Osmotic stress is the water-uptake problem caused by high solute concentration around the roots. Water moves from higher to lower water potential; dissolved salts lower the water potential of the root-zone solution. When it falls close to the plant's own, roots take up water slowly, as if the medium were drier than it is. High sodium and chloride add specific ion toxicity on top.
In practice
Signs: dull, dark leaves, crisped tips and margins, wilting in moist medium, slowing growth, and runoff EC well above input. Seedlings and fresh clones show it at EC levels that established plants tolerate. A sudden jump in feed strength or a medium left to dry hard (concentrating salts) are common triggers.
Cannabis tolerates a fairly wide EC window, but more is not better. Baas and Wijnen (2023) tested cannabis in rockwool from 1.8 to 12 mS/cm, and Yep, Gale and Zheng (2020) examined NaCl-induced stress in drug-type cannabis. Hershkowitz and colleagues (2025) found no yield benefit from elevated nutrient concentration.
Fix: lower input EC, increase runoff, and run a root-zone flush if runoff EC is far above input.
Don't confuse with
- Nutrient burn: the visible tip damage, often one symptom of osmotic stress.
- Underwatering: also wilting, but medium is dry and EC normal.
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.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.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.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.Input EC is the electrical conductivity of the nutrient solution going into the pot or system, in mS/cm, including the base EC of the source water.mS/cm (millisiemens per centimetre) is the standard unit of EC for nutrient solutions; 1 mS/cm equals 1 dS/m and 1,000 µS/cm.Running a large volume of low-EC water or mild solution through the medium to wash out accumulated salts; it treats the medium, not the plant, and suits any stage.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.Salt build-up is the accumulation of unused fertiliser and water salts in the growing medium, raising root-zone EC and causing osmotic stress and lockout-type symptoms.Runoff EC is the conductivity of solution draining from the pot; compared with input EC, it shows whether salts are building up or being used up in the root zone.Nutrient lockout is when plants show deficiency symptoms although nutrients are present, because pH, salt build-up, ion antagonism or damaged roots stop uptake.
