Short answer. Nitrogen is the engine of leaf growth. In soilless and hydro systems cannabis does well on roughly 150–200 mg/L N, supplied mostly as nitrate (NO₃⁻), with ammonium (NH₄⁺) capped at about 10–30% of total N [1][2][3]. Too little N yellows the oldest leaves first and cuts yield hard; too much gives dark, glossy, clawed leaves and soft, leafy growth. Ammonium above about 30% of total N caused toxicity in a controlled cannabis trial, and 100% ammonium killed the plants [2].
What's happening
N is part of every amino acid, protein, nucleic acid and chlorophyll molecule. Plants take it up as nitrate or ammonium.
- Nitrate is the safe workhorse. Its uptake makes roots release hydroxide or bicarbonate, so the root zone pH rises on a nitrate feed (see pH drift).
- Ammonium is taken up fast and assimilated in the roots, which costs sugars. Its uptake releases protons, so the root zone pH falls. In excess it is toxic: it competes with K⁺, Ca²⁺ and Mg²⁺ for uptake and can damage roots [2].
In the Saloner and Bernstein NH₄/NO₃ trial (uniform 200 mg/L N, 0–100% ammonium), yield fell 46% as ammonium went from 0 to 50%, plants at 50% showed toxicity, and plants at 100% died. Cannabinoids were highest under pure nitrate; 10–30% ammonium did not substantially harm secondary metabolism, and the authors advise against ratios above 30% NH₄ [2].
The nitrogen and potency trade-off
In flower, Saloner and Bernstein found that cannabinoid and terpenoid concentrations were suppressed at higher N supply, while inflorescence yield rose with N up to about 160 mg/L [8]. So lower N can raise cannabinoid and terpene percentage while lowering total yield. It's a trade-off to set deliberately with your flower recipe, not a reason to flush: a pre-harvest flush starves the plant while it is packing on yield and resin, and is a Myth (see the flush debate).
Late-flower N taper (instead of flushing)
GenetixBay tapers N in the last 3–4 weeks, towards zero at the finish, and keeps P, K, Ca, Mg, S and the micros at normal late-flower strength. It puts the trade-off above to deliberate use: a little potency percentage for a little bulk, with the plant otherwise fully fed. Grower practice, widely recommended by experienced growers; this ramp hasn't been tested in a controlled trial.
| Weeks before harvest | N (mg/L) | Everything else |
|---|---|---|
| −4 | 100–150 (normal late flower) | Full late-flower feed |
| −3 | ~75–110 (about 75 %) | P, K, Ca, Mg, S and micros unchanged |
| −2 | ~50–75 (about 50 %) | Unchanged |
| −1 | ~25–40 (about 25 %) | Unchanged |
| Final days | 0–20 | Unchanged; keep normal runoff |
To do it you need per-element control: single salts with an N-free Ca source (see hydroponic salts) or a per-element dosing controller. Diluting a 2-part cuts everything, not just N.
N is mobile in the phloem. When supply falls short, the plant strips old leaves to feed new ones, so deficiency starts at the bottom (see mobile vs immobile nutrients).
How to recognise it
Deficiency. Slight stunting, then even pale-yellowing of the lower foliage that climbs upward; advanced leaves go fully yellow, necrotic and drop [4]. In DWC flower trials, N deficiency appeared in flower week 3 at the lower fan-leaf tips, spread upward, fan leaves senesced by week 5, and inflorescence fresh weight was about 72% lower than control [5]. Red or purple petioles often accompany it, but that alone is not diagnostic (genetics and cold do it too).
Excess (and ammonium toxicity). Very dark green, glossy leaves; leaf tips that hook down ("the claw"); thin, soft stems; delayed or leafy flowering. Ammonium toxicity adds root browning, leaf-margin necrosis and symptoms of Ca/Mg/K shortage because ammonium blocks their uptake [2]. Claw also has other causes (overwatering, heat, pH), so see taco and canoeing leaves.
Numbers that matter
| Parameter | Target | Conditions / notes |
|---|---|---|
| Solution N, seedlings and fresh clones | 50–100 mg/L | Grower practice; low EC (0.6–1.0 mS/cm) |
| Solution N, veg | ~160 mg/L optimum; 150–200 mg/L working range | Pot trial, long photoperiod; 30 mg/L deficient, 240–320 mg/L no better [1] |
| Solution N, flower (mineral, DWC) | ~194 mg/L predicted optimum for yield | Response-surface trial, one high-THC cultivar [3] |
| Solution N, flower (liquid organic, coir) | 212–261 mg/L N | Organic N is less available; one substrate [6] |
| NH₄⁺ share of total N | ≤10–30% | >30% risks toxicity; 50% cut yield 46% [2] |
| Leaf N, healthy (most recent mature leaf) | 3.3–5.0% dry weight | NC State floral hemp range [7] |
| Leaf N at first deficiency symptoms | ~1.6% | Cannabis, sand culture [4] |
What to check first
- Stage. A slow bottom-up fade in the last 2–3 weeks of flower is normal senescence, not a problem to fix.
- Root zone. Overwatering, cold solution (<18 °C / 64 °F), root rot or root aphids cause "N deficiency" in a well-fed plant. Check roots before the feed (see root browning in hydro).
- pH. Outside about 5.5–6.5 in soilless, uptake of several nutrients suffers.
- EC in and out. Low input EC means real hunger; high runoff EC means salts or poor uptake.
- Feed history. A "bloom" base with very low N switched on at flip is a classic cause of early-flower yellowing.
How to fix it
- Mineral hydro/coco: raise N by 10–20% (for example 150 → 170 mg/L), using calcium nitrate or potassium nitrate. Do not double it.
- Keep ammonium modest. Most commercial calcium nitrate contains about 1% ammonium-N (5–8% of its N); that is fine. Do not "fix" a deficiency with ammonium sulfate or urea in hydro.
- Soil/organic: liquid organic N or a top-dress (blood meal, feather meal, fish hydrolysate) at label rate; expect 1–2 weeks before response because microbes must mineralise it.
- Toxicity: cut N 20–30%, feed at lower EC for one or two cycles, check runoff EC. If ammonium-based, switch to a nitrate-dominant feed and raise root-zone pH back into range.
How to confirm it worked
- Yellowing stops climbing within 5–7 days; already-yellow leaves stay yellow.
- New leaves come in mid-green. For excess, new leaves stop clawing within about a week.
- If you can, a tissue test on the most recent mature leaf lands at 3.3–5.0% N [7].
Common mistakes
- Feeding more N to a plant with sick roots.
- Treating harvest-time fade as a deficiency.
- Running high-ammonium "organic-style" salts in DWC and then chasing a falling pH.
- Assuming dark green always means healthy.
Evidence
- Bottom-up yellowing as the N deficiency signature in cannabis: Established (two controlled studies in different systems [4][5]).
- Veg optimum near 160 mg/L N and flower optimum near 190–200 mg/L N in mineral feeds: Supported (single-cultivar trials [1][3]); organic feeds need higher N because availability is lower [6].
- Ammonium above ~30% of N harms cannabis: Supported (one rigorous trial, consistent with other crops [2]).
- Purple petioles as a reliable N sign: Contested.
- Lower N in flower raising cannabinoid and terpene percentage while lowering yield: Supported (one controlled trial [8]); a trade-off, not a reason to flush.
- Tapering N towards zero over the last 3–4 weeks while feeding everything else: Grower practice (GenetixBay house approach).
