Short answer
Macronutrients are the elements cannabis uses in large amounts: nitrogen, phosphorus and potassium (the primaries) and calcium, magnesium and sulfur (the secondaries), fed at roughly 30–250 mg/L each. Micronutrients are just as essential but needed in traces, under about 3 mg/L: iron, manganese, zinc, copper, boron, molybdenum, chloride and nickel. Established.
Why
An element is essential when the plant cannot complete its life cycle without it and nothing else can replace it [1]. That gives 17 for higher plants: carbon, hydrogen and oxygen from air and water, plus 14 mineral elements from the root zone [1].
Macro and micro describe quantity, not importance. Macronutrients build bulk: nitrogen in proteins and chlorophyll, calcium in cell walls, potassium as the main cell solute [1]. Micronutrients mostly run enzymes, so a little goes a long way and too much is toxic.
Cannabis shows a distinct, recognisable deficiency for each element when it is withheld [3], which is why diagnosis starts with knowing the list.
Beneficial elements such as silicon and sodium can help some crops but are not essential [1].
The numbers
| Group |
Elements |
Typical cannabis feed (mg/L) [2] |
| Primary macro |
N, P, K |
N 150–200, P 30–60, K 175–250 |
| Secondary macro |
Ca, Mg, S |
Ca 150–200, Mg 40–70, S 60–120 |
| Micro |
Fe, Mn, Zn, Cu, B, Mo |
Fe 1.5–3, Mn 0.3–0.8, Zn 0.1–0.5, B 0.2–0.5, Cu 0.03–0.1, Mo 0.01–0.05 |
| Micro (rarely added) |
Cl, Ni |
Supplied by water and impurities |
| Beneficial |
Si, Na |
Si 20–50 if used; Na as low as possible |
What to do
- Use a complete base feed that lists both macros and micros.
- Check that calcium and magnesium are covered, especially with RO or soft water.
- Never add single micronutrients by guesswork.
- Diagnose by where symptoms start: old leaves (mobile) or new growth (immobile).
Common mistakes
- Thinking NPK is the whole feed.
- Adding iron or zinc "just in case" and causing toxicity.
- Calling silica essential.
- Ignoring what the water already supplies.
A macronutrient is an essential element plants need in large amounts, measured in tens to hundreds of mg/L: nitrogen, phosphorus and potassium, plus calcium, magnesium and sulfur.Sulfur (S) is a secondary nutrient taken up as sulfate, used in the amino acids cysteine and methionine; deficiency yellows new growth fairly evenly.A micronutrient is an essential element plants need only in trace amounts, below a few mg/L: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel.Manganese (Mn) is a micronutrient that splits water in photosynthesis and activates many enzymes; low pH can raise it to toxic levels.Zinc (Zn) is a micronutrient needed for auxin production and many enzymes; deficiency gives small, twisted new leaves and shortened internodes.Copper (Cu) is a micronutrient used in photosynthetic electron transport and lignin formation; plants need only traces, and excess is toxic to roots.Molybdenum (Mo) is the micronutrient needed in the smallest amount; plants need it to convert nitrate into usable nitrogen.As a plant nutrient, chlorine is used as the chloride ion (Cl-) in tiny amounts for photosynthesis and osmotic balance; excess from water or salts is the usual problem.The root zone is the volume of medium or solution where a plant's roots live, including its water, air, temperature, pH, EC and microbes.The green pigment in chloroplasts that absorbs mainly blue and red light to drive photosynthesis; chlorophyll a and b are the main forms in plants.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.A beneficial element is one that improves growth or stress tolerance in some plants without being strictly essential, such as silicon, sodium, cobalt and selenium.Sodium (Na) is not essential for cannabis; it enters through water, softeners and some inputs, and at high levels causes salt stress and crowds out potassium and calcium.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.The N-P-K ratio is the three numbers on a fertiliser label giving percent nitrogen, phosphate (P2O5) and potash (K2O) by weight, not elemental P and K.Nutrient toxicity is damage from an excess of an element in the plant, either directly or by blocking uptake of others; boron, manganese, copper and chloride are common culprits.
