Runoff PPM: What Your Drainage Water Tells You
Runoff is a free diagnostic. Comparing what goes in against what comes out tells you whether the plant is eating, starving, or drowning in salt.
How to take a reading
Water to at least 20% runoff, collect from the middle of the flow rather than the first drips, and measure both your input and output ppm with the same meter.
What the numbers mean
Runoff close to input means the plant is consuming roughly what you provide. Runoff much higher than input means salts are accumulating and you should reduce feed strength. Runoff much lower than input means the plant is hungry and you can increase.
When to flush
Runoff persistently 400-600 ppm above input, combined with burnt tips or lockout symptoms, justifies a flush with three pot volumes of pH-adjusted water, followed by a light feed.
Frequently asked questions
- Does runoff testing work in living soil?
- Less usefully. Living soil buffers heavily and readings are noisy — judge by plant appearance instead.
Keep reading
Legal notice: We do not promote or encourage breaking the law. This site is for adults 21+ growing in states and jurisdictions that have legalized cannabis cultivation, and who are following all legal growing guidelines, plant count limits, and permit requirements that apply to them.