排液（或称排废式）是指每次灌溉循环后从栽培基质中流出且不再循环利用的营养液。监测排液的pH和EC是诊断根区实际状况的重要手段，其值可能与输入液差异显著。
关键事实
- Drain-to-waste systems use runoff to flush salt build-up — typically 10–20% runoff per irrigation
- Runoff EC higher than input EC indicates salt accumulation in the medium
- Runoff EC lower than input EC indicates the plant is consuming more nutrients than it receives
- Runoff pH outside 5.5–6.5 signals a root zone chemistry problem requiring correction
- Recirculating systems capture and reuse runoff — less waste but requires regular solution changes
- Coco coir growers typically target 20–30% runoff per feeding to maintain a clean root zone