Short answer: mainly because plants take up nitrate, and alkaline water keeps pushing pH up. Both are normal. A gradual rise in a Kratky container is usually not a problem unless plants start showing symptoms.
Nitrate uptake
Most hydroponic nutrients supply nitrogen as nitrate, which carries a negative charge. When roots absorb nitrate, they release negatively charged ions such as hydroxide or bicarbonate to stay balanced. Those ions make the solution more alkaline, so pH rises as the plant grows. Nutrients that contain some ammonium have the opposite effect, which is one reason formulas differ in how much pH drift they cause.
Alkaline water
Hard tap water contains bicarbonate, which buffers against acid. After you lower the pH at mixing time, that bicarbonate keeps pulling it back up. Oklahoma State University Extension notes that water with alkalinity above about 75 ppm causes pH to increase in nutrient solutions and needs checking more often.
Algae
If light reaches the solution and algae grow, their photosynthesis removes carbon dioxide from the water during the day, which also raises pH. A lightproof container prevents this.
When to act
UF/IFAS notes that once a Kratky solution is balanced at mixing, there is rarely a need to keep adjusting. Check pH if you see:
- New leaves turning pale between the veins, a common sign of iron shortage at high pH
- Growth slowing for no other obvious reason
Bringing it back down
Add a small amount of pH down, stir gently without disturbing the roots, and retest. In a container with an established air gap, add acid diluted in a little solution rather than raising the level.
Related guide: managing pH in Kratky nutrient solution.
Sources
- Dunn, B. and Singh, H. Oklahoma State University Extension. Electrical Conductivity and pH Guide for Hydroponics (HLA-6722).
- University of Florida IFAS Extension. Set It and Forget It Hydroponics (HS1488).
- Greenhouse Grower, with Neil Mattson (Cornell University). Tips to Avoid Nutrient Issues in Greenhouse Lettuce.
