pH for Kratky Nutrient Solution
pH measures how acidic or alkaline the solution is. It matters because it controls whether nutrients in the water can be absorbed. A solution can contain everything a plant needs and still starve it if the pH is far off.
The range to aim for
Most hydroponic crops do best in a slightly acidic solution. UF/IFAS recommends about 5.5 to 6.0 for Kratky growing. Oklahoma State University Extension gives 5 to 6 for nutrient solutions, usually 5.5, and lists crop-specific ranges that mostly fall between 5.5 and 6.5. A target of 5.5 to 6.0 suits most Kratky crops.
Why pH matters
Nutrients are most available to plants in this slightly acidic range (Oklahoma State University Extension). When pH runs high, iron and some other micronutrients become harder to absorb, and the newest leaves can turn pale between the veins (Greenhouse Grower, with Neil Mattson of Cornell University).
How to test pH
- Test strips: cheap and simple; read the color against the chart.
- Liquid drops: a few drops of indicator in a sample of solution; easier to read than strips for many people.
- pH meter: the most precise. Calibrate it with buffer solutions (commonly pH 7 and pH 4) and store the probe as the maker recommends (Oklahoma State University Extension).
Lowering pH
Most tap water and many nutrients leave the solution above the target. Products sold as pH down (usually phosphoric acid), citric acid or white vinegar all lower pH (Oklahoma State University Extension). UF/IFAS notes that vinegar typically takes around 2 teaspoons per gallon, depending on the water. Add a little at a time, stir, wait several minutes and test again.
Raising pH
If you overshoot, pH up products (usually potassium hydroxide or potassium carbonate) raise it again (Oklahoma State University Extension). Use very small amounts; they are strong.
Always adjust pH after strength
Adding nutrient changes pH, so set the EC first and adjust pH last (Oklahoma State University Extension).
pH drift during the crop
pH changes over time as plants take up nutrients. Hard water with high alkalinity tends to push pH up (Oklahoma State University Extension). In a well-mixed Kratky container there is rarely a need to keep adjusting (UF/IFAS), but if plants show symptoms, check pH before adding anything else.
Sources
- University of Florida IFAS Extension. Set It and Forget It Hydroponics (HS1488).
- Dunn, B. and Singh, H. Oklahoma State University Extension. Electrical Conductivity and pH Guide for Hydroponics (HLA-6722).
- Greenhouse Grower, with Neil Mattson (Cornell University). Tips to Avoid Nutrient Issues in Greenhouse Lettuce.
