pH measures how acidic or alkaline the nutrient solution is. It matters because a tomato can only absorb each nutrient within a certain pH range. Outside it, the nutrients are still in the water but the roots can't take them up, which is called nutrient lockout.
The Range for Tomatoes
Keep a Kratky tomato's solution between pH 5.5 and 6.5. Many growers aim for the middle, around 5.8 to 6.2, because it leaves room for drift in both directions.
| pH | What happens |
|---|---|
| Below 5.5 | Calcium and magnesium become harder to absorb |
| 5.5 to 6.5 | The range where tomatoes take up nutrients best |
| Above 6.5 | Iron and other micronutrients lock out; new leaves turn yellow |
How to Test It
- pH test drops or strips are cheap and fine for a few containers. Compare the color against the chart.
- A pH pen (meter) gives a number and is faster for many containers. Calibrate it with buffer solution as the maker recommends.
Test after mixing a fresh batch, before every top-up, and whenever leaves change color.
How to Adjust It
- Mix the nutrients into the water first, then test. Nutrients change the pH.
- Add pH Down or pH Up a few drops at a time, stir, and test again.
- Stop when you are inside the range. Overshooting and correcting back stresses the plant.
Adjust the top-up solution in a separate bucket before you add it, not inside the container where the roots are.
Why pH Drifts in Kratky
As the tomato drinks and feeds, it takes up some nutrients faster than others, and the pH of what's left moves. Hard tap water also pushes pH up over time. Small changes are normal; check at each top-up and correct the new solution rather than chasing the reservoir every day.
When pH is right but the plant still looks hungry, check strength with an EC meter: see Kratky tomato nutrients. Yellow leaves have several causes besides pH: see yellow tomato leaves.
Back to the Kratky method for tomatoes.
Frequently Asked Questions
What pH do Kratky tomatoes need?
Between 5.5 and 6.5, ideally around 5.8 to 6.2.
What happens if the pH is too high for tomatoes?
Iron and other micronutrients lock out, and the newest leaves turn yellow between the veins even though the nutrients are in the water.
Can you use tap water for Kratky tomatoes?
Usually, yes. Hard tap water can push pH up, so test after mixing and adjust. Let chlorinated water stand or use a dechlorinator if your nutrient maker advises it.