Hydroponic vegetables – mineral nutrition and the law of the minimum
BiologyPlant PhysiologyAges 16–17
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Plant Physiology
Mix a nutrient solution for hydroponic vegetables by adjusting N, P, K concentrations and pH. The plant shows the exact deficiency symptom for each element (yellowing of old leaves, purple leaves, scorched leaf margins, yellow young leaves with green veins when high pH precipitates iron), growth is capped by the scarcest element like a barrel with one short stave, and an over-concentrated solution wilts the plant through osmotic water loss.
Lesson: Mineral nutrition, crop yield and sustainable agriculture
What it shows
Plants need mineral ions in the right amounts: nitrogen for proteins and chlorophyll, phosphorus for ATP and nucleic acids, and potassium for stomatal control and water balance. Liebig's law of the minimum states that growth is limited by the scarcest essential nutrient, however plentiful the others are. Deficiencies produce typical symptoms, and mobile nutrients such as nitrogen show up first in older leaves. The pH of the solution affects availability, since iron precipitates at high pH, and an over-concentrated solution has a lower water potential than root cells, so roots cannot take up water.
How to use
Start from a balanced solution, then lower Nitrogen (N), Phosphorus (P) or Potassium (K) one at a time and note the leaf symptom and which stave of Liebig's barrel becomes the shortest. While one nutrient is deficient, raise another to show that growth does not improve. Finally, push Solution pH above 7, then raise all three nutrients together and watch EC rise and the plant wilt.
Parameters you can change
- Nitrogen (N) 0–400 mg/L
- Phosphorus (P) 0–120 mg/L
- Potassium (K) 0–500 mg/L
- Solution pH 4–8.5
Questions to explore
- Why do nitrogen deficiency symptoms appear in older leaves before young ones?
- According to Liebig's barrel, will a potassium-deficient plant grow faster if more nitrogen is added?
- Why does a very concentrated nutrient solution make the plant wilt?
