Short answer
Find the cause before you treat it. Most "lockout" is root-zone pH out of range, salt build-up or failing roots, not a nutrient fight. If runoff EC sits about 0.8–1.0 mS/cm or more above what you feed, flush the root zone with 2–3 times the pot volume, then go straight back to normal feeding. Supported.
Why
pH controls solubility. At high pH iron, manganese and zinc drop out of solution; at low pH calcium and magnesium uptake falls [3][4]. Alkaline water pushes the root zone up week by week even if you pH every feed [3].
Real antagonism exists but is narrower than charts suggest. A review of 117 interactions found synergy far more often than antagonism; the ones that matter are among the cations [1]. In cannabis, high potassium supply lowered calcium and magnesium uptake [2], which is what stacked PK boosters do.
Salt build-up raises root-zone EC until roots struggle to take up water and ions. That is the one case where a flush helps, because it treats the medium.
The numbers
| Check |
Normal |
Act if |
| Runoff pH, coco or rockwool |
5.8–6.5 |
Outside, or >0.5 from input and rising |
| Runoff pH, soil |
6.0–7.0 |
Below 5.8 or above 7.3 |
| Runoff EC vs input |
Equal to +0.5 mS/cm |
+0.8–1.0 mS/cm or more |
| Roots |
White to cream, firm |
Brown, slimy, smelly |
What to do
- Calibrate the pH meter in fresh pH 7 and pH 4 buffer.
- Measure input and runoff pH and EC.
- Check roots and watering: soggy pots and rot mimic lockout.
- Salty root zone: flush 2–3× pot volume, slowly, then feed normally.
- pH off: correct the input gradually; acidify alkaline water.
- Stop stacking boosters and Cal-Mag on a complete base.
Common mistakes
- Flushing with plain water for every leaf spot.
- Adding more of the "missing" nutrient, which raises EC further.
- Trusting an uncalibrated pen.
- Expecting damaged leaves to turn green again.
Nutrient lockout is when plants show deficiency symptoms although nutrients are present, because pH, salt build-up, ion antagonism or damaged roots stop uptake.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.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 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.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.mS/cm (millisiemens per centimetre) is the standard unit of EC for nutrient solutions; 1 mS/cm equals 1 dS/m and 1,000 µS/cm.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.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.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 antagonism is when an excess of one element reduces the uptake or use of another, such as high potassium suppressing magnesium and calcium.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.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.A pH meter measures solution pH with a glass electrode whose voltage changes with hydrogen ion activity; it needs regular calibration and a moist probe.A calibration solution is a standard liquid of known pH or EC used to calibrate meters, such as pH 4.01, 7.00 and 10.01 buffers or 1.413 mS/cm EC standard.Cal-Mag is a supplement supplying calcium and magnesium, usually as nitrates; need depends on cultivar, water source (RO vs tap) and medium (coco), not on LED lighting.Any disease that causes discrete dead spots on leaves; on cannabis, mainly fungi such as Septoria cannabis and Alternaria, plus some bacteria.
