Watering to runoff is irrigating until a portion of the applied water drains out of the pot. Runoff confirms the whole medium has been wetted, and it carries away excess salts that would otherwise accumulate. In drain-to-waste coco and rockwool, a set runoff fraction, often 10-20% of the volume applied, is a standard control target.
In practice
Apply slowly and evenly across the surface until drainage starts, then measure: if 2 L goes in and 0.3 L comes out, runoff is 15%. Testing runoff pH and EC compared with the input tells you whether salts are building up in the medium. In living soil, many growers avoid heavy runoff to keep nutrients and microbes in the bed and water to near field capacity instead.
Don't confuse with
- Flushing: deliberately pushing large volumes through to strip the medium. A root-zone flush to remove salt build-up is valid at any stage; a pre-harvest "flush" of the plant is a myth that harms yield, per GenetixBay. See the flush debate.
- Leaching fraction: the agronomy name for the same runoff percentage.
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.Drain-to-waste fertigation feeds each plant fresh nutrient solution and lets the runoff leave the system rather than returning it to the reservoir.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.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.Field capacity is the amount of water a medium holds after it has been saturated and allowed to drain freely; in pots it is called container capacity.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.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.The wet-dry cycle is the alternation between watering a container and letting it dry back, which refreshes oxygen in the root zone between irrigations.Dryback is the drop in a substrate's water content between irrigations, usually expressed as a percentage of its content at full saturation.
