- N-P-K (nitrogen, phosphorus, potassium) are the three primary macronutrients; every nutrient label lists them in this order.
- Vegetative growth is nitrogen-heavy: high N drives leaf and stem growth. Typical veg ratio: 3-1-2.
- Flowering requires phosphorus and potassium dominance: N is reduced sharply as buds develop. Typical bloom ratio: 1-3-2.
- Calcium and magnesium are secondary macronutrients that are critical in coco coir and soft-water grows; deficiencies are extremely common.
- Deficiencies show in leaves first — learning to read leaf symptoms is the fastest diagnostic tool a grower has.
- pH is the root cause of most apparent nutrient deficiencies: wrong pH locks out nutrients even when they’re present in the medium.
- Overfeeding is as damaging as underfeeding — nutrient burn (brown leaf tips) is one of the most common beginner mistakes.
The NPK System Explained
Every cannabis nutrient product lists three numbers on the label: N-P-K. These represent the percentage by weight of nitrogen, phosphorus, and potassium in that order. A product labelled 3-1-2 contains 3% nitrogen, 1% phosphorus, and 2% potassium.
- Nitrogen (N) — drives vegetative growth: leaf area, stem diameter, internodal development. It is the primary building block of chlorophyll and amino acids. Cannabis is a heavy nitrogen consumer during veg.
- Phosphorus (P) — fuels root development in seedlings and energy transfer during flower initiation and bud development. High-P bloom formulas target this phase.
- Potassium (K) — regulates overall plant health: water transport, enzyme activation, bud density, and resistance to stress. Elevated K in late flower is associated with denser, heavier buds.
Stage-by-Stage NPK Requirements
| Stage | N Level | P Level | K Level | Notes |
|---|---|---|---|---|
| Seedling | Very low | Low | Low | Cotyledons feed the plant; no feeding for 1–2 weeks |
| Early Veg | Medium–High | Medium | Medium | Start at 1/4 strength; build gradually |
| Late Veg | High | Medium | Medium | Full veg feed; training and topping here |
| Early Flower (Wk 1–3) | Medium | Medium–High | Medium–High | Transition; reduce N, increase P+K gradually |
| Mid Flower (Wk 4–7) | Low | High | High | Peak bloom feed; bud swell phase |
| Late Flower / Flush | None | Very low | Low | Taper off; flush 1–2 weeks before harvest |
Secondary Macronutrients: Calcium, Magnesium, Sulfur
These are called secondary only because plants need slightly less of them than NPK — not because they are less important. Calcium and magnesium deficiencies are among the most frequently seen nutrient problems in cannabis cultivation.
- Calcium (Ca) — strengthens cell walls and is essential for root tip development. Deficiency shows as brown spots and spots on new leaves. Especially critical in coco coir, which binds calcium naturally, and in any grow using reverse osmosis or soft water.
- Magnesium (Mg) — the central atom of every chlorophyll molecule. Without adequate Mg, photosynthesis is impaired. Deficiency shows as interveinal yellowing — yellow leaf with green veins remaining — starting on older lower leaves and moving upward.
- Sulfur (S) — involved in protein synthesis and enzyme function. Deficiency is less common but presents as pale yellow new growth, similar to nitrogen deficiency but starting on new leaves rather than old.
Cal-mag supplementation is standard practice in coco coir, hydroponic, and soft-water soil grows. Most pre-buffered soils contain adequate calcium and magnesium for the first 4–6 weeks.
Micronutrients: Small Amounts, Critical Roles
Cannabis requires trace amounts of iron, zinc, manganese, copper, boron, and molybdenum. These are usually present in quality soils and balanced nutrient formulas, but become deficient when pH is outside the correct range.
