Lesson guide · Lesson 10 · Grades 3–8 · 6 min

pH: the nutrient lock

Why plant food in the water can be out of reach, and how pH unlocks it.

Open the lessonPrintable worksheet

Students will

  • Explain that nutrients can be in the water but out of reach.
  • Name the pH range most hydroponic plants like.
  • Describe one sign that pH is too high.

New words

pH
A number that tells you how sour or how soapy water is.

Talk about it

Can you think of something you can see but can't use until something changes?

Check questions and answers

  1. The water has plenty of plant food. The plant still looks hungry. What do you check first?

    • The pH (answer) Right. Food can be in the water and still be locked.
    • Add more plant food Not quite. The food is already there. Something is locking it.
    • Give it more light Not quite. Light can't unlock food in the water.

  2. Which pH do most hydroponic plants like?

    • About 6 (answer) Right. Between 5.5 and 6.5, the locks are open.
    • About 2 Not quite. That is as sour as lemon juice. It is far too sour for plants.
    • About 9 Not quite. That is too soapy. Iron and other foods lock up.

  3. The pH climbs too high. Which leaves show it first?

    • The newest leaves turn yellow (answer) Right. Iron locks up, and new leaves need iron to turn green.
    • The oldest leaves fall off Not quite. Look at the top of the plant, where new leaves grow.
    • The roots turn green Not quite. The first sign is in the new leaves.

Sources

  • U.S. Geological Survey, Water Science School. pH and Water.
  • U.S. Environmental Protection Agency. What is pH?
  • Clark, M. A., Douglas, M., and Choi, J. Biology 2e. OpenStax, 2018. Section 2.2, Water (pH scale); section 5.2, Passive Transport (osmosis).
  • Cabrera-Garcia, J. Hydroponic Nutrient Solutions. Publication G6984. University of Missouri Extension.
  • 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.
  • 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.
  • Camberato, D. M., Lopez, R. G., and Mickelbart, M. V. pH and Electrical Conductivity Measurements in Soilless Substrates. HO-237-W. Purdue Extension.
  • 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.
  • 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.
  • 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.