Nutrient Management: NPK, Micronutrients and Feeding Schedules

Nutrient management is one of the most complex yet important aspects of cannabis cultivation. A plant optimally supplied with all required nutrients grows faster, flowers more productively and produces a fuller terpene profile. At the same time, overfeeding is one of the most common beginner mistakes. This guide explains all fundamentals – from chemical elements through pH and EC to concrete feeding schedules.

## Macronutrients: The Building Blocks

Like all plants, cannabis requires three primary macronutrients in large quantities:

**Nitrogen (N):** The engine of vegetative growth. Nitrogen is a major component of amino acids, proteins and chlorophyll. During the vegetative phase, cannabis has its highest nitrogen demand – it promotes the production of leaves, stems and roots. Typical uptake forms: nitrate (NO₃⁻) and ammonium (NH₄⁺). Plants prefer nitrate, as ammonium in high concentrations can be toxic.

**Phosphorus (P):** Essential for energy transfer (ATP), DNA/RNA synthesis and flower formation. During the flowering phase, phosphorus demand increases significantly. Phosphorus is taken up as phosphate (H₂PO₄⁻, HPO₄²⁻) and at incorrect pH is often the first element to precipitate and become unavailable to the plant.

**Potassium (K):** Regulates water transport (osmotic regulation), stomatal function, enzyme activation and carbohydrate transport. Potassium is taken up as K⁺ ions and is particularly important during flowering for resin production and bud density. Adequate potassium levels also increase the plant's stress resistance.

**NPK ratios by growth phase:**

- Seedling/clone: No or minimal fertilisation (week 1–2) - Early veg: NPK 2-1-2 or 3-1-2 (low dose) - Late veg: NPK 3-1-2 (full dose) - Transition to bloom (week 1–2): NPK 2-2-2 (transition) - Early bloom (week 3–4): NPK 1-2-3 - Mid bloom (week 5–6): NPK 0-3-3 - Late bloom (week 7–8): NPK 0-2-3 (reduced dose) - Final 1–2 weeks: Flush (water only)

## Secondary Macronutrients

**Calcium (Ca):** Structural element of cell walls (calcium pectinate), important for cell division and cell elongation. Calcium deficiency appears on new leaves (immobile nutrient). Particularly critical in coco substrate, as coco binds calcium and exchanges it for sodium and potassium. Tap water often contains sufficient calcium; when using reverse osmosis water, calcium must always be supplemented.

**Magnesium (Mg):** Central atom in the chlorophyll molecule – without magnesium, no photosynthesis. Also important for enzyme activation and phosphorus metabolism. Magnesium deficiency shows interveinal chlorosis (yellowing between leaf veins) on middle to lower leaves. Epsom salt (magnesium sulphate, MgSO₄) is the simplest supplement.

**Sulphur (S):** Component of amino acids (cysteine, methionine), vitamins and enzymes. Sulphur is also involved in terpene synthesis. Sulphur deficiency resembles nitrogen deficiency but appears on younger leaves (sulphur is poorly mobile). Most fertiliser lines contain adequate sulphur.

## Micronutrients: Small but Essential

Micronutrients are needed only in traces but are indispensable for enzymatic processes:

**Iron (Fe):** Electron transfer in photosynthesis. Deficiency: interveinal chlorosis on the youngest leaves (white-yellow with green veins). Almost always a pH problem – iron becomes rapidly insoluble at pH >6.5 (soil) or >6.0 (hydro).

**Manganese (Mn):** Involved in photosynthesis (water splitting) and as an enzyme cofactor. Deficiency: interveinal chlorosis with brown spots on younger leaves. Excess: brown dots (manganese toxicity) at pH <5.5.

**Zinc (Zn):** Important for auxin synthesis (growth hormones) and enzyme functions. Deficiency: disrupted growth, small, twisted young leaves. Common at high pH and excess phosphorus (phosphorus blocks zinc uptake).

**Copper (Cu):** Involved in photosynthesis and lignin formation (stem stability). Deficiency rare, but excess is toxic. New leaves become dark green and wilt.

**Boron (B):** Important for cell division, calcium transport and cell wall stability. Deficiency: thickened, deformed growing points, hollow stems. Excess: leaf tip burn.

**Molybdenum (Mo):** Essential for nitrate reductase (conversion of nitrate to ammonium). Deficiency at low pH (<5.5), showing as pale green leaves with curled edges.

## pH Value: The Key to Nutrient Availability

The pH value of irrigation water and substrate determines which nutrients are available to the plant. This is the single most important factor in nutrient management.

**Optimal pH ranges:**

- Soil/compost: 6.0–6.5 (sweet spot: 6.3) - Coco: 5.5–6.5 (sweet spot: 5.8–6.0) - Hydroponics: 5.5–6.0 (sweet spot: 5.8)

**Why allow fluctuation?** Within the optimal range, it is advisable to slightly vary the pH with each watering (e.g. between 5.8 and 6.2 in coco). Different nutrients have their maximum availability at different pH values. Through fluctuation, all elements can be optimally absorbed at various times.

