Chlorine is an essential micronutrient, taken up and used as the chloride ion (Cl-). It helps the water-splitting step of photosynthesis, balances charge and osmotic pressure in cells and supports stomatal movement. Chloride is highly mobile and is present almost everywhere, so true deficiency is rare.
In practice
The practical issue is excess. Tap water, coco that was not washed, calcium chloride or potassium chloride, and some organic inputs can push chloride high enough to burn leaf margins and tips, starting on older leaves. Moderate chloride is tolerated; problems tend to appear when it climbs into the low hundreds of mg/L (ppm)being reviewed. A water report or runoff test shows where it comes from.
Don't confuse with
- Chlorine as water treatment: dissolved chlorine gas or hypochlorite used as a disinfectant, a separate topic covered in pH and water quality.
- Chloramine: a longer-lasting disinfectant in many municipal supplies.
- Nutrient burn: tip burn from total EC rather than chloride specifically.
A micronutrient is an essential element plants need only in trace amounts, below a few mg/L: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel.The process in chloroplasts that uses light energy to turn carbon dioxide and water into sugars and oxygen; cannabis uses the C3 pathway.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 chloride (CaCl2) adds calcium without nitrogen, useful for late-flower nitrogen tapering, but it adds chloride, so doses must stay modest.Free chlorine is the disinfectant (hypochlorous acid and hypochlorite) added to many tap waters; it dissipates from standing or aerated water within about a day.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.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.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.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.Nutrient toxicity is damage from an excess of an element in the plant, either directly or by blocking uptake of others; boron, manganese, copper and chloride are common culprits.A mobile nutrient is one the plant can move from older leaves to new growth, so its deficiency shows first on lower, older leaves: N, P, K, Mg, Cl, Mo.
