Nutrient toxicity is injury caused by too much of a particular element in the plant tissue. Some elements are directly toxic at high levels: boron, manganese, copper, chloride, sodium and ammonium. Others, such as potassium or phosphorus in excess, do harm mainly by suppressing the uptake of other nutrients and inducing a deficiency.
In practice
Toxicity symptoms often look like deficiencies of a different element: excess manganese induces iron chlorosis, excess phosphorus induces zinc deficiency, excess potassium induces magnesium or calcium deficiency. Direct toxicity often appears as burning of older leaf margins (boron, chloride) or brown speckling (manganese at low pH). Causes include overdosing micronutrients, water with high boron, sodium or chloride, pH drift that frees metals, and copper or zinc from plumbing.
Don't confuse with
- Nutrient burn: tip burn from high total EC rather than a single element.
- Nutrient deficiency: too little rather than too much, though toxicity can cause one.
- Pesticide phytotoxicity: damage from a spray, not from nutrition.
Boron (B) is a micronutrient needed for cell-wall structure and growing tips; the gap between deficient and toxic levels is narrow.Manganese (Mn) is a micronutrient that splits water in photosynthesis and activates many enzymes; low pH can raise it to toxic levels.As a plant nutrient, chlorine is used as the chloride ion (Cl-) in tiny amounts for photosynthesis and osmotic balance; excess from water or salts is the usual problem.Sodium (Na) is not essential for cannabis; it enters through water, softeners and some inputs, and at high levels causes salt stress and crowds out potassium and calcium.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.Calcium (Ca) is an immobile secondary nutrient that builds cell walls and membranes; it travels only in the transpiration stream, so deficiency appears in new growth.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.Nutrient burn is browning and crisping of leaf tips, then margins, caused by a root-zone salt concentration (EC) higher than the plant can handle.Nutrient antagonism is when an excess of one element reduces the uptake or use of another, such as high potassium suppressing magnesium and calcium.