**Nutrient lockout:** When pH is outside the optimal range, certain nutrients become chemically bound in the substrate and can no longer be taken up by the roots – even if present in sufficient quantities. Typical lockout scenarios:

- pH >7.0: Iron, manganese, zinc, copper, boron become insoluble - pH <5.5: Calcium, magnesium, phosphorus become less available - pH >6.5 in hydro: almost all micronutrients become problematic

**pH measurement:** Digital pH meters (Bluelab, Apera, Milwaukee) are standard. Calibrate before each use (pH 4.0 and 7.0 reference solutions). Also measure the pH of runoff water – this indicates the actual pH in the substrate. A large difference between input and runoff indicates salt accumulation.

**pH correction:** pH-Down (phosphoric acid or nitric acid), pH-Up (potassium hydroxide or potassium carbonate). In organic living soil systems, pH correction is usually unnecessary – the microbial ecosystem naturally buffers pH.

## EC Value: Measuring Nutrient Concentration

EC stands for Electrical Conductivity and indicates how many dissolved salts (ions) are in the water. The more fertiliser dissolved, the higher the EC value. The unit is mS/cm (millisiemens per centimetre) or µS/cm.

**Guidelines by growth phase:**

- Tap water (base): 0.2–0.5 mS/cm - RO water: 0.0–0.05 mS/cm - Seedling/clone: 0.4–0.8 mS/cm (total) - Early veg: 0.8–1.2 mS/cm - Late veg: 1.2–1.6 mS/cm - Early bloom: 1.4–1.8 mS/cm - Mid/late bloom: 1.6–2.2 mS/cm - Flush: 0.0–0.3 mS/cm

**Runoff EC:** As with pH, the EC of runoff water should also be measured. If runoff EC is significantly higher than input EC (e.g. input 1.4, runoff 2.8), it means salts have accumulated in the substrate. A flush with pH-adjusted water (EC 0.0–0.3) dissolves these salts.

**TDS vs. EC:** Some meters display TDS (Total Dissolved Solids) in ppm rather than EC. The conversion factor varies: 1 mS/cm ≈ 500 ppm (US scale, Hanna) or 700 ppm (EU scale, Truncheon). It is best to always work in EC.

## Organic vs. Mineral Fertilisation

**Mineral (synthetic) fertilisers:**

Advantages: Immediately plant-available (ions are directly in soluble form). Precisely dosable. Reproducible results. EC and pH easily measurable and correctable. Cost-effective. Faster nutrient uptake.

Disadvantages: Can quickly cause damage when overdosed. Do not build soil life. Salts can accumulate in the substrate. Flush before harvest recommended/necessary. The resulting product may have a "chemical" taste if not flushed correctly.

Well-known brands: General Hydroponics (Flora series), Advanced Nutrients, CANNA, BioBizz (organic), Plagron.

**Organic fertilisers:**

Advantages: Released slowly by microorganisms – lower burn risk. Promote soil life (microbes, mycorrhizae, bacteria). Produce more complex terpene profiles (many growers consistently report this). Flush before harvest less necessary. More sustainable and environmentally friendly.

Disadvantages: Slower availability – not suitable for quick corrections. More difficult to dose precisely. EC/pH management less predictable. May contain pathogens if improperly composted. Problematic in hydro systems (clog lines, promote algae).

Typical organic fertilisers: Worm castings, bat guano, blood meal (N), bone meal (P), kelp/seaweed (K + trace elements), fish emulsion, compost tea, mycorrhizal inoculants.

**Recommendation:** For beginners, mineral fertilisers are easier to manage. For advanced growers and cannabis social clubs that prioritise terpene quality and sustainability, organic cultivation with living soil is the superior method long-term.

## Recognising Overfeeding and Corrective Action

Overfeeding (nutrient burn) is one of the most common mistakes and shows characteristic symptoms:

**Early signs:** Dark green, almost black-green leaves (nitrogen excess). Leaf tips turn brown and dry (nutrient burn). Leaf tips bend downward like eagle claws (nitrogen toxicity = "The Claw"). Slow, stunted growth despite seemingly good conditions.

**Advanced symptoms:** Brown spots and necrosis on leaves. Leaf edges curl inward. Roots turn brown (salt stress). Nutrient lockout: overdosing one element blocks uptake of others (e.g. too much potassium blocks calcium and magnesium).

**Immediate measures:** Flush with pH-adjusted water – run 2–3× the pot volume through. Measure runoff EC – should drop below 0.5 mS/cm. Give only water for 2–3 days, then restart at 50% of normal fertiliser dose.

## Recognising Underfeeding

Underfeeding develops more slowly but just as clearly:

**General picture:** Pale, yellowish-green leaves (nitrogen deficiency). Slowed growth. Thin stems. Small buds. Premature death of lower leaves.

