Runoff (drainage, leachate) is the solution that leaves a container after watering. Its pH and EC reflect the conditions in the medium more than the input solution does, because it has passed through the root zone and picked up or lost ions. Runoff fraction is the share of applied water that drains, for example 1 L out of 5 L applied is 20% runoff.
In practice
In coco and rockwool drain-to-waste, growers usually aim for about 10–20% runoff per day to stop salts accumulating. In soil and living soil, many growers water to little or no runoff and test it only occasionally.
How to test: water as normal, collect the first runoff in a clean container, and measure EC and pH promptly. For more consistent numbers, use the PourThru method developed for containers by NC State: wait about an hour after irrigation, then pour just enough distilled water on top to collect roughly 50 mL of leachate (Cavins and colleagues). Compare trends, not single readings.
Don't confuse with
- Root-zone flush: deliberately running large volumes through to strip salts; runoff is the normal drainage from routine watering.
- Input solution: what goes into the pot.
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.
Watering to runoff means applying enough water or solution that some drains from the bottom of the container, typically 10-20% of the volume applied.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.The root zone is the volume of medium or solution where a plant's roots live, including its water, air, temperature, pH, EC and microbes.Drain-to-waste fertigation feeds each plant fresh nutrient solution and lets the runoff leave the system rather than returning it to the reservoir.Living soil is an organic growing medium managed to sustain an active soil food web, so microbes and invertebrates release nutrients and the soil is reused across cycles.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.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.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.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.
