the recipe intended.
Three rules follow from this:
- Know your meter’s conversion factor. If you cannot find it, buy a meter that states its factor. Otherwise your ppm numbers cannot be compared with anyone else’s.
- Record and share EC, not ppm. EC is a physical measurement with no scale attached, so anyone can compare it with their own reading.
- Whenever you see a ppm figure, including on this forum, ask which scale it uses. If nobody says, treat the figure as approximate.
What to do: write your meter’s scale on a strip of tape on the meter. From now on, log EC in mS/cm next to every ppm figure.
Choosing and calibrating a meter
Choose a meter that reads at least 0–2000 ppm (or 0–1999). You can stretch a 0–999 meter: dilute the sample with an equal volume of clean water, then double the reading. The result is only rough, though. Waterproof meters cost more, but around a reservoir they are worth it.
Calibrate regularly. Once a week is a sensible habit. Calibrating means checking the meter against a conductivity standard, which is a solution sold with a known EC. You do this because meters drift over time, and a drifting meter makes every later reading wrong.
- The most widely used standard is 1.413 mS/cm (1413 µS/cm at 25 °C). Standards of 2.77 mS/cm are also sold for hydroponic meters.
- Calibrate in EC if your meter allows it. If it shows only ppm, the number to expect depends on its scale. In the 1.413 mS/cm standard, a linear 500-scale meter should show about 707 ppm. A 700-scale meter should show about 990 ppm.
What to do: buy a 1.413 mS/cm standard and check your meter this week. Write the date and the reading in your log.
How strong the feed should be
The figures below are typical ranges only. The right value depends on the cultivar, the growing medium, the climate and the growth stage.
| Stage | Typical feed EC (mS/cm) |
|---|---|
| Seedlings / clones | 0.4–0.8 |
| Vegetative | 1.0–1.6 |
| Flowering | 1.4–2.2 |
Growers using hydroponics or coco often stay near the top of the flowering range. Start lower and raise the EC step by step as you watch how the plants respond.
A separate number is worth watching. Runoff is the water that drains out of the pot. A runoff or root-zone reading of around 4.0 mS/cm is far above any normal feed. It means salt has built up in the medium. Leach the medium: run a weaker solution or plain water through it so the excess salt drains out.
Translator’s note: in Thailand and Southeast Asia, many growers use tap water, well water or stored rainwater. The amount of dissolved minerals in these waters differs from place to place, and your meter adds those minerals to every reading. Measure your own water before you mix anything. If it already reads high, you have less room for fertiliser at the same target EC. The topic Irrigation Water for Living Soil: Chloramine, Hard Water, Softened Water and Why You Should Measure Your Own explains hard water and why your own measurement matters.
What to do: measure and record the EC of your source water. Measure it again whenever you change water source. Measure runoff once a week so you catch salt build-up early.
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.
A fertiliser salt is not pure nutrient. Say a salt contains P% of an element by weight. Then 1 g/L of that salt delivers:
1000 × (P ÷ 100) = 10 × P ppm of the element.
Turn this around to get the number you need:
grams per litre = target ppm ÷ (10 × P)
P is the percentage of that element in the salt. (In this formula, P stands for any percentage, not for phosphorus.)
Example 1: nitrogen. You want 150 ppm of nitrogen (N) from calcium nitrate, which is 15.5% N.
grams per litre = 150 ÷ (10 × 15.5) = 150 ÷ 155 = 0.97 g/L
Check: 0.97 g/L × 15.5% = 0.15 g/L of N = 150 mg/L = 150 ppm. Correct.
Example 2: magnesium. You want 50 ppm of magnesium (Mg) from Epsom salt. Epsom salt is magnesium sulphate heptahydrate, a crystal form that carries seven molecules of water. It is about 9.8% Mg.
grams per litre = 50 ÷ (10 × 9.8) = 0.51 g/L
The formula has two limits, and both of them matter.
