This guide covers four things you can use in your grow:
- what each nutrient does in a cannabis plant
- how much of each nutrient the plant takes up at each stage
- how to turn the numbers on a fertiliser label into the amount in your tank
- how to trace a leaf symptom back to its likely cause
The figures come from several published studies, listed at the end. Paragraphs marked “Translator’s note” explain what changes in Thailand and Southeast Asia: hard water, 30–35 °C heat and rainy-season humidity.
How the numbers in this guide are written
- mg/L means milligrams of an element dissolved in one litre of nutrient solution. In water, 1 mg/L equals 1 ppm (part per million). The arithmetic is covered in mixing a nutrient solution from raw salts.
- EC (electrical conductivity, in mS/cm) shows how much fertiliser salt is dissolved in the water. It tells you how strong the solution is. It does not tell you which elements are in it. See pH and EC basics.
- Element symbols: N nitrogen, P phosphorus, K potassium, Ca calcium, Mg magnesium, S sulfur, Fe iron, Mn manganese, Zn zinc, Cu copper, B boron, Mo molybdenum, Cl chlorine (the plant takes it up as chloride).
- Veg is the vegetative stage, when the plant grows leaves and stems. Flower is the flowering stage.
1. The three main nutrients: nitrogen, phosphorus and potassium
Nitrogen (N)
Nitrogen builds proteins, enzymes, amino acids and chlorophyll, the green pigment that captures light. The plant needs the most nitrogen during veg.
- Too little: the lower leaves turn evenly yellow. This yellowing from lost chlorophyll is called chlorosis. Growth slows and stems stay thin.
- Too much: leaves turn unusually dark green. Leaf tips bend downward like a claw. Veg drags on and the plant makes fewer flowers.
- Forms the plant absorbs: ammonium (NH4+) and nitrate (NO3−). Cannabis prefers nitrate.
What to do: some labels show how the nitrogen is split between nitrate and ammonium. When you compare products, check that split. Cannabis takes up the nitrate share more readily.
Phosphorus (P)
Phosphorus is part of ATP, the molecule that carries energy inside cells. It is also part of DNA and RNA. It supports root growth, flowering and seed production.
- Too little: the lower leaves turn purple or red, roots stay weak and flowers stay small.
- Too much: it blocks the uptake of zinc and iron. The plant then shows symptoms of those shortages.
- Cannabis needs less phosphorus than many growers think. N. Bernstein and colleagues run a research group that publishes cannabis feeding trials in the journal Frontiers in Plant Science. In their 2021 flowering trial (Shiponi and Bernstein), 30 mg/L P was enough for about 80% of the maximum yield.
What to do: compare the P in your current programme with Table 1 before you add a bloom booster. This shows whether you are paying for phosphorus the plant cannot use and risking zinc and iron lockout.
Potassium (K)
Potassium controls the stomata, the tiny pores on the leaf that release water vapour and take in carbon dioxide. It moves water and nutrients through the plant, keeps photosynthesis running at full rate and supports resin production.
- Too little: leaf edges turn brown and look burnt. This dead tissue is called necrosis. The plant is weak and the flowers are loose.
- Too much: it blocks calcium and magnesium.
- The plant needs the most potassium in late flower, while the buds ripen.
Translator’s note: at 30–35 °C, burnt leaf edges often come from heat or high EC rather than a lack of potassium (Table 4 lists both causes). Before you add potassium, measure the temperature at canopy height and the EC of the water draining from the pot. If heat is the cause, extra potassium raises EC further and can block calcium and magnesium.
2. N : P : K ratios by growth stage
A 2026 study by K. Powell and W. Bauerle in Frontiers in Plant Science, also free to read on PubMed Central (PMC), the archive of research papers run by the US National Library of Medicine, measured how much of each nutrient cannabis actually absorbs. The method is called “transpiration-driven mass balance”. It calculates uptake from the water the plants lose through their leaves (transpiration) and the nutrients that water carries in. In veg the plants took up 78–155 mg/L N, 13–49 mg/L P and 111–216 mg/L K. At the top of those ranges, the ratio is close to N:P:K = 3:1:4. The paper itself does not state a ratio; this is our calculation from its figures.
