Salt build-up happens when more dissolved ions go into the medium than the plant takes up or the drainage carries away. Water evaporates and is transpired, salts stay behind, and the concentration in the pore water climbs. Sodium, chloride, sulfate and bicarbonate from hard water accumulate especially, because plants use little of them.
In practice
Signs: runoff EC well above input EC (often by 1 mS/cm or more), white crust on the medium surface or pot edges, crisped leaf tips and margins, slowing growth, drooping despite moist medium, and deficiency-like symptoms because high salinity and imbalanced ions block uptake.
Causes: low or no runoff in coco and soilless media, high-EC feeding, hard tap water and infrequent large waterings that let the medium dry hard between them.
Fix: a root-zone flush with enough volume (often two to three times the pot volume) of a mild, pH-adjusted nutrient solution or plain water, then resume feeding at a moderate EC with regular runoff of about 10–20%. Check that runoff EC falls back towards input.
Don't confuse with
Myth check
GenetixBay house position: a root-zone flush to wash accumulated salts out of the medium is the one valid flush. It treats the medium, not the plant, and can be done at any stage. A pre-harvest flush is a Myth and actively harmful: it starves the plant while it is packing on its final weight and resin. See the flush debate.
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.General hardness (GH) is the calcium and magnesium dissolved in water, expressed as mg/L CaCO3 or German degrees (dGH); hard water supplies some Ca and Mg.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.Tap water is municipal or well water used for irrigation; its EC, alkalinity, hardness, sodium and disinfectant vary widely, so it should be tested before use.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.Pot size is the volume of medium a plant roots into; bigger pots give bigger plants within reason, but feeding frequency can let small pots punch well above their size.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.Feeding only plain water for days or weeks before harvest to "clean" the plant; GenetixBay rates it a Myth that harms yield, unlike a root-zone salt flush.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.Osmotic stress occurs when root-zone salt concentration is so high that roots struggle to draw in water, slowing growth and burning leaf tips even in wet media.Nutrient lockout is when plants show deficiency symptoms although nutrients are present, because pH, salt build-up, ion antagonism or damaged roots stop uptake.
