Short answer. Nickel is a genuine essential micronutrient: Brown, Welch and Cary showed in 1987 that barley grown for three generations in Ni-depleted conditions failed to produce viable grain [1]. Its known role is activating urease, which breaks urea into ammonium [2]. The requirement is so small that contamination in fertiliser salts, water and media normally covers it. There is no published report of nickel deficiency in cannabis. Do not add nickel unless you feed urea in an unusually pure system.
What's happening
Ni is the metal centre of urease. Without it, urea (from urea fertiliser and from the plant's own nitrogen recycling) builds up in leaves to toxic levels and causes tip necrosis in sensitive species [2]. Ni is mobile in the phloem and moves into seeds [2].
The amount needed is in the range of tens to a few hundred micrograms per kilogram of leaf dry weight [1][2].being reviewed Ordinary reagent and fertiliser-grade salts carry more than enough as impurities; that is also why Ni went unrecognised for so long (Epstein made the same point about contaminants and silicon [3]).
How to recognise it
- Deficiency (other crops): leaf tip and margin necrosis linked to urea accumulation; in barley, failure of seed viability [1][2]. Not documented in cannabis.
- Toxicity: interveinal chlorosis similar to iron deficiency (Ni competes with Fe), stunted roots and growth [2]. Cannabis and hemp are effective accumulators of heavy metals from contaminated soils, which is a reason to keep Ni out of the flower, not add more.being reviewed
Numbers that matter
| Parameter | Target | Conditions / notes |
|---|---|---|
| Solution Ni (added) | 0 mg/L for normal feeds | Contaminants suffice; grower practice and greenhouse standards |
| Solution Ni, if feeding urea in ultra-pure systems | ≤0.01–0.05 mg/L | being reviewed |
| Leaf Ni, sufficiency | ~0.01–0.1 mg/kg range | Order of magnitude from crop literature [1][2]being reviewed |
| Leaf Ni, toxicity in sensitive plants | >10 mg/kg | [2]being reviewed |
What to check first
- Tip burn on a urea-containing feed: first rule out overfeeding, high EC, ammonium toxicity and K shortage. Ni is the last suspect.
- Suspected Ni toxicity: test water and soil (industrial sites, some biosolids, stainless or plated fittings in poor condition).
How to fix it
- Deficiency (rare): switch from urea to nitrate-based N, which sidesteps urease entirely. If you must use urea, a trace of nickel sulfate hexahydrate (≈22% Ni) in the micronutrient stock.
- Toxicity: remove the source; in soil, raising pH reduces Ni availability [2].
- Safety: nickel salts are skin sensitisers and suspected carcinogens; gloves, no dust.
Common mistakes
- Adding "complete" trace-element products with Ni to an already complete feed for no reason.
- Assuming essential means "must be supplemented".