Table 1: Recommended N:P:K ratios by growth stage
| Stage | N | P | K | N (mg/L) | EC (mS/cm) |
|---|---|---|---|---|---|
| Seedling or rooted cutting (clone) | 1 | 0.5 | 1 | 80–100 | 0.4–0.8 |
| Early veg | 3 | 1 | 3 | 120–140 | 0.8–1.2 |
| Late veg | 3 | 1 | 3–4 | 140–160 | 1.2–1.8 |
| Transition (flower weeks 1–2) | 2 | 1.5 | 3 | 120–140 | 1.2–1.6 |
| Early flower (weeks 3–4) | 1.5 | 2 | 3 | 100–120 | 1.4–1.8 |
| Mid flower (weeks 5–6) | 1 | 2 | 3 | 80–100 | 1.6–2.0 |
| Late flower (weeks 7–8) | 0.5 | 1.5 | 3 | 60–80 | 1.2–1.6 |
| Ripening / flush | 0 | 0.5 | 1 | 0–40 | 0.2–0.6 |
Sources: Bevan, Jones and Zheng (2021, Frontiers in Plant Science); Powell and Bauerle (2026, Frontiers in Plant Science); Allred, Fatzinger and Bugbee (2025, Journal of Cannabis Research).
“Weeks” count from the day you switch the lights to the flowering schedule. The EC column covers the whole solution, including calcium, magnesium and the micronutrients.
The table and the measured uptake do not fully match. Table 1 gives 3:1:3 for early veg, while the plants in the Powell and Bauerle study took up closer to 3:1:4. The table is a guide compiled from several sources. None of the three studies gives these exact ratios, and the measured data leans further towards potassium. In flower, the P ratios in the table work out to more P than the roughly 59 mg/L at which Bevan, Jones and Zheng (2021) found the highest flower yield. If plants in late veg show potassium shortage (brown leaf edges, Table 4), move to the 3:1:4 end of the range.
Tapering nitrogen in late flower
Table 1 cuts nitrogen in steps:
- late veg: 140–160 mg/L
- flower weeks 1–2: 120–140 mg/L
- weeks 3–4: 100–120 mg/L
- weeks 5–6: 80–100 mg/L
- weeks 7–8: 60–80 mg/L
- final ripening: 0–40 mg/L
From week 3 on, that is a cut of about 20 mg/L every two weeks.
The cut is deliberate. A. Saloner and N. Bernstein are co-authors from the same research group. They found that cannabinoid concentration is highest at low nitrogen and falls as nitrogen rises. More nitrogen gives more plant mass (Section 7), so the table gives up some mass to gain potency. The last row is the “flush”: a final period on plain water or a very weak solution. Whether flushing improves quality is debated; see pre-harvest flushing.
Translator’s note: Table 1 assumes about eight weeks of flowering. Many cultivars grown in the tropics, especially sativa-leaning ones, flower for longer. Keep the order of the steps, but space them over your cultivar’s real flowering time instead of following the calendar weeks above.
What to do: before you switch to flower, write down the nitrogen target for every week of your cultivar’s flowering period. Then you know in advance when each cut is due, and you won’t have to guess late in the run.
3. Reading a fertiliser label: from oxide to element to mg/L
A fertiliser label shows three main numbers in the order N–P–K. On most labels, including Thai ones, P is given as phosphorus pentoxide (P₂O₅) and K as potassium oxide (K₂O), not as the pure elements. Research papers and Table 1 use the pure elements. Convert the label numbers before you compare them:
- P = P₂O₅ × 0.436
- K = K₂O × 0.830
- N needs no conversion.
From percentage to mg/L: percentage of the element × grams of product per litre × 10 = mg/L of that element.
Example (an imaginary product labelled 4–6–8, used at 2 g/L):
| Label % | Element % | mg/L at 2 g/L | |
|---|---|---|---|
| N | 4 | 4 | 80 |
| P | 6 (P₂O₅) | 2.6 | 52 |
| K | 8 (K₂O) | 6.6 | 133 |
The label looks rich in phosphorus. As elements, though, the ratio is about 3:2:5. The 52 mg/L of P is already above the top of the measured veg uptake (49 mg/L).
Liquid fertilisers are dosed in mL, but their percentages are often by weight. Multiply the mL by the product’s density (grams per mL) to get grams. Density is often printed on the label or the safety data sheet. Without it, treat your result as an estimate. If you mix several products, add up what each one supplies, plus what is already in your water.