**Important:** Always check pH before increasing fertiliser! A pH problem can cause identical symptoms to a nutrient deficiency, and more fertiliser would worsen the problem.

## Feeding Schedules: Veg and Bloom

Below is an example feeding schedule for mineral fertilisation in coco substrate. All EC values are totals (including base EC of the water). ml/L figures are guidelines – always measure EC and adjust accordingly.

**Vegetative Phase (Week 1–4):**

Week 1 (after germination): Water only with Cal-Mag 0.5 ml/L. EC: 0.4–0.6. pH: 5.8–6.0. Watering frequency: When pot feels light.

Week 2: Base-A + Base-B 0.5 ml/L each + Cal-Mag 0.5 ml/L. EC: 0.8–1.0. pH: 5.8–6.2. Daily light watering.

Week 3: Base-A + Base-B 1.0 ml/L each + Cal-Mag 0.5 ml/L + root stimulator. EC: 1.0–1.3. pH: 5.8–6.2.

Week 4: Base-A + Base-B 1.5 ml/L each + Cal-Mag 0.5 ml/L. EC: 1.2–1.5. pH: 5.8–6.2. 2–3× daily fertigation in coco.

**Flowering Phase (Week 1–8+):**

Week 1 (bloom trigger 12/12): Base-A + Base-B 1.5 ml/L each + Cal-Mag 0.5 ml/L + PK boost 0.25 ml/L. EC: 1.3–1.6. pH: 5.8–6.2.

Week 2–3: Base-A + Base-B 1.5 ml/L each + Cal-Mag 0.5 ml/L + PK boost 0.5 ml/L. EC: 1.5–1.8. pH: 5.8–6.0.

Week 4–5 (bulk phase): Base-A + Base-B 1.5 ml/L each + Cal-Mag 0.3 ml/L + PK boost 1.0 ml/L. EC: 1.6–2.0. pH: 5.8–6.0.

Week 6–7 (ripening): Base-A + Base-B 1.0 ml/L each + PK boost 0.5 ml/L. EC: 1.2–1.6. pH: 5.8–6.0. Reduce or eliminate Cal-Mag.

Week 8+ (final 7–14 days): Flush – pH-adjusted water only. EC: 0.0–0.3.

**Important notes on the feeding schedule:** This is an example plan – every strain responds differently. Always start at 50% of the manufacturer's recommendation and increase based on plant response. Check runoff EC regularly – if runoff EC exceeds input EC by more than 50%, perform a flush. Coco must always be kept moist – never let it dry out completely. In soil, less fertilisation is needed as the soil itself buffers nutrients.

## Flushing: Purpose and Procedure

The end-of-bloom flush is a controversial topic. With mineral fertilisation it is standard practice; with organic growing it is largely unnecessary.

**Purpose:** Removal of excess salts from the substrate and plant. Promotes degradation of chlorophyll and residual nutrients in the leaves. Improves taste and smoke quality of the final product (less harsh, whiter ash). Leaves begin to yellow – a sign that the plant is mobilising its reserves.

**Procedure:** 7–14 days before the planned harvest, give only pH-adjusted water (pH 5.8–6.0 in coco, 6.0–6.5 in soil). On the first flush day, rinse generously (3× pot volume). Then water normally but without fertiliser. Runoff EC should drop below 0.5 mS/cm within 2–3 days.

**Critical perspective:** A study from the University of Guelph (2020) found no significant difference in taste, ash colour or chemical composition between flushed and unflushed plants with mineral fertilisation. In practice, most experienced growers still report an improvement in taste – possibly due to the natural ripening process during the flush phase rather than the flushing itself.

## Deficiency Symptoms: Quick Reference

| Nutrient | Mobile? | Symptom | Where first? | |----------|---------|---------|-------------| | N | Yes | Uniform yellowing | Lower leaves | | P | Yes | Dark green/purple | Lower leaves | | K | Yes | Brown leaf margins | Lower leaves | | Ca | No | Brown spots, deformed | Upper leaves | | Mg | Yes | Interveinal chlorosis | Middle leaves | | Fe | No | Yellow leaves, green veins | Youngest leaves | | Mn | No | Chlorosis + brown spots | Younger leaves | | Zn | No | Small, twisted leaves | New growth | | S | Low | Uniform yellowing | Younger leaves | | B | No | Deformed shoot tips | Growing points |

**Mobile vs. Immobile:** Mobile nutrients (N, P, K, Mg) are translocated by the plant from older leaves to young ones – therefore deficiencies first appear on lower, older leaves. Immobile nutrients (Ca, Fe, Mn, Zn, B) cannot be translocated – deficiencies appear on the youngest, uppermost leaves.

Consistently measuring pH and EC and following a structured feeding plan will prevent most nutrient problems from the outset. Reading the plant – regularly inspecting leaves for discolouration and deformations – nonetheless remains the most important skill of any grower.