Cannabis Nutrients Guide:
Feeding, EC Targets & Deficiency Diagnosis

What Each Nutrient Actually Does

NPK Ratios by Growth Stage

The 500 vs. 700 Scale Problem

Two different PPM conversion factors are used by different meter brands: the 500 scale (used by Hanna, Milwaukee) and the 700 scale (used by Bluelab, Oakton). The same solution that reads 1,000 PPM on a 500-scale meter will read approximately 1,400 PPM on a 700-scale meter. A grower comparing their readings to a feeding chart without knowing which scale their meter uses will systematically under- or over-feed. EC in mS/cm is the universal unit — the EC targets in this guide are unambiguous regardless of your meter brand.

EC Targets by Growth Stage

Starting Point by Medium

How to Start Feeding Without Burning

• 1Mix your first nutrient solution at 25–50% of the EC target for the current stage. A seedling target is 0.4–0.6 EC — start your first feeding at 0.4 or below.
• 2Observe the plant response for 3–5 days. Look for signs of overfeeding (tip burn, dark claw leaves) or underfeeding (pale color, slow growth). Adjust accordingly.
• 3Ramp up gradually as the plant develops its root system and canopy. The more root mass the plant has, the more nutrient solution it can efficiently process.

The feed-water-water schedule in soil gives roots a chance to access oxygen between waterings and prevents salt accumulation from continuous synthetic feeding. In coco, there is no natural buffering — every watering must include nutrients or salt balance drifts rapidly at the root zone.

DWC reservoir management: Top off daily with pH-adjusted plain water to replace what plants consume — topping off with nutrients spikes EC as the reservoir shrinks. Do a full drain-and-remix every 7–14 days to prevent salt accumulation, pathogen buildup, and nutrient imbalance drift.

Core Veg Nutrients

• Base veg nutrients (3-1-2 or 4-2-3 NPK): Drives rapid leaf and stem development — the foundation of every veg feeding program
• Cal-mag supplement: Essential in coco (strips calcium) and with RO water (zero baseline minerals); also valuable in any setup showing interveinal yellowing or tip curl
• Silica (potassium silicate or monosilicic acid): Deposits in epidermal cell walls — improves physical resistance to sucking insects, fungal penetration, and water loss under heat stress. Must be added to water first, before any other nutrients
• Humic and fulvic acids: Humic acid improves cation exchange capacity; fulvic acid chelates micronutrients, improving their bioavailability — especially useful in RO water setups where baseline mineral complexity is absent
• Root stimulators (mycorrhizae, beneficial bacteria): Most effective when introduced at transplant, not after root establishment is already complete. See Module 08 for mechanism and the phosphorus suppression note

Optimizing Nutrient Uptake in Veg

• Root zone temperature 68–72°F (20–22°C): The primary environmental variable for nutrient absorption — cold roots below 60°F significantly impair phosphorus and calcium uptake regardless of feeding program
• pH dialed in for medium: Soil 6.2–6.5; coco/hydro 5.8–6.2 — pH is the gatekeeper that determines which nutrients are chemically available regardless of what's in the reservoir
• Adequate canopy airflow: Oscillating fans cause thigmomorphogenesis (stem strengthening) and ensure even CO₂ distribution across all leaves — both support efficient photosynthesis and growth

What Each Nutrient Does in Flower

When to Use Bloom Boosters

The Mobile vs. Immobile Framework — With Mechanism

Mobile nutrients (nitrogen, phosphorus, potassium, magnesium) can be translocated — when supply is insufficient, the plant actively moves these elements from older leaf tissue to actively growing new tissue. The older, lower leaves are essentially cannibalized to feed the canopy. This is why N, P, K, and Mg deficiencies always appear first on lower, older leaves — the plant is moving nutrients upward.

Immobile nutrients (calcium, iron, boron, manganese) are deposited into cell walls and cannot be moved once placed. When supply is insufficient, the plant has no reserves to draw from — so new growth is the first affected. Calcium deficiency always appears at the top of the plant; iron deficiency always shows as yellowing in the youngest leaves.

Symptom Location → Diagnosis Direction

Toxicity Symptoms (Too Much)

• Dark green, clawed leaves: Nitrogen toxicity — leaves point downward at the tips (eagle claw) and develop an unusually dark, waxy green color. Reduce N immediately and flush lightly
• Brown or crispy leaf tips: Nutrient burn from high EC or salt accumulation — check runoff EC; flush if runoff is significantly above input
• Widespread interveinal chlorosis on new growth: Phosphorus toxicity causing iron and zinc lockout — reduce P, check pH (iron uptake especially pH-sensitive at 6.0–6.5)

Understanding the key antagonism pairs explains why many experienced growers' "deficiencies" are actually caused by their feeding program itself. When you see apparent magnesium deficiency in a plant that is well-fed, the first question is not "am I underdosing magnesium?" — it's "do I have too much calcium or potassium blocking it?"