Limit 1: it handles one element at a time
Most salts carry more than one nutrient. Calcium nitrate supplies calcium and nitrogen. Potassium nitrate supplies potassium and nitrogen. So you build a recipe step by step:
- Start with the nutrient you have the fewest ways to supply. In the Part 2 recipe, only calcium nitrate supplies calcium, so it goes first.
- Calculate the grams of that salt.
- Work out what the same salt adds to the other nutrients, and subtract that from their targets.
- Move on to the next salt. Repeat until every target is met.
Part 2 shows a finished recipe built this way.
Limit 2: labels do not all use the same basis
By convention, NPK labels state phosphorus as P₂O₅ (phosphorus pentoxide) and potassium as K₂O (potassium oxide). They do not state the elements P and K directly. Convert before you calculate:
- P = P₂O₅ × 0.436
- K = K₂O × 0.830
For example, a bag of monopotassium phosphate (MKP) with the analysis 0-52-34 contains 52 × 0.436 = 22.7% P and 34 × 0.830 = 28.2% K. A bag of potassium nitrate labelled 13-0-46 contains 46 × 0.830 = 38% K. These are the elemental figures used in Part 2.
Skipping this step puts you out by more than a factor of two. Suppose you want 50 ppm of P from MKP and wrongly enter 52 as P: 50 ÷ 520 = 0.096 g/L. That amount actually gives 0.096 × 10 × 22.7 ≈ 22 ppm of P, which is less than half your target.
Calcium and magnesium are sometimes printed as oxides too (CaO, MgO). Convert them with Ca = CaO × 0.715 and Mg = MgO × 0.603.
What to do: for every bag you own, write down the label analysis. Note whether it is on the oxide basis or the elemental basis. Convert it to elemental % before you do any calculation.
Buying and storing raw salts in Thailand and Southeast Asia
Translator’s note: the arithmetic is the easy part. The hard part is buying a bag whose label you can trust.
- What is available. Calcium nitrate, MKP and magnesium sulphate (Epsom salt) are common farm fertilisers in Thailand and Vietnam, and agricultural supply stores sell them. In Thailand, potassium sulphate (0-0-50) is also sold in 1 kg bags. For chelated micronutrient blends in small packs, ask your supplier before you plan a recipe around one.
- Insist on a stated analysis. Buy only bags with a printed guaranteed analysis, from a supplier who will confirm it in writing. Without a reliable percentage, the formula has nothing trustworthy to work with.
- Check that the fertiliser is legally on sale. In Thailand, fertiliser falls under the Fertilizer Act B.E. 2518 (1975), amended by the Fertilizer Act (No. 2) B.E. 2550 (2007). For how to check a registration and read a Thai label, see Verifying a Fertiliser Is Legally in Thailand.
- Where to ask. Ask the official distributor of the salt brand in Thailand. You can also ask other growers in the General Nutrients section.
- Storage in the humid season. Calcium nitrate pulls moisture from the air and turns sticky or caked. The water it absorbs adds weight, so each gram you weigh carries less nutrient. Keep bags sealed in airtight containers, off the floor, in a dry and shaded place.
What to do: before you use a new bag for a whole crop, mix a small test batch and measure its EC. Part 2 shows how to estimate the EC in advance. A reading far from the estimate is a reason to suspect the bag.
Safety and legal note: this is technical horticultural information for readers operating legally where they live. Fertiliser salts are chemicals. Store them safely, keep them away from children and follow the supplier’s safety data sheet (SDS). 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, in force since 26 June 2025. It may be sold only against a PT 33 prescription, for a maximum of 30 days. It may not be sold or advertised online. Research, export, sale and processing of a controlled herb need a licence under the Thai Traditional Medicine Protection Act B.E. 2542. The Cannabis Control Act is still a draft, not law. Nothing here is legal or medical advice.
Continued in Part 2: Mixing Your Own Nutrient Solution From Raw Salts: A Worked Recipe, A/B Stocks and a Safe Mixing Order (Part 2 of 2)