Total vs available nutrients. “Total” is everything in the bag. “Available” or “water-soluble” is the part the roots can take up. Thai labels commonly list ไนโตรเจนทั้งหมด (total N), ฟอสเฟตที่เป็นประโยชน์ (available P₂O₅) and โพแทชที่ละลายน้ำ (water-soluble K₂O). In organic products, much of the nitrogen is bound in organic matter until soil microbes break it down. The total-N figure on such a bag overstates what the plant can use this week; see how nitrogen release differs between organic fertilisers.
Translator’s note: fertilisers in Thailand fall under the Fertilizer Act B.E. 2518 (1975). Its only amendment is the Fertilizer Act (No. 2) B.E. 2550 (2007). How to check that a product is registered and how to read the Thai label has its own topic.
What to do: work out the N, P and K in mg/L for the mix you use now and write the figures down. Then compare them with Table 1. This shows whether you over-feed phosphorus or under-feed potassium.
4. Secondary nutrients: calcium, magnesium and sulfur
Calcium (Ca)
- Role: builds cell walls and carries signals inside the plant.
- Too little: brown spots on new leaves, deformed leaf tips and short roots.
- Target: 100–200 mg/L calcium in the solution.
Translator’s note: calcium travels with the water the plant pulls up and releases through its leaves. In the rainy season the air is humid, so leaves release less water, and less calcium reaches the new leaves. Calcium deficiency can then appear even when there is enough in the solution. Improve airflow and lower the humidity before you add more calcium; see grow-room climate control.
Magnesium (Mg)
- Role: sits at the centre of the chlorophyll molecule and activates enzymes.
- Too little: yellowing between the veins while the veins stay green. Magnesium is mobile (Section 6), so the yellowing starts on older leaves in the lower and middle part of the plant and spreads upward.
- Sources: Epsom salt (magnesium sulfate, MgSO₄), CalMag products and dolomite lime.
Translator’s note: Epsom salt is sold in Thailand as ดีเกลือ, and dolomite as ปูนโดโลไมท์. Both are easy to find at agricultural supply stores. Check that the ดีเกลือ label says magnesium sulfate. The common heptahydrate form (MgSO₄·7H₂O) contains about 9.9% Mg and 13% S. At 1 g/L it adds roughly 99 mg/L magnesium and 130 mg/L sulfur. Dolomite dissolves slowly. It suits soil beds and outdoor ground, not a hydroponic or coco nutrient tank.
Sulfur (S)
- Role: builds proteins and enzymes. The original article also links sulfur to “terpenes (thiol structures)”. More precisely, sulfur forms the thiols, the sulfur-containing aroma compounds behind some skunky smells. Terpenes themselves contain no sulfur.
- Too little: new upper leaves turn evenly yellow. It looks like nitrogen shortage, but it shows on the TOP leaves.
- Sulfur shortage is rare if you use a good mineral fertiliser.
What to do: measure the pH and EC of your raw water before you buy CalMag. A clearly raised EC in plain water means dissolved minerals, often calcium and magnesium. Hard water is explained in irrigation water: hard water and chloramine. For exact figures, have a water-testing laboratory run a Ca/Mg analysis. This keeps you from adding calcium and magnesium your water already supplies.
5. Micronutrients: Fe, Mn, Zn, Cu, B, Mo, Cl
Table 2: Micronutrients — roles and deficiency symptoms
| Element | Main role | Deficiency symptom | pH that blocks uptake |
|---|---|---|---|
| Fe (iron) | Chlorophyll, respiration | Yellowing between veins on NEW leaves | pH > 6.5 |
| Mn (manganese) | Photosynthesis, enzymes | Small yellow spots between veins | pH > 6.5 |
| Zn (zinc) | Hormones, enzymes | Small, deformed, yellow leaves | pH > 7.0 or excess P |
| Cu (copper) | Enzymes, lignin (stiffening material in stems) | Wilted, purple leaves | pH > 6.5 |
| B (boron) | Cell walls, cell division | Dead tips, hollow stems | pH > 6.5 or dry root zone |
| Mo (molybdenum) | Converting nitrate into usable nitrogen; enzymes | Leaf edges curl upward, yellowing | pH < 5.5 |
| Cl (chlorine) | Water pressure inside cells | Wilting, short roots | Rarely deficient |
The original listed molybdenum’s role as “N fixation”. Fixing nitrogen from the air is done by bacteria in the roots of legumes such as beans, not by cannabis. In cannabis, molybdenum is needed by the enzyme that converts nitrate (nitrate reductase).