The Four Most Impactful Antagonisms for Cannabis

Non-Nutrient Causes of Yellowing to Rule Out First

• Natural leaf senescence in flower: Lower fan leaves yellowing and dropping in weeks 4–8 of flower is completely normal — the plant is translocating mobile nutrients from old tissue to actively developing buds. If the yellowing is limited to lower leaves and is not progressing rapidly up the canopy, it is almost certainly natural
• Overwatering: The most common cause of apparent deficiency in soil. Waterlogged media prevents oxygen from reaching roots, impairing all nutrient uptake simultaneously — the plant looks deficient in everything at once despite adequate feeding
• Root zone temperature too low (below 60°F): Cold roots impair phosphorus and calcium absorption specifically, producing those deficiency symptoms even when the feeding program is correct
• Rootbound container: A plant whose roots have filled the container cannot absorb water and nutrients efficiently — the feeding solution runs through channels rather than being absorbed. Symptoms look like deficiency but the fix is transplanting
• Nutrient antagonism: As described in the previous section — too much of one element blocking another is the most common non-environment cause of deficiency-looking symptoms in well-fed plants

pH Targets by Medium

Runoff EC Interpretation

When Flushing Makes Sense

• Heavy synthetic nutrient programs: Where salt accumulation over a long flower cycle is likely — reducing EC to near zero for the final 5–14 days reduces medium EC regardless of flavor benefit
• High runoff EC in coco or hydro: If runoff EC is significantly above input EC in the final weeks, a flush corrects salt balance before harvest
• Nutrient lockout or toxicity symptoms: Emergency flush to reset the root zone if plants are showing severe stress

When NOT to Flush

• Organic or living soil: Flushing with large volumes of water damages beneficial microbial communities — the organisms that make living soil work are disrupted by aggressive water-only irrigation
• Healthy EC and pH in runoff: If your readings are in target range, flushing provides no documented benefit and can leach the nutrients the plant is still using for final ripening
• Autoflowering strains on short cycles: Less time to accumulate salts means less justification for a flush period that reduces the total productive flowering time

How to Flush by Medium

• 1Confirm lockout: Check runoff EC and pH. If runoff EC is within 0.5 of input and symptoms match a single nutrient, pH lockout is more likely than actual deficiency — the nutrient is present but unavailable due to wrong pH range.
• 2Adjust pH gradually: Move pH in 0.2–0.3 increments toward the target range — don't overcorrect from one extreme to the other in a single watering. Gradual shifts are less stressful than sudden swings.
• 3Give one or two plain-water feedings: At corrected pH, without full nutrients — allows roots to access what's already available in the medium before adding more.
• 4Monitor runoff daily for 3–5 days: If pH stabilizes and symptoms stop progressing, the lockout was pH-related and you've corrected it without a full flush.
• 5Full flush only if runoff EC remains 0.5+ above input: At this point, salt accumulation is confirmed and a flush is the appropriate response.

The Nutrient Program Hierarchy

• 1Base nutrients first: A complete veg and bloom base provides all primary, secondary, and micronutrients in appropriate ratios. Dial this in before adding anything else — get pH correct, EC at target, and runoff within normal range.
• 2Cal-mag second (if needed): Add before other supplements — it's a near-essential addition in coco and RO water grows, and the most commonly needed supplement in home cannabis cultivation.
• 3Silica third (if using): Always added first in the mixing sequence (before cal-mag) despite being added to the program third conceptually.
• 4Stage-specific supplements last: Root stimulators at transplant; bloom boosters in mid-flower; enzymes throughout; PK supplements in peak flower. Add one supplement at a time and monitor response before adding the next.

When Cal-Mag Is Required vs. Optional

What Each Does

• Humic acid: A large organic molecule that improves cation exchange capacity in soil and coco — makes more nutrient ions available at root surfaces by holding them in accessible form rather than letting them leach with drainage. Most useful in soil and coco; less relevant in recirculating DWC where direct uptake dominates
• Fulvic acid: A smaller molecule related to humic acid — chelates (binds) micronutrients (especially iron, zinc, manganese) and transports them across root membranes more efficiently. Particularly useful in RO-fed systems where baseline micronutrient complexity is absent
• Enzyme products (Sensizym, Cannazym, etc.): Contain cellulase, protease, and other enzymes that break down dead root material in the root zone. This reduces the organic substrate available to root-zone pathogens (particularly Pythium) and keeps the root zone cleaner between reservoir changes. Genuinely useful in recirculating hydro and multi-cycle coco; less critical in single-cycle soil grows