pH: the nutrient lock
The food is right there. So why is this plant hungry?
- A · net pot & seedling
- B · roots
- C · air stone
The water is full of food. Something is in the way.
Think of a snack inside a locked box.
You can see it. You can’t eat it.
Water can lock up plant food the same way.
The food floats right past the roots. The roots can’t take it in.
What works the lock? How sour or how soapy the water is.
Growers measure that with a number called pHA number that tells you how sour or how soapy water is.
See it in the glossary.
The pH scale runs from 0 to 14. Pure water sits in the middle, at 7.
Sour lemon juice is low, and soapy water is high.
Most hydroponic plants like a pH between 5.5 and 6.5.
That is a tiny bit sour. In that zone, every lock is open.
If the pH climbs too high, iron is among the first to lock.
New leaves need iron to turn green. Without it, they come in yellow.
- A · net pot & seedling
- B · roots
- C · air stone
Your turn. Open every lock.
Move the pH up and down. Watch the locks and the leaves.
Open every lock.3 of 5 open
- Nitrogen, for leaves: Open
- Phosphorus, for roots: Locked
- Calcium, for strong stems: Open
- Magnesium, for green: Open
- Iron, for new leaves: Locked
Between about 5.5 and 6.5, all five food locks are open. The plant stays green.
When the pH goes too high, iron locks first. New leaves come in yellow.
Real growers test their water often, because pH drifts as plants eat.
If it drifts, they add a special liquid to nudge it back.
Check 1 of 3
The water has plenty of plant food. The plant still looks hungry. What do you check first?
Show the answer
The pH. Right. Food can be in the water and still be locked.
Check 2 of 3
Which pH do most hydroponic plants like?
Show the answer
About 6. Right. Between 5.5 and 6.5, the locks are open.
Check 3 of 3
The pH climbs too high. Which leaves show it first?
Show the answer
The newest leaves turn yellow. Right. Iron locks up, and new leaves need iron to turn green.
You just learned how pH locks and unlocks plant food.
Next up: Nutrient strength: the soup test.
Go deeper: the chemistryOpenClose
pH measures how many hydrogen ions are in a liquid. More hydrogen ions means a lower pH and a more acidic liquid.
The scale is not even steps. Each whole number is ten times stronger than the one before. So water at pH 5 is ten times more acidic than water at pH 6.
Above about pH 7, iron can join with other things in the water and form tiny solid bits. Plants take in nutrients that are dissolved, so the solid iron is out of reach. Phosphorus also gets harder to take in at high pH.
At low pH, plants take in less calcium and magnesium. That is why those two lock first on the low side of the slider.
The locks in the slider are a simple picture. Real nutrients fade out slowly as the pH changes, and sources draw the edges in slightly different places.
Sources also give slightly different target ranges. Many say 5.5 to 6.5. Others say 5.8 to 6.4, or as wide as 5.0 to 7.0. That is why growers aim for “about 6” and then check what their own crop likes.
Sources
- U.S. Geological Survey, Water Science School. pH and Water. Link
- U.S. Environmental Protection Agency. What is pH? Link
- Clark, M. A., Douglas, M., and Choi, J. Biology 2e. OpenStax, 2018. Section 2.2, Water (pH scale); section 5.2, Passive Transport (osmosis). Link
- Cabrera-Garcia, J. Hydroponic Nutrient Solutions. Publication G6984. University of Missouri Extension. Link
- Singh, H., Dunn, B., and Payton, M. Hydroponic pH modifiers affect plant growth and nutrient content in leafy greens. Journal of Horticultural Research 27(1): 31-36, 2019. Link
- Gillespie, D. P., Kubota, C., and Miller, S. A. Effects of low pH of hydroponic nutrient solution on plant growth, nutrient uptake, and root rot disease incidence of basil (Ocimum basilicum L.). HortScience 55(8): 1251-1258, 2020. Link
- Camberato, D. M., Lopez, R. G., and Mickelbart, M. V. pH and Electrical Conductivity Measurements in Soilless Substrates. HO-237-W. Purdue Extension. Link
- Sánchez, E., F. Di Gioia, R. Berghage, N. Flax, and T. Ford. Hydroponics Systems and Principles of Plant Nutrition: Essential Nutrients, Function, Deficiency, and Excess. Penn State Extension, 2026. Link
- Somerville, C., Cohen, M., Pantanella, E., Stankus, A. and Lovatelli, A. Small-scale aquaponic food production: Integrated fish and plant farming. FAO Fisheries and Aquaculture Technical Paper No. 589. Food and Agriculture Organization of the United Nations, Rome, 2014. Link
- Koenig, R. T., and Koenig, T. C. Chapter 4: Plant Mineral Nutrition and Fertilizers. In The Pacific Northwest Gardener's Handbook. Washington State University Extension. Link