The pH limits in Table 2 apply to soil. In coco and hydroponics, uptake problems start at a lower pH, above 6.0 (Section 8).
What to do: if you see a micronutrient symptom, measure the pH of the solution and of the runoff before you buy a micronutrient product. In this table, most shortages come from the wrong pH, not from a missing element.
6. Mobile vs immobile nutrients: where the symptom appears
This is the most important rule for diagnosing a nutrient shortage.
Table 3: Mobile vs immobile nutrients
| Type | Elements | Symptoms appear on | Why |
|---|---|---|---|
| Mobile | N, P, K, Mg, Mo | OLD (lower) leaves | The plant moves these from old leaves to new ones |
| Immobile | Ca, Fe, Mn, Cu, B, S, Zn | NEW (upper) leaves | The plant cannot move these once they are in place |
Yellowing that moves from the bottom up points to a mobile nutrient, usually N, Mg or K. Yellowing that moves from the top down points to an immobile nutrient, usually Fe, Ca or Mn.
Zinc is often listed as mobile in grower charts. Plant-nutrition references describe it as relatively immobile, and its deficiency shows first on young leaves (small, deformed leaves, Table 2). Look for it at the top of the plant.
What to do: when leaves start yellowing, note which leaves showed it first and take a photo every two days. Post the photos in the Plant Infirmary section. This record of where symptoms started is what other growers need to tell a mobile from an immobile shortage.
7. How much the plant actually takes up
The Powell and Bauerle (2026) study measured real uptake:
- N: 78–155 mg/L, depending on cultivar and stage
- P: 13–49 mg/L, much lower than most feeding charts suggest
- K: 111–216 mg/L, the nutrient absorbed in the largest amount
Saloner and Bernstein found that about 160 mg/L N gave the best growth in veg (2020 paper). In flower (2021 paper), yield dropped when N was below about 160 mg/L. Table 1 sets lower nitrogen in flower on purpose (Section 2). The 160 mg/L figure gives the most plant mass, while the table aims for the highest cannabinoid concentration.
Many manufacturers’ feeding charts set phosphorus too high in flower. Feeding close to the research figures saves money and avoids lockout.
Translator’s note: uptake equals the nutrient concentration multiplied by the water the plant uses. At 30–35 °C, plants drink much more water. In hydroponics the water level in the tank drops faster than the salts are used up, so EC climbs. The topic why temperate feeding plans break at 31 °C and 85% humidity covers this in detail.
What to do: in hot weeks, measure tank EC every day and write it down. If EC rises above your target, top up with plain water instead of more nutrient solution. This keeps the plant at the strength in Table 1 instead of a creeping overdose.
8. Lockout and interactions between elements
Nutrient lockout happens when too much of one element, or the wrong pH, stops the plant from taking up another element. That element can be present in the solution, and the plant still goes short of it.
- Excess P blocks Zn and Fe
- Excess K blocks Ca and Mg
- Excess Ca blocks Mg and K
- pH too high (above 6.5 in soil, above 6.0 in hydro) blocks Fe, Mn, Zn, Cu and B
- pH too low (below 5.5) blocks Ca, Mg, P and Mo
Mix the nutrients into the water FIRST, then measure and adjust pH. Target ranges: soil 6.0–6.5, coco and hydro 5.5–6.0. Nutrient salts shift the pH on their own, so a pH set before mixing will not hold once the fertiliser goes in.
Translator’s note: many growers in Thailand use well water or hard tap water. Hard water often has a high starting pH and contains carbonates, which push the pH back up after you adjust it. Measure the mixed solution again a few hours after adjusting. If the pH has crept up, the carbonates in your water are the likely cause.
What to do: use the same order every time. Water → each nutrient, stirring after each → measure EC → adjust pH → measure pH again a few hours later. A fixed order lets you compare readings from week to week and spot a problem early.
