pH Lockout

pH lockout is not a deficiency. It is the root zone sitting outside the pH band where cannabis can absorb nutrients, so the plant shows hunger symptoms while surrounded by food. In soil, most apparent deficiencies are this.

What you are seeing#

The pattern is that there is no single clean pattern. That is the diagnosis.

You will see two or three deficiency signatures on the same plant at once: pale yellowing on lower leaves, interveinal yellowing on new top growth, rust spots in the middle, tips browning. Any one of them would point at a specific element. All of them together points at pH.

Growth slows across the whole plant. Symptoms keep appearing and spreading even though you fed the plant this week and the feed is correct.

High pH, above about 7.0 in soil, yellows the new top growth first, because iron, manganese and zinc lock out first. Low pH, below about 5.5, hits calcium and magnesium, so you get spotting and lower leaf yellowing.

Why it happens#

Nutrients are only absorbable within a narrow band. Soil wants 6.0 to 7.0. Coco wants 5.5 to 6.5. Hydro wants 5.5 to 6.2. Outside that range, specific elements bind into forms roots cannot take in.

Beginner causes:

• Watering with untreated tap water at pH 7.5 to 8.2, which drifts the root zone up over weeks.
• Never measuring runoff, only the water going in. The pot's pH is what matters, not the jug's.
• Salt buildup from feeding without runoff, which shifts pH on its own.
• An uncalibrated pH pen reporting a confident wrong number.
• Adding pH Down without measuring, overshooting to 5.2 and creating a lockout at the other end.

How to fix it#

1. Calibrate your pH pen today with 4.0 and 7.0 solution. An uncalibrated pen is why people end up on this page twice.
2. Water with pH-corrected water, collect the runoff from the bottom, and measure that. This number is your diagnosis.
3. If runoff reads above 7.0 or below 5.5, flush with 2 to 3 pot volumes adjusted to the center of your range: pH 6.5 in soil, 5.9 in coco, 5.8 in hydro.
4. Flushing also removes accumulated salt, usually part of the cause. Check runoff EC and get it under 1.2.
5. Resume feeding at half strength with every input pH-corrected. Do not add nutrients to chase symptoms.
6. Over the next week, measure runoff at every watering until it holds in range three times running.

How to tell it apart#

The honest answer is that you cannot tell it apart by looking. Every deficiency page on this site describes symptoms that pH lockout can reproduce exactly. The discriminator is a measurement, and there is no visual substitute for it.

Versus nitrogen deficiency. Nitrogen deficiency yellows the bottom of the plant evenly and nothing else is wrong. Lockout adds a second or third symptom elsewhere on the plant. If runoff pH reads 6.4 in soil and the only issue is pale lower leaves, it is a real nitrogen shortage and you should feed.

Versus calcium and magnesium deficiency. Both of these are genuinely common in coco and with reverse osmosis water, so they are the two most likely to be real rather than lockout. But low pH below 5.8 blocks calcium specifically. If you have been using pH Down heavily, suspect lockout.

Versus iron deficiency. Bright yellow new growth is the classic high pH signature. If the top of the plant is lemon yellow, check runoff first and expect a reading above 6.8.

How long until it looks better#

Damaged leaves stay damaged. Spots, scorch and yellowing do not reverse, and a plant locked out for two weeks will carry those marks to harvest.

Recovery is faster than most people expect once pH is right, because the nutrients were already there and waiting. New growth should look normal within five to seven days, and the spread should stop within three. If symptoms keep appearing on fresh growth after ten days at confirmed correct runoff pH, the problem is root health rather than chemistry, so look at drainage and watering frequency next.