Nutrient Management
Feed the plant. Not the schedule.
Educational use only. Cannabis cultivation is subject to federal, state/provincial, and local laws. Verify your local laws before proceeding. Nothing here constitutes legal or medical advice. For adults 21+ (18+ in medical jurisdictions).
Cannabis is a heavy feeder with specific needs that change across the grow cycle. Get nutrients right and plants push explosive growth and dense, resinous flowers. Get it wrong and you will spend weeks chasing symptoms that compound each other. The good news: most nutrient problems trace back to a small number of root causes — pH, overfeeding, and calcium deficiency account for the vast majority of what growers encounter. Understand those three, and you will solve 80% of issues before they start.
The macronutrient triad: NPK
Every nutrient label shows three numbers: N-P-K (nitrogen, phosphorus, potassium). These are the primary macronutrients that cannabis consumes in the largest quantities.
Nitrogen (N) is the engine of vegetative growth. It is a core component of chlorophyll and amino acids — without adequate nitrogen, new growth is pale and slow. During the vegetative stage, you want a nitrogen-forward feed. Once you flip to flower, you begin reducing nitrogen progressively: excess nitrogen in late flower suppresses resin production and delays maturation.
Phosphorus (P) supports root development and energy transfer within the plant, and becomes especially important during the flowering stage for bud development and terpene synthesis. Potassium (K) regulates stomata, protein synthesis, and overall plant resilience. Both P and K are ramped up during the flowering stage in what most feeding charts call the "bloom" or "PK boost" phase.
Macronutrient role and demand by stage
| Nutrient | Primary Role | Vegetative Demand | Flowering Demand |
|---|---|---|---|
| Nitrogen (N) | Chlorophyll, amino acids, vigorous growth | High | Medium → Low (taper at week 4+) |
| Phosphorus (P) | Root development, energy transfer, bud formation | Medium | High |
| Potassium (K) | Stomatal regulation, disease resistance, yield | Medium | High |
Secondary and micronutrients
Beyond NPK, cannabis requires calcium (Ca), magnesium (Mg), and sulfur (S) in significant amounts — these are called secondary macronutrients. Calcium is essential for cell wall structure and is almost entirely taken up through the transpiration stream. Magnesium sits at the center of every chlorophyll molecule. Deficiencies in either are extremely common and frequently misdiagnosed as NPK issues.
Micronutrients — iron, manganese, zinc, copper, boron, molybdenum — are needed in trace amounts but are still critical. Most quality nutrient lines include micronutrients in their base formula. Deficiencies in micros usually appear as interveinal chlorosis (yellowing between leaf veins on newer growth) and are almost always pH-related rather than an actual absence of the nutrient.
Tip
Cal-Mag is the single most commonly needed supplement across all growing mediums. In coco coir, it is essentially non-negotiable — coco has a high cation exchange capacity that actively holds calcium and magnesium from your nutrient solution. Add cal-mag at every watering in coco.
pH: the hidden lever behind most deficiencies
Every nutrient has a pH range at which it is soluble and available for root uptake. Outside that range, the nutrient precipitates out of solution or becomes chemically bound in the root zone — a condition called nutrient lockout. The plant can be sitting in a perfectly formulated feed, but if pH is wrong, it cannot access what it needs.
The target pH range depends on your medium. Soil buffers a wider range and performs best between 6.0 and 7.0, with 6.2–6.8 being ideal. Coco coir and hydro systems are more sensitive, with an optimal range of 5.5–6.2. Never let hydro pH drift above 6.5 for extended periods — iron and manganese become locked out quickly above that threshold.
Always check your runoff pH, not just your input pH. Nutrient and fertilizer salts accumulate in the root zone over time, and runoff pH tells you what the plant is actually experiencing. A significant gap between input and runoff is a signal to flush.
Optimal pH by growing medium
| Medium | Optimal pH Range | Notes |
|---|---|---|
| Soil | 6.2–6.8 | Biological activity buffers minor swings; wider window than coco/hydro |
| Coco coir | 5.8–6.2 | Less buffering than soil; monitor runoff closely |
| DWC / hydro | 5.5–6.1 | Tightest window; pH swings affect nutrient availability fastest |
| Perlite / soilless | 5.8–6.3 | Similar to coco; no biological buffering |
EC and feeding strength
EC (electrical conductivity) measures the total dissolved salt content of your nutrient solution — a proxy for feed strength that works across any nutrient brand or line. The higher the EC, the more concentrated the solution. This is measured in mS/cm (millisiemens per centimeter).
EC targets vary by stage and strain vigor. A conservative approach: seedlings at 0.4–0.8 mS/cm, vegetative at 1.2–2.0, early flower at 1.8–2.4, mid-late flower at 2.0–2.8. Heavy feeders and commercial growers may push higher. The best feedback is the plant itself: dark glossy leaves with slight claw often signal nitrogen excess; pale new growth points to deficiency or lockout.
Running a small amount of runoff at each watering (10–20% of input volume) and measuring its EC tells you if salts are accumulating. Runoff EC significantly higher than input EC suggests it is time to flush with plain pH-balanced water.
Pro tips
- →Always pH after adding nutrients — nutrient concentrates affect pH significantly
- →Start new strains at 50–70% of the recommended feed chart dose, then adjust based on response
- →Pure water (EC near 0) flushes help reset a salt-accumulated root zone but are not a substitute for correct ongoing feeding
- →Leaves cannot directly absorb nitrogen deficiency from foliar spray — fix it at the root zone
- →Yellowing of lower (oldest) leaves is normal in late flower as the plant cannibalizes nitrogen to finish buds
Reading deficiency symptoms
Most deficiency identification guides work from the location of symptoms on the plant — old growth vs new growth. Nutrients that the plant can relocate internally (mobile nutrients) show deficiency symptoms on older, lower leaves first. Nutrients the plant cannot move (immobile nutrients) affect newer growth first.
Mobile nutrients: nitrogen, phosphorus, potassium, magnesium. Symptoms on lower/older leaves first. Immobile nutrients: calcium, iron, zinc, manganese, copper. Symptoms on upper/newer growth first.
Before adjusting nutrients, always: (1) check pH, (2) verify VPD and transpiration are normal, (3) consider whether a flush is due. Address these first. Adding nutrients to a plant with pH lockout and salt buildup makes the situation worse.
Warning
Nutrient burn (brown, crispy leaf tips) is caused by overfeeding, not underfeeding. The most common mistake new growers make is seeing any leaf change and adding more nutrients. When in doubt, back off — plants recover from underfeeding faster than from salt stress.