9. Quick diagnosis from leaf symptoms
Table 4: Quick diagnosis from leaf symptoms
| Symptom | Where | Possible cause | What to do |
|---|---|---|---|
| Even yellowing starting at the bottom | Old leaves | N deficiency | Raise N, check pH |
| Yellowing between veins, veins green | Middle to old leaves | Mg deficiency | Epsom salt 1 g/L |
| Brown, burnt leaf edges | Old leaves | K deficiency, or heat / high EC | Raise K, lower EC |
| Purple stems and leaf stalks | Whole plant | P deficiency or cold | Check temperature and pH |
| Yellow new leaves, green veins | Top | Fe deficiency | If pH > 6.5, lower the pH |
| Brown spots and deformed new leaves | Top | Ca deficiency | CalMag, check pH |
| Dark green leaves, claw-shaped tips | Whole plant | EXCESS N | Flush, reduce N |
| Burnt leaf tips | Whole plant | Nutrient burn (EC too high) | Flush, lower EC |
“Flush” in this table means running plain water through the pot to wash out excess salts.
Translator’s note: cold is a likely cause of purple stems only in highland areas during the cool season. Elsewhere, check pH and phosphorus first. Some cultivars are also purple by genetics, and a purple plant that is growing well needs no treatment.
What to do: change only one thing at a time, and try it first on 2–3 plants. Record the date, the change and how the plants look one week later. If two things change at once, you cannot tell which one solved the problem or caused a new one.
Where to start this week
- Measure the pH and EC of your raw water. This shows what your water already contains before any fertiliser goes in.
- Convert your fertiliser labels to element mg/L (Section 3). This shows your real N, P and K levels.
- Compare your figures with Table 1. This shows where you are over- or under-feeding, especially phosphorus in flower.
- Test every change on a few plants for one full cycle. This keeps one mistake from costing the whole crop.
Questions and your own feeding figures are welcome below this topic or in General Nutrients. Include your water source, growing medium, cultivar and room temperature so others can compare.
References
- Powell K, Bauerle WL (2026). “Predicting vegetative phase nutrient uptake in Cannabis sativa L. via transpiration-driven mass-balance.” Frontiers in Plant Science. PMC12872892.
- Bevan L, Jones M, Zheng Y (2021). “Optimisation of Nitrogen, Phosphorus, and Potassium for Soilless Production of Cannabis sativa in the Flowering Stage Using Response Surface Analysis.” Frontiers in Plant Science.
- Allred J, Fatzinger B, Bugbee B (2025). “Crop steering through osmotic stress…” Journal of Cannabis Research. (Crop steering means controlling irrigation and EC on purpose to push the plant towards leafy growth or towards flowering.)
- Saloner A, Bernstein N (2020). “Response of Medical Cannabis (Cannabis sativa L.) to Nitrogen Supply Under Long Photoperiod.” Frontiers in Plant Science.
- Cockson P et al. (2019). “Characterization of Nutrient Disorders of Cannabis sativa.” Applied Sciences (MDPI).
- Kalinowski J et al. (2020). “Augmenting Nutrient Acquisition Ranges of Greenhouse Grown CBD (Cannabidiol) Hemp (Cannabis sativa) Cultivars.” Horticulturae.
- Shiponi S, Bernstein N (2021). “The Highs and Lows of P Supply in Medical Cannabis: Effects on Cannabinoids, the Ionome, and Morpho-Physiology.” Frontiers in Plant Science.
- Saloner A, Bernstein N (2021). “Nitrogen supply affects cannabinoid and terpenoid profile in medical cannabis (Cannabis sativa L.).” Industrial Crops and Products.
Legal note
This article is for education and reference only. Growing cannabis must follow the laws in force where you live.
In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. The notification was issued under the Thai Traditional Medicine Protection Act B.E. 2542 and has been in force since 26 June 2025. Flower may be sold only against a PT 33 (ภ.ท.33) prescription, for at most 30 days’ supply. Advertising and online sale of flower are prohibited.
Two Ministerial Regulations of B.E. 2569 also apply:
- The regulation on cannabis or hemp extracts (in force 26 April 2026) allows licences for extracts above 0.2% THC only to Thai juristic persons that are not “foreign” under the Foreign Business Act B.E. 2542, state agencies and the Thai Red Cross Society.
- The Controlled Herbs (No. 2) regulation (in force 30 April 2026) tightens the licence criteria for cannabis flower.
The draft Cannabis Control Act is not law yet. Cabinet approved the draft on 22 September 2026 and sent it to the House.
This forum does not allow direct sale, promotion of recreational use, or content aimed at countries where cannabis is prohibited.