A rising pH is normal in a Kratky container. The plant itself pushes it up as it feeds, and hard water and algae add to the effect. The fix is regular testing, small corrections and a full change of solution when it will not hold.
Cause 1: The Plant Is Feeding
Most of the nitrogen in hydroponic nutrients is in the nitrate form. As roots absorb nitrate they release compounds that raise the pH of the surrounding water. A healthy, fast-growing plant therefore drives pH upward, and the effect is strongest when the plant is growing hardest.
Cause 2: Hard Water
Hard tap water contains dissolved carbonates, which resist acid and pull pH back up after you lower it. If your pH rebounds within a day or two of adjusting, your water is the likely reason. Every top-up with the same water adds more.
Cause 3: Algae
Algae growing in the solution raises pH during the day as it uses up dissolved carbon dioxide. If the solution is green and the pH is climbing, block the light first.
What to Do
- Test at every top-up, and at least weekly while the plant is fruiting.
- Correct with a few drops of pH down, stir, wait and retest.
- Aim for the lower half of the range, around 5.8, so the pH has room to rise before it leaves the safe zone.
- If you are correcting every couple of days, replace the whole solution.
Slowing It Down
A larger container drifts more slowly than a small one. Mixing tap water with rainwater or filtered water lowers its hardness. Keeping the container lightproof removes algae from the picture.
A pH that falls instead of rising is less common and usually points to a solution that has gone stale or roots that are in trouble.
Related guide: PH for Kratky strawberries.
