Nutrient strength: the soup test
Can a plant get too much food?
- A · net pot & seedling
- B · roots
- C · air stone
This tub is full of plant food. So why are the leaf tips brown?
Think about a bowl of soup. With too little salt, it tastes like plain water.
With far too much, you can’t drink it.
Plant food in water works the same way.
Too weak, and the plant stays hungry. Too strong, and the roots struggle to drink.
Food that is too weak leaves the plant hungry.
The leaves turn pale, and the plant grows slowly. There isn’t enough food to go around.
Food that is too strong is a different problem. Very salty water makes it hard for roots to drink.
The leaf tips dry out and turn brown.
Plant food is a mix of salts that vanish into the water.
The more food, the better the water carries a tiny bit of electricity. Growers call that number ECA number that tells you how strong the plant food in water is.
See it in the glossary.
A meter reads the EC for you. Dip its metal tips in a cup of water from the tub.
A bigger number means stronger food.
- A · EC meter
- B · metal tips
- C · water from the tub
- D · plant food
- E · plain water
Your turn. Make the food just right.
This plant is lettuce. Turn the strength up and down, and watch the leaves.
Find the strength where the lettuce grows best.
0.5 · Too weak
Too weak. The plant turns pale and grows slowly.
Find the strength where the lettuce grows best.
Plant food strength (EC): what happens in each zone.
- 0.0 to 0.5 · Too weak. The plant turns pale and grows slowly.
- 1.0 to 2.0 · Just right. The plant is green and growing well. Right Not quite
- 2.5 to 4.0 · Too strong. The roots can't drink well, and leaf tips turn brown.
If the food is too strong, growers add plain water. If it is too weak, they add more food.
Then they check the number again.
Check 1 of 3
The leaf tips are brown and crispy. The EC reads very high. What do you do?
Show the answer
Add plain water. Right. Plain water makes the plant food weaker.
Check 2 of 3
What does an EC meter tell you?
Show the answer
How strong the plant food is. Right. More food in the water gives a bigger number.
Check 3 of 3
The whole plant is pale and grows slowly. The EC is very low. What is going on?
Show the answer
The food is too weak. Right. There isn't enough food in the water, so the plant stays hungry.
You just learned how strong plant food should be, and how growers measure it.
Next up: Oxygen at the roots.
Go deeper: what EC really measuresOpenClose
EC stands for electrical conductivity. It measures how easily an electric current passes through a liquid.
Pure water carries very little current. When plant food dissolves, its salts split into charged particles called ions, and the ions carry the current.
Growers often give EC in millisiemens per centimetre (mS/cm), which is the same as dS/m. Some meters show parts per million (ppm) instead.
Meters turn EC into ppm using different factors, from about 0.5 to 0.7. So ppm numbers from two meters may not match.
Sources give different targets for lettuce, mostly between about 1.0 and 2.0 mS/cm. In one university trial, lettuce grew less at 2.9 and 3.9 than at 1.3 and 2.0.
Fruiting crops such as tomatoes often run higher, but sources disagree. One guide gives tomatoes 2 to 4 mS/cm, while others stay between about 1 and 2.4.
Roots take in water by osmosis: water moves toward the side with more dissolved salts. When the water outside the roots is too salty, roots struggle to take water in.
EC tells you how much food is in the water, not which nutrients. A mix can read just right and still be short of one nutrient.
Plants take in water and nutrients at different rates, so the EC drifts up or down between checks.
Sources
- Singh, H., and Dunn, B. Electrical Conductivity and pH Guide for Hydroponics. HLA-6722. Oklahoma State University Extension, 2016. Link
- Miller, A., Langenhoven, P., and Nemali, K. Optimal Fertilizer Solution Concentration for Hydroponic Lettuce Production. HO-309-W. Purdue Extension. Link
- Hooks, T. Module 3: Hydroponic Nutrient Solutions. Greenhouse Hydroponic Tomato Workshop, University of Arizona Controlled Environment Agriculture Center, 2026. Link
- Vought, K., Bayabil, H., and Martin-Ryals, A. How Efficient Are Hydroponic Lettuce Production Systems in Nutrient Usage Under Low and High Salinity? AE610. UF/IFAS Extension. Link
- Hochmuth, G., and Hochmuth, R. Fertilizer Management for Greenhouse Vegetables. Florida Greenhouse Vegetable Production Handbook, Vol. 3. CV265. UF/IFAS Extension. Link
- Cabrera-Garcia, J. Hydroponic Nutrient Solutions. Publication G6984. University of Missouri Extension. Link
- Hoidal, N., Reardon, A., Worth, L., and Rogers, M. Small-scale hydroponics. University of Minnesota 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
- Boser, S., and Rizzo, D. A Water Quality Toolkit for Greenhouse and Nursery Production. Penn State Extension. Link
- Mattson, N. Testing Container Media. Cornell University. Link
- Sonon, L. S., Saha, U., and Kissel, D. E. Soil Salinity: Testing, Data Interpretation and Recommendations. Circular 1019. University of Georgia Cooperative Extension. Link
- Prasad, L., and Meehan, M. Using Electrical Conductivity and Total Dissolved Solids Meters to Field Test Water Quality. WQ1923. NDSU Extension. Link
- Ayers, R. S., and Westcot, D. W. Water quality for agriculture. FAO Irrigation and Drainage Paper 29 Rev. 1. FAO, 1985. Section 2.3, Salinity effects on crops. 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
- Houston, L., Dickson, R., and Machesney, L. Fine-tuning nutrient replenishment in hydroponics. e-GRO Edible Alert 6(11), 2021. Link
- Lee, J.-Y., Rahman, A., Azam, H., Kim, H. S., and Kwon, M. J. Characterizing nutrient uptake kinetics for efficient crop production during Solanum lycopersicum var. cerasiforme Alef. growth in a closed indoor hydroponic system. PLoS ONE 12(5): e0177041, 2017. Link