- Iron (Fe) — required for chlorophyll synthesis; deficiency causes pale yellow new growth (interveinal yellowing on young leaves)
- Zinc (Zn) — involved in enzyme systems and growth hormone production; deficiency causes small, distorted new leaves and short internodes
- Manganese (Mn) — supports photosynthesis and nitrogen metabolism; deficiency resembles iron deficiency but progresses differently
- Copper (Cu) — enzyme cofactor; deficiency causes bluish-green leaves that curl under, eventually turning yellow
- Boron (B) — cell division and sugar transport; deficiency causes thick, hollow stems and malformed new growth
- Molybdenum (Mo) — nitrogen processing; deficiency causes cupped leaves and pale yellow older leaves
Nutrient Deficiency Identification
| Deficiency | Symptoms | Where It Shows First | Common Cause | Fix |
|---|---|---|---|---|
| Nitrogen | Uniform yellowing, leaf drop | Older lower leaves | Under-feeding, late flower (normal) | Increase N feed; check pH |
| Phosphorus | Purple stems, dark green leaves | Older leaves, stems | Cold temps, low pH | Raise pH; warm root zone |
| Potassium | Brown leaf edges, yellowing | Older leaves first | Under-feeding in late flower | Add bloom booster; check pH |
| Calcium | Brown spots, crinkling | New growth | Coco, soft water, low pH | Add cal-mag; raise pH slightly |
| Magnesium | Interveinal yellowing | Older leaves midway up | Soft water, coco, low pH | Add cal-mag or Epsom salt |
| Iron | Pale yellow new growth | New leaves (top of plant) | High pH (most common) | Lower pH to unlock iron |
| Zinc | Small, twisted new leaves | New growth | High pH | Lower pH; trace element supplement |
Nutrient Lockout: Why pH Matters More Than Nutrients
Nutrient lockout occurs when nutrients are present in the medium but the plant cannot absorb them. The number one cause is pH outside the optimal range. Cannabis roots absorb different nutrients at different pH levels — when pH drifts too high or too low, specific nutrients become chemically unavailable regardless of how much you have added.
The classic beginner mistake: seeing a deficiency, adding more of that nutrient, seeing no improvement, adding more again — while the real cause is a pH problem locking everything out. Always check and correct pH before diagnosing a nutrient deficiency.
pH and Nutrient Availability
| Growing Medium | Optimal pH Range | Notes |
|---|---|---|
| Soil | 6.0–7.0 | Buffered by organic matter; most forgiving medium |
| Coco Coir | 5.5–6.5 | More like hydro; no buffer; pH must be managed every feed |
| Hydro / DWC | 5.5–6.5 | Precise control needed; fluctuating between 5.8–6.2 is ideal |
A calibrated digital pH meter is not optional — it is essential equipment. Test the pH of your feed water after adding nutrients, not before, as nutrients change the pH of water significantly.
Organic vs. Synthetic Nutrients
Both approaches can produce excellent cannabis. The key difference is speed and control:
- Organic nutrients are derived from natural sources (worm castings, bat guano, kelp, bone meal, fish hydrolysate). They release slowly, feeding soil biology which in turn feeds the plant. More forgiving — harder to overfeed. Slower to correct deficiencies. Better suited to soil.
- Synthetic (mineral) nutrients are immediately available to roots. Fast correction of deficiencies. Precise dose control. Higher risk of overfeeding and salt buildup. Better suited to coco and hydro. Require regular flushing to prevent salt accumulation.
Many growers combine both: a base synthetic nutrient formula supplemented with organic additives for terpene development and root health.
Reading Feed Charts
Every major nutrient brand publishes a feed chart showing recommended doses per litre or gallon at each growth stage. These charts are starting points, not gospel. Most professional growers run nutrient lines at 60–80% of the recommended dose and monitor plants closely. Cannabis in a healthy medium with good root development needs fewer nutrients than manufacturer charts suggest.
Always track your nutrient schedule and make incremental changes. Changing multiple variables at once makes it impossible to identify what caused a problem or a positive response.
Overfeeding: How to Recognise It
Nutrient toxicity (overfeeding) is as common as deficiency. Signs include:
- Nutrient burn — brown or rust-coloured tips on leaf edges; starts at tips and works inward if severe
- Dark, clawed leaves — nitrogen toxicity causes leaves to curl downward (“the claw”) with very dark green colour
- Light green new growth — can indicate calcium or nitrogen toxicity blocking other nutrient uptake
- Twisted or deformed new leaves — excess zinc or copper; often caused by over-supplementing with trace elements
- Salt crust on medium surface — visible white crust indicates salt buildup; flush immediately
Flushing Before Harvest
Flushing means running plain pH-adjusted water through the medium for 1–2 weeks before harvest, stopping all nutrient feeding. The rationale is that residual nutrients in plant tissue affect the taste and smoothness of the final product — particularly the harshness of smoke.
The scientific evidence is mixed. A 2019 study found no significant difference in cannabinoid content or perceived quality between flushed and unflushed samples. Despite this, flushing remains standard practice in most grow communities. In hydro and coco, flushing is more clearly beneficial for removing salt buildup from the medium itself.