Nutrient lockout is when the nutrients are in the soil or the feed, but the roots cannot absorb them. The leaves show deficiency symptoms even though you have been feeding on schedule, because the problem is uptake, not supply. The usual cause is a root-zone pH that has moved outside the range where those nutrients stay in a usable form. Nutrient antagonism, where too much of one element blocks the uptake of another, is the second common cause, and salt build-up or waterlogged roots make things worse. Since the nutrient is already there, adding more fertilizer usually deepens the problem. The first step is to measure the pH and EC of what goes in and what drains out.
What is happening in the root zone
pH controls availability. Penn State Extension explains that pH plays an important role in plant nutrient availability. Purdue University adds that the pH of the growing medium affects the availability of nutrients, especially micronutrients. The University of Delaware gives the practical meaning: when pH is too acidic, potassium, calcium and magnesium can exist in a form roots cannot absorb, even when enough is present. At alkaline pH, iron, manganese, zinc and boron become unavailable.
Penn State notes that the most common cause of iron deficiency is high pH in the medium or irrigation water, and that magnesium deficiency can also be caused by high medium pH.
Antagonism. Plants take up nutrients roughly in proportion to what is in the solution. But when one nutrient is in excess, it can be taken up at the expense of another. Penn State calls this nutrient antagonism. Its examples:
| Nutrient in excess | Can reduce uptake of |
|---|---|
| Nitrogen | Potassium |
| Phosphorus | Zinc |
| Potassium | Nitrogen, calcium, magnesium |
| Sodium | Potassium, calcium, magnesium |
| Calcium | Magnesium, boron |
| Magnesium | Calcium |
| Iron | Manganese |
Penn State describes a case where excess potassium caused a nitrogen deficiency even though plenty of nitrogen was supplied.
Water and air around the roots. Virginia Tech’s diagnostic guide explains that waterlogging leaves roots short of oxygen. Without oxygen, root metabolism stops, so water and nutrient uptake is restricted and the leaves show wilting and deficiency symptoms. This looks like lockout but is a watering problem.
Growers’ books say the same thing in their own words. A Vietnamese translation of a well-known grow guide warns that when pH is too high or too low, roots cannot absorb nutrients, and that this often makes new growers believe their plants are short of fertilizer when that is not the problem. The Vietnamese edition of True Living Organics notes that deficiency is often a sign of nutrients locked in the soil by pH changes the grower caused, for example from hard, high-pH well or tap water.
A humid-climate lookalike: calcium
In a humid grow room or in the monsoon season, calcium problems can appear on new growth even when the medium contains enough calcium. Cornell Cooperative Extension explains that calcium is not mobile within plants and is carried to new leaves with the water that moves through the plant by transpiration. When humidity is high and light is low, plants transpire very slowly and calcium does not reach the leaf edges. Ohio State University notes that in tipburn there is often enough calcium in the root zone, and that in crops like lettuce it can be reduced by raising daytime transpiration. Better airflow and lower humidity address the cause; more calcium fertilizer does not.
Target pH ranges by growing medium
Sources give slightly different numbers. What they agree on is that soil runs higher and soilless systems run lower.
| Medium | Target pH | Source |
|---|---|---|
| Soil | 6.2 to 7.0 | Vietnamese grow guide (casaman translation) |
| Soil | best at 6.5 | VietGrower handbook |
| Coco | best at 5.8 | VietGrower handbook |
| Soilless substrates in general | all nutrients readily available at 5.4 to 6.2 | Purdue University Extension |
| Hydroponics | 5.5 to 5.8 | Vietnamese grow guide (casaman translation) |
Stability matters as much as the exact number. The same guide warns that pH in hydroponic systems moves much faster than in soil, so it needs checking often.
How to tell it is lockout
- The plant is fed regularly at a normal strength, yet shows deficiency signs.
- Several deficiencies appear at once, or the symptoms change from week to week.
- Symptoms get worse after you add more fertilizer or a supplement.
- Runoff pH is far from the pH of the feed, or runoff EC is much higher than the feed EC.
- You recently changed your water source, medium, or nutrient brand.
Remember where the symptoms appear. The University of Delaware notes that mobile nutrients (nitrogen, phosphorus, potassium, magnesium) show deficiency on old leaves first, and immobile nutrients (calcium, copper, boron, iron, manganese, zinc) on new growth first. Penn State adds that magnesium deficiency usually shows first on lower to middle leaves, which helps separate it from iron deficiency. Yellowing between the veins of new leaves in a high-pH system points to iron.
| Lockout | True deficiency | Nutrient burn | |
|---|---|---|---|
| Feeding | Normal or heavy | Light or none | Heavy |
| Runoff pH | Often far off target | Usually near target | Can be anything |
| Runoff EC | Often high | Low | High |
| More fertilizer | Makes it worse | Helps | Makes it worse |
How to fix nutrient lockout
- Stop adding extra nutrients or supplements until you know the cause.
- Measure input and runoff. Check pH and EC of the feed, then of the water that drains from the pot. Purdue describes the PourThru test as a nondestructive method that samples solution from the entire root zone. It recommends recording pH and EC over time, because both change with medium, fertilizer, water quality and plant.
- Calibrate your meters. Purdue notes that standard solutions for pH and conductivity are needed to calibrate instruments. An uncalibrated pen can send you in the wrong direction.
- Correct the feed pH. Penn State’s cure for pH-related deficiencies is to correct the pH of the nutrient solution. If your water has high alkalinity, its pH will creep up; Penn State notes acid can be added to lower high-pH water. Purdue recommends having irrigation water tested by a laboratory before starting a monitoring program.
- Flush if salts are high. If runoff EC is well above the feed, leach the pot with pH-adjusted water, then resume feeding at a moderate strength.
- Fix the root environment. Let waterlogged pots dry back, improve drainage and airflow, and avoid watering on a fixed schedule in cool or humid weather.
- Watch new growth. Damaged leaves stay damaged. Clean new leaves over the following week show the fix is working.
Common mistakes
- Adding cal-mag, iron or more base nutrient without measuring first.
- Checking only the feed pH and never the runoff.
- Pushing pH up and down in big jumps instead of correcting it steadily.
- Overwatering, then reading the resulting deficiency signs as lockout from pH alone.
- Using a soil pH target for coco or hydroponics, or the reverse.